diff --git a/.github/ISSUE_TEMPLATE/bug-report.md b/.github/ISSUE_TEMPLATE/bug-report.md
index 5352feb5461e7..7bdba491ceb3f 100644
--- a/.github/ISSUE_TEMPLATE/bug-report.md
+++ b/.github/ISSUE_TEMPLATE/bug-report.md
@@ -15,7 +15,7 @@ A minimal repro, with source-code provided, is ideal.  Using [sharplab](https://
 
 **Diagnostic Id**:
 
-If this is a report about a bug in an analyzer, please include the diagnostic if possible (e.g. `"IDE0030"`).
+If this is a report about a bug in an analyzer, please include the diagnostic ID and message if possible (e.g. `"IDE0030: Use coalesce expression"`).
 
 **Expected Behavior**:
 
diff --git a/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 9.md b/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 9.md
new file mode 100644
index 0000000000000..0b9cd956cdf28
--- /dev/null
+++ b/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 9.md	
@@ -0,0 +1,27 @@
+# This document lists known breaking changes in Roslyn after .NET 8 all the way to .NET 9.
+
+## Iterators introduce safe context in C# 13 and newer
+
+***Introduced in Visual Studio 2022 version 17.11***
+
+Although the language spec states that iterators introduce a safe context, Roslyn does not implement that in C# 12 and lower.
+This will change in C# 13 as part of [a feature which allows unsafe code in iterators](https://github.com/dotnet/roslyn/issues/72662).
+The change does not break normal scenarios as it was disallowed to use unsafe constructs directly in iterators anyway.
+However, it can break scenarios where an unsafe context was previously inherited into nested local functions, for example:
+
+```cs
+unsafe class C // unsafe context
+{
+    System.Collections.Generic.IEnumerable<int> M() // an iterator
+    {
+        yield return 1;
+        local();
+        void local()
+        {
+            int* p = null; // allowed in C# 12; error in C# 13
+        }
+    }
+}
+```
+
+You can work around the break simply by adding the `unsafe` modifier to the local function.
diff --git a/docs/compilers/CSharp/Warnversion Warning Waves.md b/docs/compilers/CSharp/Warnversion Warning Waves.md
index 65d25e8b1b528..f80c80e658838 100644
--- a/docs/compilers/CSharp/Warnversion Warning Waves.md	
+++ b/docs/compilers/CSharp/Warnversion Warning Waves.md	
@@ -13,6 +13,14 @@ In a typical project, this setting is controlled by the `AnalysisLevel` property
 which determines the `WarningLevel` property (passed to the `Csc` task).
 For more information on `AnalysisLevel`, see https://devblogs.microsoft.com/dotnet/automatically-find-latent-bugs-in-your-code-with-net-5/
 
+## Warning level 9
+
+The compiler shipped with .NET 9 (the C# 13 compiler) contains the following warnings which are reported only under `/warn:9` or higher.
+
+| Warning ID | Description |
+|------------|-------------|
+| CS9237 | ['yield return' should not be used in the body of a lock statement](https://github.com/dotnet/roslyn/issues/72443) |
+
 ## Warning level 8
 
 The compiler shipped with .NET 8 (the C# 12 compiler) contains the following warnings which are reported only under `/warn:8` or higher.
diff --git a/src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs b/src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs
index e54d02e929a84..6ab6724af1d2a 100644
--- a/src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs
+++ b/src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs
@@ -872,12 +872,24 @@ private static bool CheckNotNamespaceOrType(BoundExpression expr, BindingDiagnos
             }
         }
 
-        private bool CheckLocalValueKind(SyntaxNode node, BoundLocal local, BindValueKind valueKind, bool checkingReceiver, BindingDiagnosticBag diagnostics)
+        private void CheckAddressOfInAsyncOrIteratorMethod(SyntaxNode node, BindValueKind valueKind, BindingDiagnosticBag diagnostics)
         {
-            if (valueKind == BindValueKind.AddressOf && this.IsInAsyncMethod())
+            if (valueKind == BindValueKind.AddressOf)
             {
-                Error(diagnostics, ErrorCode.WRN_AddressOfInAsync, node);
+                if (this.IsInAsyncMethod())
+                {
+                    Error(diagnostics, ErrorCode.WRN_AddressOfInAsync, node);
+                }
+                else if (this.IsDirectlyInIterator && Compilation.IsFeatureEnabled(MessageID.IDS_FeatureRefUnsafeInIteratorAsync))
+                {
+                    Error(diagnostics, ErrorCode.ERR_AddressOfInIterator, node);
+                }
             }
+        }
+
+        private bool CheckLocalValueKind(SyntaxNode node, BoundLocal local, BindValueKind valueKind, bool checkingReceiver, BindingDiagnosticBag diagnostics)
+        {
+            CheckAddressOfInAsyncOrIteratorMethod(node, valueKind, diagnostics);
 
             // Local constants are never variables. Local variables are sometimes
             // not to be treated as variables, if they are fixed, declared in a using, 
@@ -968,10 +980,8 @@ internal partial class Binder
         private bool CheckParameterValueKind(SyntaxNode node, BoundParameter parameter, BindValueKind valueKind, bool checkingReceiver, BindingDiagnosticBag diagnostics)
         {
             Debug.Assert(!RequiresAssignableVariable(BindValueKind.AddressOf));
-            if (valueKind == BindValueKind.AddressOf && this.IsInAsyncMethod())
-            {
-                Error(diagnostics, ErrorCode.WRN_AddressOfInAsync, node);
-            }
+
+            CheckAddressOfInAsyncOrIteratorMethod(node, valueKind, diagnostics);
 
             ParameterSymbol parameterSymbol = parameter.ParameterSymbol;
 
diff --git a/src/Compilers/CSharp/Portable/Binder/BinderFactory.BinderFactoryVisitor.cs b/src/Compilers/CSharp/Portable/Binder/BinderFactory.BinderFactoryVisitor.cs
index 39e48112f4f15..86c3bbd550cf6 100644
--- a/src/Compilers/CSharp/Portable/Binder/BinderFactory.BinderFactoryVisitor.cs
+++ b/src/Compilers/CSharp/Portable/Binder/BinderFactory.BinderFactoryVisitor.cs
@@ -171,6 +171,7 @@ public override Binder VisitMethodDeclaration(MethodDeclarationSyntax methodDecl
                     }
 
                     SourceMemberMethodSymbol method = null;
+                    bool isIteratorBody = false;
 
                     if (usage != NodeUsage.Normal && methodDecl.TypeParameterList != null)
                     {
@@ -181,10 +182,11 @@ public override Binder VisitMethodDeclaration(MethodDeclarationSyntax methodDecl
                     if (usage == NodeUsage.MethodBody)
                     {
                         method = method ?? GetMethodSymbol(methodDecl, resultBinder);
+                        isIteratorBody = method.IsIterator;
                         resultBinder = new InMethodBinder(method, resultBinder);
                     }
 
-                    resultBinder = resultBinder.WithUnsafeRegionIfNecessary(methodDecl.Modifiers);
+                    resultBinder = resultBinder.SetOrClearUnsafeRegionIfNecessary(methodDecl.Modifiers, isIteratorBody: isIteratorBody);
                     binderCache.TryAdd(key, resultBinder);
                 }
 
@@ -222,7 +224,7 @@ public override Binder VisitConstructorDeclaration(ConstructorDeclarationSyntax
                         }
                     }
 
-                    resultBinder = resultBinder.WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    resultBinder = resultBinder.SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
 
                     binderCache.TryAdd(key, resultBinder);
                 }
@@ -249,7 +251,7 @@ public override Binder VisitDestructorDeclaration(DestructorDeclarationSyntax pa
                     SourceMemberMethodSymbol method = GetMethodSymbol(parent, resultBinder);
                     resultBinder = new InMethodBinder(method, resultBinder);
 
-                    resultBinder = resultBinder.WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    resultBinder = resultBinder.SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
 
                     binderCache.TryAdd(key, resultBinder);
                 }
@@ -311,6 +313,10 @@ public override Binder VisitAccessorDeclaration(AccessorDeclarationSyntax parent
                         if ((object)accessor != null)
                         {
                             resultBinder = new InMethodBinder(accessor, resultBinder);
+
+                            resultBinder = resultBinder.SetOrClearUnsafeRegionIfNecessary(
+                                modifiers: default,
+                                isIteratorBody: accessor.IsIterator);
                         }
                     }
 
@@ -338,12 +344,14 @@ private Binder VisitOperatorOrConversionDeclaration(BaseMethodDeclarationSyntax
                     resultBinder = VisitCore(parent.Parent);
 
                     MethodSymbol method = GetMethodSymbol(parent, resultBinder);
+                    bool isIteratorBody = false;
                     if ((object)method != null && inBody)
                     {
+                        isIteratorBody = method.IsIterator;
                         resultBinder = new InMethodBinder(method, resultBinder);
                     }
 
-                    resultBinder = resultBinder.WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    resultBinder = resultBinder.SetOrClearUnsafeRegionIfNecessary(parent.Modifiers, isIteratorBody: isIteratorBody);
 
                     binderCache.TryAdd(key, resultBinder);
                 }
@@ -363,24 +371,24 @@ public override Binder VisitConversionOperatorDeclaration(ConversionOperatorDecl
 
             public override Binder VisitFieldDeclaration(FieldDeclarationSyntax parent)
             {
-                return VisitCore(parent.Parent).WithUnsafeRegionIfNecessary(parent.Modifiers);
+                return VisitCore(parent.Parent).SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
             }
 
             public override Binder VisitEventDeclaration(EventDeclarationSyntax parent)
             {
-                return VisitCore(parent.Parent).WithUnsafeRegionIfNecessary(parent.Modifiers);
+                return VisitCore(parent.Parent).SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
             }
 
             public override Binder VisitEventFieldDeclaration(EventFieldDeclarationSyntax parent)
             {
-                return VisitCore(parent.Parent).WithUnsafeRegionIfNecessary(parent.Modifiers);
+                return VisitCore(parent.Parent).SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
             }
 
             public override Binder VisitPropertyDeclaration(PropertyDeclarationSyntax parent)
             {
                 if (!LookupPosition.IsInBody(_position, parent))
                 {
-                    return VisitCore(parent.Parent).WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    return VisitCore(parent.Parent).SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
                 }
 
                 return VisitPropertyOrIndexerExpressionBody(parent);
@@ -390,7 +398,7 @@ public override Binder VisitIndexerDeclaration(IndexerDeclarationSyntax parent)
             {
                 if (!LookupPosition.IsInBody(_position, parent))
                 {
-                    return VisitCore(parent.Parent).WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    return VisitCore(parent.Parent).SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
                 }
 
                 return VisitPropertyOrIndexerExpressionBody(parent);
@@ -403,12 +411,16 @@ private Binder VisitPropertyOrIndexerExpressionBody(BasePropertyDeclarationSynta
                 Binder resultBinder;
                 if (!binderCache.TryGetValue(key, out resultBinder))
                 {
-                    resultBinder = VisitCore(parent.Parent).WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    resultBinder = VisitCore(parent.Parent).SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
 
                     var propertySymbol = GetPropertySymbol(parent, resultBinder);
                     var accessor = propertySymbol.GetMethod;
                     if ((object)accessor != null)
                     {
+                        // Expression body cannot be an iterator, otherwise we would need to pass
+                        // `isIteratorBody` to `SetOrClearUnsafeRegionIfNecessary` above.
+                        Debug.Assert(!accessor.IsIterator);
+
                         resultBinder = new InMethodBinder(accessor, resultBinder);
                     }
 
@@ -663,7 +675,7 @@ public override Binder VisitDelegateDeclaration(DelegateDeclarationSyntax parent
                         resultBinder = new WithClassTypeParametersBinder(container, resultBinder);
                     }
 
-                    resultBinder = resultBinder.WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    resultBinder = resultBinder.SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
 
                     binderCache.TryAdd(key, resultBinder);
                 }
@@ -691,7 +703,7 @@ public override Binder VisitEnumDeclaration(EnumDeclarationSyntax parent)
 
                     resultBinder = new InContainerBinder(container, outer);
 
-                    resultBinder = resultBinder.WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    resultBinder = resultBinder.SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
 
                     binderCache.TryAdd(key, resultBinder);
                 }
@@ -773,7 +785,7 @@ internal Binder VisitTypeDeclarationCore(TypeDeclarationSyntax parent, NodeUsage
                         }
                     }
 
-                    resultBinder = resultBinder.WithUnsafeRegionIfNecessary(parent.Modifiers);
+                    resultBinder = resultBinder.SetOrClearUnsafeRegionIfNecessary(parent.Modifiers);
 
                     binderCache.TryAdd(key, resultBinder);
                 }
diff --git a/src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs b/src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs
index 95ab2164a2f99..23fef4ab05257 100644
--- a/src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs
+++ b/src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs
@@ -3182,21 +3182,13 @@ private BoundExpression BindOutVariableDeclarationArgument(
         /// <summary>
         /// Reports an error when a bad special by-ref local was found.
         /// </summary>
-        internal static void CheckRestrictedTypeInAsyncMethod(Symbol containingSymbol, TypeSymbol type, BindingDiagnosticBag diagnostics, SyntaxNode syntax, ErrorCode errorCode = ErrorCode.ERR_BadSpecialByRefLocal)
+        internal static void CheckRestrictedTypeInAsyncMethod(Symbol containingSymbol, TypeSymbol type, BindingDiagnosticBag diagnostics, SyntaxNode syntax)
         {
-            Debug.Assert(errorCode is ErrorCode.ERR_BadSpecialByRefLocal or ErrorCode.ERR_BadSpecialByRefUsing or ErrorCode.ERR_BadSpecialByRefLock);
             if (containingSymbol.Kind == SymbolKind.Method
                 && ((MethodSymbol)containingSymbol).IsAsync
                 && type.IsRestrictedType())
             {
-                if (errorCode == ErrorCode.ERR_BadSpecialByRefLock)
-                {
-                    Error(diagnostics, errorCode, syntax);
-                }
-                else
-                {
-                    Error(diagnostics, errorCode, syntax, type);
-                }
+                CheckFeatureAvailability(syntax, MessageID.IDS_FeatureRefUnsafeInIteratorAsync, diagnostics);
             }
         }
 
diff --git a/src/Compilers/CSharp/Portable/Binder/Binder_Flags.cs b/src/Compilers/CSharp/Portable/Binder/Binder_Flags.cs
index 7374ede9d1163..e594a2999aea2 100644
--- a/src/Compilers/CSharp/Portable/Binder/Binder_Flags.cs
+++ b/src/Compilers/CSharp/Portable/Binder/Binder_Flags.cs
@@ -91,11 +91,24 @@ internal Binder WithAdditionalFlagsAndContainingMemberOrLambda(BinderFlags flags
             return new BinderWithContainingMemberOrLambda(this, this.Flags | flags, containing);
         }
 
-        internal Binder WithUnsafeRegionIfNecessary(SyntaxTokenList modifiers)
+        internal Binder SetOrClearUnsafeRegionIfNecessary(SyntaxTokenList modifiers, bool isIteratorBody = false)
         {
-            return (this.Flags.Includes(BinderFlags.UnsafeRegion) || !modifiers.Any(SyntaxKind.UnsafeKeyword))
-                ? this
-                : new Binder(this, this.Flags | BinderFlags.UnsafeRegion);
+            // In C# 13 and above, iterator bodies define a safe context even when nested in an unsafe context.
+            // In C# 12 and below, we keep the (spec violating) behavior that iterator bodies inherit the safe/unsafe context
+            // from their containing scope. Since there are errors for unsafe constructs directly in iterators,
+            // this inherited unsafe context can be observed only in nested non-iterator local functions.
+            var withoutUnsafe = isIteratorBody && this.Compilation.IsFeatureEnabled(MessageID.IDS_FeatureRefUnsafeInIteratorAsync);
+
+            if (this.Flags.Includes(BinderFlags.UnsafeRegion))
+            {
+                return withoutUnsafe
+                    ? new Binder(this, this.Flags & ~BinderFlags.UnsafeRegion)
+                    : this;
+            }
+
+            return !withoutUnsafe && modifiers.Any(SyntaxKind.UnsafeKeyword)
+                ? new Binder(this, this.Flags | BinderFlags.UnsafeRegion)
+                : this;
         }
 
         internal Binder WithCheckedOrUncheckedRegion(bool @checked)
diff --git a/src/Compilers/CSharp/Portable/Binder/Binder_Statements.cs b/src/Compilers/CSharp/Portable/Binder/Binder_Statements.cs
index 6c3c6d41eae46..936a3bba5ce26 100644
--- a/src/Compilers/CSharp/Portable/Binder/Binder_Statements.cs
+++ b/src/Compilers/CSharp/Portable/Binder/Binder_Statements.cs
@@ -176,9 +176,7 @@ private BoundStatement BindUnsafeStatement(UnsafeStatementSyntax node, BindingDi
             }
             else if (this.IsIndirectlyInIterator) // called *after* we know the binder map has been created.
             {
-                // Spec 8.2: "An iterator block always defines a safe context, even when its declaration
-                // is nested in an unsafe context."
-                Error(diagnostics, ErrorCode.ERR_IllegalInnerUnsafe, node.UnsafeKeyword);
+                CheckFeatureAvailability(node.UnsafeKeyword, MessageID.IDS_FeatureRefUnsafeInIteratorAsync, diagnostics);
             }
 
             return BindEmbeddedBlock(node.Block, diagnostics);
@@ -268,6 +266,15 @@ private BoundStatement BindYieldReturnStatement(YieldStatementSyntax node, Bindi
             {
                 Error(diagnostics, ErrorCode.ERR_YieldNotAllowedInScript, node.YieldKeyword);
             }
+            else if (InUnsafeRegion && Compilation.IsFeatureEnabled(MessageID.IDS_FeatureRefUnsafeInIteratorAsync))
+            {
+                Error(diagnostics, ErrorCode.ERR_BadYieldInUnsafe, node.YieldKeyword);
+            }
+            // NOTE: Error conditions should be checked above this point; only warning conditions below.
+            else if (this.Flags.Includes(BinderFlags.InLockBody))
+            {
+                Error(diagnostics, ErrorCode.WRN_BadYieldInLock, node.YieldKeyword);
+            }
 
             CheckRequiredLangVersionForIteratorMethods(node, diagnostics);
             return new BoundYieldReturnStatement(node, argument);
@@ -1165,15 +1172,9 @@ protected BoundLocalDeclaration BindVariableDeclaration(
 
         protected bool CheckRefLocalInAsyncOrIteratorMethod(SyntaxToken identifierToken, BindingDiagnosticBag diagnostics)
         {
-            if (IsInAsyncMethod())
-            {
-                Error(diagnostics, ErrorCode.ERR_BadAsyncLocalType, identifierToken);
-                return true;
-            }
-            else if (IsDirectlyInIterator)
+            if (IsDirectlyInIterator || IsInAsyncMethod())
             {
-                Error(diagnostics, ErrorCode.ERR_BadIteratorLocalType, identifierToken);
-                return true;
+                return !CheckFeatureAvailability(identifierToken, MessageID.IDS_FeatureRefUnsafeInIteratorAsync, diagnostics);
             }
 
             return false;
diff --git a/src/Compilers/CSharp/Portable/Binder/Binder_Unsafe.cs b/src/Compilers/CSharp/Portable/Binder/Binder_Unsafe.cs
index ec8cd5d3885b7..2c4d9b6a7fb88 100644
--- a/src/Compilers/CSharp/Portable/Binder/Binder_Unsafe.cs
+++ b/src/Compilers/CSharp/Portable/Binder/Binder_Unsafe.cs
@@ -56,18 +56,16 @@ private CSDiagnosticInfo GetUnsafeDiagnosticInfo(TypeSymbol sizeOfTypeOpt)
             {
                 return null;
             }
-            else if (this.IsIndirectlyInIterator)
-            {
-                // Spec 8.2: "An iterator block always defines a safe context, even when its declaration
-                // is nested in an unsafe context."
-                return new CSDiagnosticInfo(ErrorCode.ERR_IllegalInnerUnsafe);
-            }
             else if (!this.InUnsafeRegion)
             {
                 return ((object)sizeOfTypeOpt == null)
                     ? new CSDiagnosticInfo(ErrorCode.ERR_UnsafeNeeded)
                     : new CSDiagnosticInfo(ErrorCode.ERR_SizeofUnsafe, sizeOfTypeOpt);
             }
+            else if (this.IsIndirectlyInIterator && MessageID.IDS_FeatureRefUnsafeInIteratorAsync.GetFeatureAvailabilityDiagnosticInfo(Compilation) is { } unsafeInIteratorDiagnosticInfo)
+            {
+                return unsafeInIteratorDiagnosticInfo;
+            }
             else
             {
                 return null;
diff --git a/src/Compilers/CSharp/Portable/Binder/ExecutableCodeBinder.cs b/src/Compilers/CSharp/Portable/Binder/ExecutableCodeBinder.cs
index edffa7f5535d3..e08152589f822 100644
--- a/src/Compilers/CSharp/Portable/Binder/ExecutableCodeBinder.cs
+++ b/src/Compilers/CSharp/Portable/Binder/ExecutableCodeBinder.cs
@@ -129,7 +129,7 @@ public static void ValidateIteratorMethod(CSharpCompilation compilation, MethodS
             if (((iterator as SourceMemberMethodSymbol)?.IsUnsafe == true || (iterator as LocalFunctionSymbol)?.IsUnsafe == true)
                 && compilation.Options.AllowUnsafe) // Don't cascade
             {
-                diagnostics.Add(ErrorCode.ERR_IllegalInnerUnsafe, errorLocation);
+                MessageID.IDS_FeatureRefUnsafeInIteratorAsync.CheckFeatureAvailability(diagnostics, compilation, errorLocation);
             }
 
             var returnType = iterator.ReturnType;
diff --git a/src/Compilers/CSharp/Portable/Binder/LocalBinderFactory.cs b/src/Compilers/CSharp/Portable/Binder/LocalBinderFactory.cs
index 0695fe3f94395..1e6ee6bfc4953 100644
--- a/src/Compilers/CSharp/Portable/Binder/LocalBinderFactory.cs
+++ b/src/Compilers/CSharp/Portable/Binder/LocalBinderFactory.cs
@@ -416,7 +416,9 @@ public override void VisitLocalFunctionStatement(LocalFunctionStatementSyntax no
                     ? new WithMethodTypeParametersBinder(match, _enclosing)
                     : _enclosing;
 
-                binder = binder.WithUnsafeRegionIfNecessary(node.Modifiers);
+                binder = binder.SetOrClearUnsafeRegionIfNecessary(node.Modifiers,
+                    isIteratorBody: match.IsIterator);
+
                 binder = new InMethodBinder(match, binder);
             }
 
diff --git a/src/Compilers/CSharp/Portable/Binder/LockBinder.cs b/src/Compilers/CSharp/Portable/Binder/LockBinder.cs
index c29d038662a25..0899da0cfdb69 100644
--- a/src/Compilers/CSharp/Portable/Binder/LockBinder.cs
+++ b/src/Compilers/CSharp/Portable/Binder/LockBinder.cs
@@ -64,13 +64,6 @@ internal override BoundStatement BindLockStatementParts(BindingDiagnosticBag dia
                 _ = diagnostics.ReportUseSite(lockTypeInfo.EnterScopeMethod, exprSyntax) ||
                     diagnostics.ReportUseSite(lockTypeInfo.ScopeType, exprSyntax) ||
                     diagnostics.ReportUseSite(lockTypeInfo.ScopeDisposeMethod, exprSyntax);
-
-                CheckRestrictedTypeInAsyncMethod(
-                    originalBinder.ContainingMemberOrLambda,
-                    lockTypeInfo.ScopeType,
-                    diagnostics,
-                    exprSyntax,
-                    errorCode: ErrorCode.ERR_BadSpecialByRefLock);
             }
 
             BoundStatement stmt = originalBinder.BindPossibleEmbeddedStatement(_syntax.Statement, diagnostics);
diff --git a/src/Compilers/CSharp/Portable/Binder/UsingStatementBinder.cs b/src/Compilers/CSharp/Portable/Binder/UsingStatementBinder.cs
index e93e456b8d9cf..9d4659c8e3637 100644
--- a/src/Compilers/CSharp/Portable/Binder/UsingStatementBinder.cs
+++ b/src/Compilers/CSharp/Portable/Binder/UsingStatementBinder.cs
@@ -119,7 +119,7 @@ internal static BoundStatement BindUsingStatementOrDeclarationFromParts(SyntaxNo
                 Debug.Assert(expressionOpt is not null);
                 if (expressionOpt.Type is not null)
                 {
-                    CheckRestrictedTypeInAsyncMethod(originalBinder.ContainingMemberOrLambda, expressionOpt.Type, diagnostics, expressionOpt.Syntax, errorCode: ErrorCode.ERR_BadSpecialByRefUsing);
+                    CheckRestrictedTypeInAsyncMethod(originalBinder.ContainingMemberOrLambda, expressionOpt.Type, diagnostics, expressionOpt.Syntax);
                 }
             }
             else
diff --git a/src/Compilers/CSharp/Portable/CSharpResources.resx b/src/Compilers/CSharp/Portable/CSharpResources.resx
index d681f0b302910..4c09a80debc1a 100644
--- a/src/Compilers/CSharp/Portable/CSharpResources.resx
+++ b/src/Compilers/CSharp/Portable/CSharpResources.resx
@@ -2947,9 +2947,6 @@ A catch() block after a catch (System.Exception e) block can catch non-CLS excep
   <data name="ERR_AnonDelegateCantUse" xml:space="preserve">
     <value>Cannot use ref, out, or in parameter '{0}' inside an anonymous method, lambda expression, query expression, or local function</value>
   </data>
-  <data name="ERR_IllegalInnerUnsafe" xml:space="preserve">
-    <value>Unsafe code may not appear in iterators</value>
-  </data>
   <data name="ERR_BadYieldInCatch" xml:space="preserve">
     <value>Cannot yield a value in the body of a catch clause</value>
   </data>
@@ -3807,7 +3804,7 @@ Give the compiler some way to differentiate the methods. For example, you can gi
     <value>__arglist is not allowed in the parameter list of async methods</value>
   </data>
   <data name="ERR_ByRefTypeAndAwait" xml:space="preserve">
-    <value>'await' cannot be used in an expression containing the type '{0}'</value>
+    <value>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</value>
   </data>
   <data name="ERR_UnsafeAsyncArgType" xml:space="preserve">
     <value>Async methods cannot have pointer type parameters</value>
@@ -3848,15 +3845,12 @@ Give the compiler some way to differentiate the methods. For example, you can gi
   <data name="ERR_BadAsyncLacksBody" xml:space="preserve">
     <value>The 'async' modifier can only be used in methods that have a body.</value>
   </data>
-  <data name="ERR_BadSpecialByRefLocal" xml:space="preserve">
-    <value>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</value>
-  </data>
-  <data name="ERR_BadSpecialByRefUsing" xml:space="preserve">
-    <value>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</value>
-  </data>
   <data name="ERR_BadSpecialByRefIterator" xml:space="preserve">
     <value>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</value>
   </data>
+  <data name="ERR_BadSpecialByRefParameter" xml:space="preserve">
+    <value>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</value>
+  </data>
   <data name="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync" xml:space="preserve">
     <value>Security attribute '{0}' cannot be applied to an Async method.</value>
   </data>
@@ -5287,12 +5281,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_AnonDelegateCantUseLocal" xml:space="preserve">
     <value>Cannot use ref local '{0}' inside an anonymous method, lambda expression, or query expression</value>
   </data>
-  <data name="ERR_BadIteratorLocalType" xml:space="preserve">
-    <value>Iterators cannot have by-reference locals</value>
-  </data>
-  <data name="ERR_BadAsyncLocalType" xml:space="preserve">
-    <value>Async methods cannot have by-reference locals</value>
-  </data>
   <data name="ERR_RefReturningCallAndAwait" xml:space="preserve">
     <value>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</value>
   </data>
@@ -7854,9 +7842,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_ConvertingLock_Title" xml:space="preserve">
     <value>A value of type 'System.Threading.Lock' converted to a different type will use likely unintended monitor-based locking in 'lock' statement.</value>
   </data>
-  <data name="ERR_BadSpecialByRefLock" xml:space="preserve">
-    <value>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</value>
-  </data>
   <data name="IDS_FeatureLockObject" xml:space="preserve">
     <value>Lock object</value>
   </data>
@@ -7926,6 +7911,24 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_InterceptsLocationDataInvalidPosition" xml:space="preserve">
     <value>The data argument to InterceptsLocationAttribute refers to an invalid position in file '{0}'.</value>
   </data>
+  <data name="WRN_BadYieldInLock" xml:space="preserve">
+    <value>'yield return' should not be used in the body of a lock statement</value>
+  </data>
+  <data name="WRN_BadYieldInLock_Title" xml:space="preserve">
+    <value>'yield return' should not be used in the body of a lock statement</value>
+  </data>
+  <data name="IDS_FeatureRefUnsafeInIteratorAsync" xml:space="preserve">
+    <value>ref and unsafe in async and iterator methods</value>
+  </data>
+  <data name="ERR_RefLocalAcrossAwait" xml:space="preserve">
+    <value>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</value>
+  </data>
+  <data name="ERR_BadYieldInUnsafe" xml:space="preserve">
+    <value>Cannot use 'yield return' in an 'unsafe' block</value>
+  </data>
+  <data name="ERR_AddressOfInIterator" xml:space="preserve">
+    <value>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</value>
+  </data>
   <data name="IDS_FeatureRefStructInterfaces" xml:space="preserve">
     <value>ref struct interfaces</value>
   </data>
diff --git a/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs b/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs
index bc6c2e8928216..4a532e27f6b31 100644
--- a/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs
+++ b/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs
@@ -792,7 +792,7 @@ internal enum ErrorCode
         ERR_BadYieldInTryOfCatch = 1626,
         ERR_EmptyYield = 1627,
         ERR_AnonDelegateCantUse = 1628,
-        ERR_IllegalInnerUnsafe = 1629,
+        // ERR_IllegalInnerUnsafe = 1629,
         //ERR_BadWatsonMode = 1630,
         ERR_BadYieldInCatch = 1631,
         ERR_BadDelegateLeave = 1632,
@@ -1111,7 +1111,7 @@ internal enum ErrorCode
         ERR_NonTaskMainCantBeAsync = 4009,
         ERR_CantConvAsyncAnonFuncReturns = 4010,
         ERR_BadAwaiterPattern = 4011,
-        ERR_BadSpecialByRefLocal = 4012,
+        ERR_BadSpecialByRefParameter = 4012,
         ERR_SpecialByRefInLambda = 4013,
         WRN_UnobservedAwaitableExpression = 4014,
         ERR_SynchronizedAsyncMethod = 4015,
@@ -1429,8 +1429,8 @@ internal enum ErrorCode
         ERR_RefAssignmentMustHaveIdentityConversion = 8173,
         ERR_ByReferenceVariableMustBeInitialized = 8174,
         ERR_AnonDelegateCantUseLocal = 8175,
-        ERR_BadIteratorLocalType = 8176,
-        ERR_BadAsyncLocalType = 8177,
+        // ERR_BadIteratorLocalType = 8176,
+        // ERR_BadAsyncLocalType = 8177,
         ERR_RefReturningCallAndAwait = 8178,
         #endregion diagnostics for ref locals and ref returns introduced in C# 7
 
@@ -2161,7 +2161,7 @@ internal enum ErrorCode
         ERR_UnscopedRefAttributeUnsupportedMemberTarget = 9101,
         ERR_UnscopedRefAttributeInterfaceImplementation = 9102,
         ERR_UnrecognizedRefSafetyRulesAttributeVersion = 9103,
-        ERR_BadSpecialByRefUsing = 9104,
+        // ERR_BadSpecialByRefUsing = 9104,
 
         ERR_InvalidPrimaryConstructorParameterReference = 9105,
         ERR_AmbiguousPrimaryConstructorParameterAsColorColorReceiver = 9106,
@@ -2286,7 +2286,7 @@ internal enum ErrorCode
         ERR_CollectionExpressionMissingAdd = 9215,
 
         WRN_ConvertingLock = 9216,
-        ERR_BadSpecialByRefLock = 9217,
+        ERR_RefLocalAcrossAwait = 9217,
 
         ERR_DynamicDispatchToParamsCollection = 9218,
         ERR_ParamsCollectionAmbiguousDynamicArgument = 9219,
@@ -2321,6 +2321,10 @@ internal enum ErrorCode
 
         #endregion
 
+        WRN_BadYieldInLock = 9237,
+        ERR_BadYieldInUnsafe = 9238,
+        ERR_AddressOfInIterator = 9239,
+
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)
 
diff --git a/src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs b/src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs
index 62bbe6b94e202..03a75fda22f79 100644
--- a/src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs
+++ b/src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs
@@ -207,6 +207,10 @@ internal static int GetWarningLevel(ErrorCode code)
             // docs/compilers/CSharp/Warnversion Warning Waves.md
             switch (code)
             {
+                case ErrorCode.WRN_BadYieldInLock:
+                    // Warning level 9 is exclusively for warnings introduced in the compiler
+                    // shipped with dotnet 9 (C# 13) and that can be reported for pre-existing code.
+                    return 9;
                 case ErrorCode.WRN_AddressOfInAsync:
                 case ErrorCode.WRN_ByValArraySizeConstRequired:
                     // Warning level 8 is exclusively for warnings introduced in the compiler
@@ -623,6 +627,7 @@ or ErrorCode.ERR_InterceptorCannotBeGeneric
                 or ErrorCode.ERR_InterceptableMethodMustBeOrdinary
                 or ErrorCode.ERR_PossibleAsyncIteratorWithoutYield
                 or ErrorCode.ERR_PossibleAsyncIteratorWithoutYieldOrAwait
+                or ErrorCode.ERR_RefLocalAcrossAwait
                     // Update src\EditorFeatures\CSharp\LanguageServer\CSharpLspBuildOnlyDiagnostics.cs
                     // whenever new values are added here.
                     => true,
@@ -1275,7 +1280,6 @@ or ErrorCode.ERR_UnsupportedPrimaryConstructorParameterCapturingRef
                 or ErrorCode.ERR_UnsupportedPrimaryConstructorParameterCapturingRefLike
                 or ErrorCode.ERR_AnonDelegateCantUseStructPrimaryConstructorParameterInMember
                 or ErrorCode.ERR_AnonDelegateCantUseStructPrimaryConstructorParameterCaptured
-                or ErrorCode.ERR_IllegalInnerUnsafe
                 or ErrorCode.ERR_BadYieldInCatch
                 or ErrorCode.ERR_BadDelegateLeave
                 or ErrorCode.WRN_IllegalPragma
@@ -1535,7 +1539,7 @@ or ErrorCode.ERR_BadAwaitArgVoidCall
                 or ErrorCode.ERR_NonTaskMainCantBeAsync
                 or ErrorCode.ERR_CantConvAsyncAnonFuncReturns
                 or ErrorCode.ERR_BadAwaiterPattern
-                or ErrorCode.ERR_BadSpecialByRefLocal
+                or ErrorCode.ERR_BadSpecialByRefParameter
                 or ErrorCode.WRN_UnobservedAwaitableExpression
                 or ErrorCode.ERR_SynchronizedAsyncMethod
                 or ErrorCode.ERR_BadAsyncReturnExpression
@@ -1778,8 +1782,6 @@ or ErrorCode.ERR_InitializeByReferenceVariableWithValue
                 or ErrorCode.ERR_RefAssignmentMustHaveIdentityConversion
                 or ErrorCode.ERR_ByReferenceVariableMustBeInitialized
                 or ErrorCode.ERR_AnonDelegateCantUseLocal
-                or ErrorCode.ERR_BadIteratorLocalType
-                or ErrorCode.ERR_BadAsyncLocalType
                 or ErrorCode.ERR_PredefinedValueTupleTypeNotFound
                 or ErrorCode.ERR_SemiOrLBraceOrArrowExpected
                 or ErrorCode.ERR_NewWithTupleTypeSyntax
@@ -2330,7 +2332,6 @@ or ErrorCode.WRN_ParamsArrayInLambdaOnly
                 or ErrorCode.ERR_UnscopedRefAttributeUnsupportedMemberTarget
                 or ErrorCode.ERR_UnscopedRefAttributeInterfaceImplementation
                 or ErrorCode.ERR_UnrecognizedRefSafetyRulesAttributeVersion
-                or ErrorCode.ERR_BadSpecialByRefUsing
                 or ErrorCode.ERR_InvalidPrimaryConstructorParameterReference
                 or ErrorCode.ERR_AmbiguousPrimaryConstructorParameterAsColorColorReceiver
                 or ErrorCode.WRN_CapturedPrimaryConstructorParameterPassedToBase
@@ -2419,7 +2420,6 @@ or ErrorCode.ERR_CollectionExpressionTargetNoElementType
                 or ErrorCode.ERR_CollectionExpressionMissingConstructor
                 or ErrorCode.ERR_CollectionExpressionMissingAdd
                 or ErrorCode.WRN_ConvertingLock
-                or ErrorCode.ERR_BadSpecialByRefLock
                 or ErrorCode.ERR_DynamicDispatchToParamsCollection
                 or ErrorCode.ERR_ParamsCollectionAmbiguousDynamicArgument
                 or ErrorCode.WRN_DynamicDispatchToParamsCollectionMethod
@@ -2439,6 +2439,9 @@ or ErrorCode.ERR_InterceptsLocationDuplicateFile
                 or ErrorCode.ERR_InterceptsLocationFileNotFound
                 or ErrorCode.ERR_InterceptsLocationDataInvalidPosition
                 or ErrorCode.INF_TooManyBoundLambdas
+                or ErrorCode.WRN_BadYieldInLock
+                or ErrorCode.ERR_BadYieldInUnsafe
+                or ErrorCode.ERR_AddressOfInIterator
                 or ErrorCode.ERR_RuntimeDoesNotSupportByRefLikeGenerics
                 or ErrorCode.ERR_RefStructConstraintAlreadySpecified
                 or ErrorCode.ERR_AllowsClauseMustBeLast
diff --git a/src/Compilers/CSharp/Portable/Errors/MessageID.cs b/src/Compilers/CSharp/Portable/Errors/MessageID.cs
index de89cced38570..15f4d28a240f6 100644
--- a/src/Compilers/CSharp/Portable/Errors/MessageID.cs
+++ b/src/Compilers/CSharp/Portable/Errors/MessageID.cs
@@ -283,6 +283,8 @@ internal enum MessageID
 
         IDS_FeatureParamsCollections = MessageBase + 12842,
 
+        IDS_FeatureRefUnsafeInIteratorAsync = MessageBase + 12843,
+
         IDS_FeatureRefStructInterfaces = MessageBase + 12950, // PROTOTYPE(RefStructInterfaces): Pack numbers
     }
 
@@ -468,6 +470,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureImplicitIndexerInitializer:
                 case MessageID.IDS_FeatureLockObject:
                 case MessageID.IDS_FeatureParamsCollections:
+                case MessageID.IDS_FeatureRefUnsafeInIteratorAsync:
                 case MessageID.IDS_FeatureRefStructInterfaces:
                     return LanguageVersion.Preview;
 
diff --git a/src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs b/src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs
index 62d7b4c68bec6..ac6ebf95db21f 100644
--- a/src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs
+++ b/src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs
@@ -341,6 +341,7 @@ public static bool IsWarning(ErrorCode code)
                 case ErrorCode.WRN_DynamicDispatchToParamsCollectionMethod:
                 case ErrorCode.WRN_DynamicDispatchToParamsCollectionIndexer:
                 case ErrorCode.WRN_DynamicDispatchToParamsCollectionConstructor:
+                case ErrorCode.WRN_BadYieldInLock:
                     return true;
                 default:
                     return false;
diff --git a/src/Compilers/CSharp/Portable/Lowering/StateMachineRewriter/IteratorAndAsyncCaptureWalker.cs b/src/Compilers/CSharp/Portable/Lowering/StateMachineRewriter/IteratorAndAsyncCaptureWalker.cs
index 91d75e4f81586..d8ce97fa4088f 100644
--- a/src/Compilers/CSharp/Portable/Lowering/StateMachineRewriter/IteratorAndAsyncCaptureWalker.cs
+++ b/src/Compilers/CSharp/Portable/Lowering/StateMachineRewriter/IteratorAndAsyncCaptureWalker.cs
@@ -19,7 +19,7 @@ namespace Microsoft.CodeAnalysis.CSharp
     /// </summary>
     /// <remarks>
     /// Data flow analysis is used to calculate the locals. At yield/await we mark all variables as "unassigned".
-    /// When a read from an unassigned variables is reported we add the variable to the captured set.
+    /// When a read from an unassigned variable is reported we add the variable to the captured set.
     /// "this" parameter is captured if a reference to "this", "base" or an instance field is encountered.
     /// Variables used in finally also need to be captured if there is a yield in the corresponding try block.
     /// </remarks>
@@ -76,19 +76,33 @@ public static OrderedSet<Symbol> Analyze(CSharpCompilation compilation, MethodSy
                 foreach (var kvp in lazyDisallowedCaptures)
                 {
                     var variable = kvp.Key;
-                    var type = (variable.Kind == SymbolKind.Local) ? ((LocalSymbol)variable).Type : ((ParameterSymbol)variable).Type;
 
-                    if (variable is SynthesizedLocal local && local.SynthesizedKind == SynthesizedLocalKind.Spill)
+                    if (variable is LocalSymbol local)
                     {
-                        Debug.Assert(local.TypeWithAnnotations.IsRestrictedType());
-                        diagnostics.Add(ErrorCode.ERR_ByRefTypeAndAwait, local.GetFirstLocation(), local.TypeWithAnnotations);
+                        foreach (var syntax in kvp.Value)
+                        {
+                            if (local.TypeWithAnnotations.IsRestrictedType())
+                            {
+                                // CS4007: Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.
+                                diagnostics.Add(ErrorCode.ERR_ByRefTypeAndAwait, syntax.Location, local.TypeWithAnnotations);
+                            }
+                            else
+                            {
+                                Debug.Assert(local.RefKind != RefKind.None);
+                                // CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                                diagnostics.Add(ErrorCode.ERR_RefLocalAcrossAwait, syntax.Location);
+                            }
+                        }
                     }
                     else
                     {
-                        foreach (CSharpSyntaxNode syntax in kvp.Value)
+                        var parameter = (ParameterSymbol)variable;
+                        Debug.Assert(parameter.TypeWithAnnotations.IsRestrictedType());
+
+                        foreach (var syntax in kvp.Value)
                         {
-                            // CS4013: Instance of type '{0}' cannot be used inside an anonymous function, query expression, iterator block or async method
-                            diagnostics.Add(ErrorCode.ERR_SpecialByRefInLambda, syntax.Location, type);
+                            // CS4007: Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.
+                            diagnostics.Add(ErrorCode.ERR_ByRefTypeAndAwait, syntax.Location, parameter.TypeWithAnnotations);
                         }
                     }
                 }
@@ -195,14 +209,23 @@ protected override ImmutableArray<PendingBranch> Scan(ref bool badRegion)
         private void CaptureVariable(Symbol variable, SyntaxNode syntax)
         {
             var type = (variable.Kind == SymbolKind.Local) ? ((LocalSymbol)variable).Type : ((ParameterSymbol)variable).Type;
-            if (type.IsRestrictedType())
+            if (type.IsRestrictedType() ||
+                (variable is LocalSymbol { RefKind: not RefKind.None } refLocal && !canRefLocalBeHoisted(refLocal)))
             {
                 (_lazyDisallowedCaptures ??= new MultiDictionary<Symbol, SyntaxNode>()).Add(variable, syntax);
             }
             else
             {
                 if (_variablesToHoist.Add(variable) && variable is LocalSymbol local && _boundRefLocalInitializers.TryGetValue(local, out var variableInitializer))
-                    CaptureRefInitializer(variableInitializer, syntax);
+                    CaptureRefInitializer(variableInitializer, local.SynthesizedKind != SynthesizedLocalKind.UserDefined ? variableInitializer.Syntax : syntax);
+            }
+
+            static bool canRefLocalBeHoisted(LocalSymbol refLocal)
+            {
+                return refLocal.SynthesizedKind == SynthesizedLocalKind.Spill ||
+                    (refLocal.SynthesizedKind == SynthesizedLocalKind.ForEachArray &&
+                        refLocal.Type.HasInlineArrayAttribute(out _) &&
+                        refLocal.Type.TryGetInlineArrayElementField() is not null);
             }
         }
 
diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/LocalFunctionSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Source/LocalFunctionSymbol.cs
index cd3aec6946c79..d502a7080b83c 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/LocalFunctionSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/LocalFunctionSymbol.cs
@@ -58,7 +58,7 @@ public LocalFunctionSymbol(
 
             ScopeBinder = binder;
 
-            binder = binder.WithUnsafeRegionIfNecessary(syntax.Modifiers);
+            binder = binder.SetOrClearUnsafeRegionIfNecessary(syntax.Modifiers);
 
             if (syntax.TypeParameterList != null)
             {
diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourceMethodSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourceMethodSymbol.cs
index a1771beb99a70..8cce2057e5200 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourceMethodSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourceMethodSymbol.cs
@@ -72,7 +72,7 @@ internal void ReportAsyncParameterErrors(BindingDiagnosticBag diagnostics, Locat
                 }
                 else if (parameter.Type.IsRestrictedType())
                 {
-                    diagnostics.Add(ErrorCode.ERR_BadSpecialByRefLocal, getLocation(parameter, location), parameter.Type);
+                    diagnostics.Add(ErrorCode.ERR_BadSpecialByRefParameter, getLocation(parameter, location), parameter.Type);
                 }
             }
 
diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
index af93d12c934ff..eb6c3ff553bfa 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
@@ -59,7 +59,7 @@ private static SourcePropertySymbol Create(
 
             bool isExpressionBodied = !hasAccessorList && GetArrowExpression(syntax) != null;
 
-            binder = binder.WithUnsafeRegionIfNecessary(modifiersTokenList);
+            binder = binder.SetOrClearUnsafeRegionIfNecessary(modifiersTokenList);
             TypeSymbol? explicitInterfaceType;
             string? aliasQualifierOpt;
             string memberName = ExplicitInterfaceHelpers.GetMemberNameAndInterfaceSymbol(binder, explicitInterfaceSpecifier, name, diagnostics, out explicitInterfaceType, out aliasQualifierOpt);
@@ -431,7 +431,7 @@ private Binder CreateBinderForTypeAndParameters()
             var binderFactory = compilation.GetBinderFactory(syntaxTree);
             var binder = binderFactory.GetBinder(syntax, syntax, this);
             SyntaxTokenList modifiers = GetModifierTokensSyntax(syntax);
-            binder = binder.WithUnsafeRegionIfNecessary(modifiers);
+            binder = binder.SetOrClearUnsafeRegionIfNecessary(modifiers);
             return binder.WithAdditionalFlagsAndContainingMemberOrLambda(BinderFlags.SuppressConstraintChecks, this);
         }
 
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf
index 6bc888cca671b..482f74b8c1b7e 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf
@@ -32,6 +32,11 @@
         <target state="translated">{0}: abstraktní událost nemůže používat syntaxi přístupového objektu události.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">'&amp;' pro skupiny metod se nedá použít ve stromech výrazů.</target>
@@ -282,14 +287,14 @@
         <target state="translated">Člen záznamu {0} musí být čitelná vlastnost instance nebo pole typu {1}, která se bude shodovat s pozičním parametrem {2}.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Příkaz lock pro hodnotu typu System.Threading.Lock nejde použít v asynchronních metodách nebo asynchronních výrazech lambda.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Prostředek použitého příkazu typu {0} nejde použít v asynchronních metodách ani v asynchronních výrazech lambda.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">Typ {0} není platný pro using static. Lze použít pouze třídu, strukturu, rozhraní, výčet, delegáta nebo obor názvů.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">Atribut AsyncMethodBuilder je u anonymních metod bez explicitního návratového typu zakázaný.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Pole ref lze deklarovat pouze ve struktuře ref.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">Levá strana přiřazení odkazu musí být parametr Ref.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">uvolněný operátor směny</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">Modifikátor ref pro argument odpovídající parametru in je ekvivalentem in. Místo toho zvažte použití in.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">Je nutné zadat parametr atributu SizeConst.</target>
@@ -9221,11 +9251,6 @@ Blok catch() po bloku catch (System.Exception e) může zachytit výjimky, kter
         <target state="translated">Parametr ref, out nebo in {0} nejde použít uvnitř anonymní metody, výrazu lambda, výrazu dotazu nebo lokální funkce.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Iterátory nesmí obsahovat nezabezpečený kód.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">V těle klauzule catch nejde použít hodnotu získanou příkazem yield.</target>
@@ -10610,8 +10635,8 @@ Poskytněte kompilátoru nějaký způsob, jak metody rozlišit. Můžete např
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'Operátor await nejde použít ve výrazu, který obsahuje typ {0}.</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'Operátor await nejde použít ve výrazu, který obsahuje typ {0}.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Poskytněte kompilátoru nějaký způsob, jak metody rozlišit. Můžete např
         <target state="translated">Modifikátor async se dá použít jenom v metodách, které mají tělo.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Parametry nebo lokální proměnné typu {0} nemůžou být deklarované v asynchronních metodách nebo asynchronních výrazech lambda.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">Výraz foreach nejde použít na enumerátorech typu {0} v asynchronních metodách nebo metodách iterátoru, protože {0} je struktura REF.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">Atribut zabezpečení {0} nejde použít pro metodu Async.</target>
@@ -11208,7 +11223,7 @@ Potlačení upozornění zvažte jenom v případě, když určitě nechcete če
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">Instance typu {0} nelze použít uvnitř vnořené funkce, výrazu dotazu, bloku iterátoru nebo asynchronní metody.</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ Pokud chcete odstranit toto varování, můžete místo toho použít /reference
         <target state="translated">Místní hodnotu odkazu {0} nejde použít uvnitř anonymní metody, výrazu lambda nebo výrazu dotazu.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Iterátory nemůžou mít lokální proměnné podle odkazu.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">Asynchronní metody nemůžou mít lokální proměnné podle odkazu.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">Odkaz vrácený voláním funkce {0} nelze zachovat v rámci hranice await nebo yield.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf
index 798fc08ecb8a3..458820646f0bd 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf
@@ -32,6 +32,11 @@
         <target state="translated">{0}: Das abstrakte Ereignis kann die Ereignisaccessorsyntax nicht verwenden.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">"&amp;" für Methodengruppen kann in Ausdrucksbaumstrukturen nicht verwendet werden.</target>
@@ -282,14 +287,14 @@
         <target state="translated">Das Datensatzelement "{0}" muss eine lesbare Instanzeigenschaft oder ein Feld vom Typ "{1}" sein, um dem Positionsparameter "{2}" zu entsprechen.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Eine Sperranweisung für einen Wert vom Typ „System.Threading.Lock“ kann nicht in asynchronen Methoden oder asynchronen Lambdaausdrücken verwendet werden.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Eine using-Anweisungsressource vom Typ „{0}“ kann nicht in asynchronen Methoden oder asynchronen Lambdaausdrücken verwendet werden.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">Der Typ "{0}" ist für "using static" ungültig. Nur eine Klasse, Struktur, Schnittstelle, Enumeration, ein Delegat oder ein Namespace kann verwendet werden.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">Das AsyncMethodBuilder-Attribut ist für anonyme Methoden ohne expliziten Rückgabetyp unzulässig.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Ein Verweisfeld kann nur in einer Verweisstruktur deklariert werden.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">Die linke Seite einer Ref-Zuweisung muss eine Ref-Variable sein.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">entspannter Schichtoperator</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">Der ref-Modifizierer für ein Argument, das dem in-Parameter entspricht, entspricht "in". Erwägen Sie stattdessen die Verwendung von "in".</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">Der Attributparameter "SizeConst" muss angegeben werden.</target>
@@ -9221,11 +9251,6 @@ Ein catch()-Block nach einem catch (System.Exception e)-Block kann nicht-CLS-Aus
         <target state="translated">Der ref-, out-, oder in-Parameter "{0}" kann nicht in einer anonymen Methode, einem Lambdaausdruck, einem Abfrageausdruck oder einer lokalen Funktion verwendet werden.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Unsicherer Code wird möglicherweise nicht in Iteratoren angezeigt.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">Mit "yield" kann im Text einer catch-Klausel kein Wert zurückgegeben werden.</target>
@@ -10610,8 +10635,8 @@ Unterstützen Sie den Compiler bei der Unterscheidung zwischen den Methoden. Daz
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'"await" kann nicht in einem Ausdruck verwendet werden, der den Typ "{0}" enthält</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'"await" kann nicht in einem Ausdruck verwendet werden, der den Typ "{0}" enthält</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Unterstützen Sie den Compiler bei der Unterscheidung zwischen den Methoden. Daz
         <target state="translated">Der Modifizierer "async" kann nur in Methoden verwendet werden, die über einen Textkörper verfügen.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Parameter oder lokale Variablen des Typs "{0}" können nicht in asynchronen Methoden oder in asynchronen Lambdaausdrücken deklariert werden.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">Die foreach-Anweisung kann nicht für Enumeratoren vom Typ "{0}" in asynchronen oder Iteratormethoden verwendet werden, weil "{0}" eine Referenzstruktur ist.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">Das Sicherheitsattribut "{0}" kann nicht auf eine Async-Methode angewendet werden.</target>
@@ -11208,7 +11223,7 @@ Sie sollten das Unterdrücken der Warnung nur in Betracht ziehen, wenn Sie siche
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">Eine Instanz des Typs "{0}" kann nicht in einer geschachtelten Funktion, einem Abfrageausdruck, einem Iteratorblock oder einer Async-Methode verwendet werden.</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ Um die Warnung zu beheben, können Sie stattdessen /reference verwenden (Einbett
         <target state="translated">Der lokale Verweis "{0}" kann nicht in einer anonymen Methode, einem Lambdaausdruck oder einem Abfrageausdruck verwendet werden.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Iteratoren dürfen keine lokalen by-reference-Elemente aufweisen.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">Asynchrone Methoden dürfen keine lokalen by-reference-Elemente aufweisen.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">Ein Verweis, der von einem Aufruf von "{0}" zurückgegeben wird, kann nicht über die Grenzen "await" oder "yield" hinweg beibehalten werden.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf
index 2308721ead667..de96942b43ca1 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf
@@ -32,6 +32,11 @@
         <target state="translated">"{0}": un evento abstracto no puede usar la sintaxis de descriptor de acceso de eventos</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">No se puede usar "&amp;" para los grupos de métodos en los árboles de expresión.</target>
@@ -282,14 +287,14 @@
         <target state="translated">El miembro de registro '{0}' debe ser una propiedad de instancia legible o un campo de tipo '{1}' para coincidir con el parámetro posicional '{2}'.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">No se puede usar una instrucción de bloqueo en un valor de tipo 'System.Threading.Lock' en métodos asincrónicos ni expresiones lambda asincrónicas.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Un recurso de instrucción using de tipo '{0}' no se puede usar en métodos asincrónicos ni expresiones lambda asincrónicas.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">El tipo '{0}' no es válido para 'using static'. Solo se puede usar una clase, estructura, interfaz, enumeración, delegado o espacio de nombres.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">El atributo AsyncMethodBuilder no se permite en métodos anónimos sin un tipo de valor devuelto explícito.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Un campo ref solo se puede declarar en una estructura ref.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">La parte izquierda de una asignación de referencias debe ser una variable local.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">operador de cambios relajado</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">El modificador "ref" de un argumento correspondiente al parámetro "in" es equivalente a "in". Considere la posibilidad de usar "in" en su lugar.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">Se debe especificar el parámetro de atributo "SizeConst".</target>
@@ -9221,11 +9251,6 @@ Un bloque catch() después de un bloque catch (System.Exception e) puede abarcar
         <target state="translated">No se puede usar el parámetro ref, out o in "{0}" dentro de un método anónimo, una expresión lambda, una expresión de consulta o una función local</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">No puede aparecer código no seguro en iteradores</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">No se puede proporcionar ningún valor en el cuerpo de una cláusula catch</target>
@@ -10610,8 +10635,8 @@ Indique al compilador alguna forma de diferenciar los métodos. Por ejemplo, pue
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'await' no se puede usar en una expresión que contenga el tipo '{0}'</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'await' no se puede usar en una expresión que contenga el tipo '{0}'</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Indique al compilador alguna forma de diferenciar los métodos. Por ejemplo, pue
         <target state="translated">El modificador 'async' solo se puede usar en métodos que tengan un cuerpo.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Los parámetros o locales de tipo '{0}' no pueden declararse en expresiones lambda o métodos asincrónicos.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">La instrucción foreach no puede funcionar en enumeradores de tipo "{0}" en métodos async o iterator porque "{0}" es una estructura ref.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">El atributo de seguridad '{0}' no se puede aplicar a un método Async.</target>
@@ -11208,7 +11223,7 @@ Considere la posibilidad de suprimir la advertencia solo si tiene la seguridad d
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">La instancia de tipo "{0}" no se puede usar dentro de una función anidada, una expresión de consulta, un bloque iterador ni un método asincrónico.</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ Para eliminar la advertencia puede usar /reference (establezca la propiedad Embe
         <target state="translated">No se puede usar la variable local de tipo ref '{0}' dentro de un método anónimo, una expresión lambda o una expresión de consulta.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Los iteradores no pueden tener variables locales por referencia.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">Los métodos asincrónicos no pueden tener variables locales por referencia.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">Una referencia devuelta por una llamada a '{0}' no se puede conservar a través del límite "await" o "yield".</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf
index 8585fb986186d..cd8e3b4571564 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf
@@ -32,6 +32,11 @@
         <target state="translated">'{0}' : un événement abstrait ne peut pas utiliser une syntaxe d'accesseur d'événement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">'&amp;' des groupes de méthodes ne peut pas être utilisé dans les arborescences d'expression</target>
@@ -282,14 +287,14 @@
         <target state="translated">Le membre d'enregistrement '{0}' doit être une propriété d'instance our champ lisible de type '{1}' pour correspondre au paramètre positionnel '{2}'.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Vous ne pouvez pas utiliser une instruction lock sur une valeur de type « System.Threading.Lock » dans des méthodes asynchrones ou des expressions lambda asynchrones.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Une ressource d’instruction d’utilisation de type '{0}' ne peut pas être utilisée dans des méthodes asynchrones ou des expressions lambda asynchrones.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">Le type '{0}' n'est pas valide pour 'en utilisant statique'. Seuls une classe, une structure, une interface, une énumération, un délégué ou un espace de noms peuvent être utilisés.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">L'attribut AsyncMethodBuilder n'est pas autorisé pour les méthodes anonymes sans type de retour explicite.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Un champ de référence ne peut être déclaré que dans une sructure de référence.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">Le côté gauche d’une affectation ref doit être une variable ref.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">opérateur shift souple</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">Le modificateur « ref » d’un argument correspondant au paramètre « in » est équivalent à « in ». Envisagez d’utiliser « in » à la place.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">Vous devez spécifier un paramètre d’attribut « SizeConst ».</target>
@@ -9221,11 +9251,6 @@ Un bloc catch() après un bloc catch (System.Exception e) peut intercepter des e
         <target state="translated">Impossible d'utiliser le paramètre ref, out ou in '{0}' dans une méthode anonyme, une expression lambda, une expression de requête ou une fonction locale</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Du code unsafe ne peut pas s'afficher dans des itérateurs</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">Impossible de générer une valeur dans le corps d'une clause catch</target>
@@ -10610,8 +10635,8 @@ Permettez au compilateur de différencier les méthodes. Par exemple, vous pouve
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'await' ne peut pas être utilisé dans une expression contenant le type '{0}'</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'await' ne peut pas être utilisé dans une expression contenant le type '{0}'</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Permettez au compilateur de différencier les méthodes. Par exemple, vous pouve
         <target state="translated">Le modificateur 'async' ne peut être utilisé que dans des méthodes ayant un corps.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Les paramètres ou variables locales de type '{0}' ne peuvent pas être déclarés dans des méthodes asynchrones ou des expressions asynchrones lambda.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">L'instruction foreach ne peut pas fonctionner sur les énumérateurs de type '{0}' dans les méthodes asynchrones ou les méthodes d'itérateurs, car '{0}' est un struct par référence.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">Impossible d'appliquer l'attribut de sécurité '{0}' à une méthode Async.</target>
@@ -11208,7 +11223,7 @@ Supprimez l'avertissement seulement si vous êtes sûr de ne pas vouloir attendr
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">Impossible d'utiliser une instance de type '{0}' dans une fonction imbriquée, une expression de requête, un bloc itérateur ou une méthode async</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ Pour supprimer l'avertissement, vous pouvez utiliser la commande /reference (dé
         <target state="translated">Impossible d'utiliser ref local '{0}' dans une méthode anonyme, une expression lambda ou une expression de requête</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Les itérateurs ne peuvent pas avoir de variables locales par référence</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">Les méthodes async ne peuvent pas avoir de variables locales par référence</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">Une référence renvoyée par un appel à '{0}' ne peut pas être conservée à travers la limite 'wait' ou 'yield'.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf
index bb19c5576466c..ee6c972ccf03d 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf
@@ -32,6 +32,11 @@
         <target state="translated">'{0}': l'evento astratto non può usare la sintassi della funzione di accesso agli eventi</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">Non è possibile usare '&amp;' su gruppi di metodi in alberi delle espressioni</target>
@@ -282,14 +287,14 @@
         <target state="translated">Il membro di record '{0}' deve essere una proprietà di istanza leggibile o campo di tipo '{1}' per corrispondere al parametro posizionale '{2}'.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Non è possibile usare un'istruzione lock in un valore di tipo 'System.Threading.Lock' nei metodi asincroni o nelle espressioni lambda asincrone.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Non è possibile usare una risorsa di istruzione using di tipo '{0}' in metodi asincroni o espressioni lambda asincrone.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">'{0}' tipo non valido per 'using static'. È possibile usare solo una classe, una struttura, un'interfaccia, un'enumerazione, un delegato o uno spazio dei nomi.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">L'attributo AsyncMethodBuilder non è consentito in metodi anonimi senza un tipo restituito esplicito.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Un campo ref può essere dichiarato solo in uno struct ref.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">La parte sinistra di un'assegnazione ref deve essere una variabile ref.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">operatore di spostamento rilassato</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">Il modificatore 'ref' per un argomento corrispondente al parametro 'in' equivale a 'in'. Provare a usare 'in'.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">È necessario specificare il parametro di attributo 'SizeConst'.</target>
@@ -9221,11 +9251,6 @@ Un blocco catch() dopo un blocco catch (System.Exception e) può rilevare eccezi
         <target state="translated">Non è possibile usare il parametro ref, out o in '{0}' all'interno di un metodo anonimo, di un'espressione lambda, di un'espressione di query o di una funzione locale</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Gli iteratori non possono contenere codice unsafe</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">Impossibile produrre un valore nel corpo di una clausola catch</target>
@@ -10610,8 +10635,8 @@ Impostare il compilatore in modo tale da distinguere i metodi, ad esempio assegn
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'non è possibile usare 'await' in un'espressione contenente il tipo '{0}'</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'non è possibile usare 'await' in un'espressione contenente il tipo '{0}'</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Impostare il compilatore in modo tale da distinguere i metodi, ad esempio assegn
         <target state="translated">Il modificatore 'async' può essere usato solo nei metodi con un corpo.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Non è possibile dichiarare parametri o variabili locali di tipo '{0}' in metodi asincroni o espressioni lambda asincrone.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">L'istruzione foreach non può funzionare con enumeratori di tipo '{0}' in metodi async o iterator perché '{0}' è uno struct ref.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">Non è possibile applicare l'attributo di sicurezza '{0}' a un metodo Async</target>
@@ -11208,7 +11223,7 @@ Come procedura consigliata, è consigliabile attendere sempre la chiamata.
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">L'istanza di tipo '{0}' non può essere usata all'interno di una funzione annidata, un'espressione di query, un blocco iteratore o un metodo asincrono</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ Per rimuovere l'avviso, è invece possibile usare /reference (impostare la propr
         <target state="translated">Non è possibile usare la variabile locale ref '{0}' in un metodo anonimo, in un'espressione lambda o in un'espressione di query</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Gli iteratori non possono includere variabili locali per riferimento</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">I metodi Async non possono includere variabili locali per riferimento</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">Non è possibile mantenere un riferimento restituito da una chiamata a '{0}' oltre il limite 'await' o 'yield'.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf
index a95aa54b87139..049964d81ce3a 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf
@@ -32,6 +32,11 @@
         <target state="translated">'{0}': 抽象イベントはイベント アクセサーの構文を使用できません</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">メソッド グループの '&amp;' を式ツリーで使用することはできません</target>
@@ -282,14 +287,14 @@
         <target state="translated">レコード メンバー '{0}' は、位置指定パラメーター '{2}' に一致させるための型 '{1}' の読み取り可能なインスタンス プロパティまたはフィールドである必要があります。</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">型 'System.Threading.Lock' の値に対する lock ステートメントは、非同期メソッドまたは非同期ラムダ式では使用できません。</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">型 '{0}' の using ステートメント リソースは、非同期メソッドまたは非同期ラムダ式では使用できません。</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">'{0}' 型は 'using static' では無効です。使用できるのは、クラス、構造体、インターフェイス、列挙型、デリゲート、名前空間のみです。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">AsyncMethodBuilder 属性は、明示的な戻り値の型のない匿名メソッドでは許可されていません。</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">ref フィールドは ref 構造体でのみ宣言できます。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">ref 代入の左辺は ref 変数である必要があります。</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">緩和されたシフト演算子</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">'in' パラメーターに対応する引数の 'ref' 修飾子は 'in' と同じです。代わりに 'in' を使用することを検討してください。</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">属性パラメーター 'SizeConst' を指定する必要があります。</target>
@@ -9221,11 +9251,6 @@ AssemblyInfo.cs ファイルで RuntimeCompatibilityAttribute が false に設
         <target state="translated">ref、out、in パラメーター '{0}' は、匿名メソッド、ラムダ式、クエリ式、ローカル関数の内部では使用できません</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">アンセーフ コードは反復子には記述できません</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">catch 句の本体で値を生成することはできません</target>
@@ -10610,8 +10635,8 @@ C# では out と ref を区別しますが、CLR では同じと認識します
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'await' は、型 '{0}' を含む式では使用できません</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'await' は、型 '{0}' を含む式では使用できません</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ C# では out と ref を区別しますが、CLR では同じと認識します
         <target state="translated">async' 修飾子は、本体があるメソッドでのみ使用できます。</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">'{0}' 型のパラメーターまたはローカルは、非同期メソッドまたは非同期ラムダ式で宣言することができません。</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">'{0}' は ref 構造体であるため、非同期または反復子のメソッド内で型 '{0}' の列挙子に対して foreach ステートメントは機能しません。</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">セキュリティ属性 '{0}' を非同期メソッドに適用することはできません。</target>
@@ -11208,7 +11223,7 @@ You should consider suppressing the warning only if you're sure that you don't w
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">型 '{0}' のインスタンスは、入れ子になった関数、クエリ式、反復子ブロック、または非同期メソッドの中では使用できません</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="translated">匿名メソッド、ラムダ式、クエリ式内で ref ローカル変数 '{0}' は使用できません</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">反復子は参照渡しのローカル変数を持つことができません</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">非同期メソッドは参照渡しのローカル変数を持つことができません</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">'{0}' への呼び出しによって返された参照は、'await' または 'yield' 境界を越えて保持することはできません。</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf
index 3a0a76cc1b697..dd8ef848b9a66 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf
@@ -32,6 +32,11 @@
         <target state="translated">'{0}': 추상 이벤트는 이벤트 접근자 구문을 사용할 수 없습니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">식 트리에서는 메서드 그룹에 '&amp;'를 사용할 수 없습니다.</target>
@@ -282,14 +287,14 @@
         <target state="translated">위치 매개 변수 '{0}'과(와) 일치하려면 레코드 멤버 '{1}'이(가) 유형 '{2}'의 읽을 수 있는 인스턴스 속성 또는 필드여야 합니다.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">'System.Threading.Lock' 형식의 값에 대한 lock 문은 비동기 메서드 또는 비동기 람다 식에서 사용할 수 없습니다.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">'{0}' 형식의 using 문 리소스는 비동기 메서드 또는 비동기 람다 식에 사용할 수 없습니다.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">'{0}' 유형은 '정적 사용'에 유효하지 않습니다. 클래스, 구조체, 인터페이스, 열거형, 대리자 또는 네임스페이스만 사용할 수 있습니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">AsyncMethodBuilder 특성은 명시적 반환 형식이 없는 익명 메서드에서 허용되지 않습니다.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">ref 필드는 ref 구조체에서만 선언할 수 있습니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">ref 할당의 왼쪽은 ref 변수여야 합니다.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">완화된 시프트 연산자</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">'in' 매개 변수에 해당하는 인수의 'ref' 한정자는 'in'에 해당합니다. 대신 'in'을 사용하는 것이 좋습니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">특성 매개 변수 'SizeConst'를 지정해야 합니다.</target>
@@ -9221,11 +9251,6 @@ catch (System.Exception e) 블록 뒤의 catch() 블록은 RuntimeCompatibilityA
         <target state="translated">무명 메서드, 람다 식, 쿼리 식 또는 로컬 함수 안에서는 ref, out 또는 in 매개 변수 '{0}'을(를) 사용할 수 없습니다.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">반복기에는 안전하지 않은 코드를 사용할 수 없습니다.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">catch 절 본문에서는 값을 생성할 수 없습니다.</target>
@@ -10610,8 +10635,8 @@ C#에서는 out과 ref를 구분하지만 CLR에서는 동일한 것으로 간
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'await'는 '{0}' 형식이 포함된 식에 사용할 수 없습니다.</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'await'는 '{0}' 형식이 포함된 식에 사용할 수 없습니다.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ C#에서는 out과 ref를 구분하지만 CLR에서는 동일한 것으로 간
         <target state="translated">async' 한정자는 본문이 있는 메서드에서만 사용할 수 있습니다.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">'{0}' 형식의 매개 변수 또는 로컬은 비동기 메서드나 비동기 람다 식에서 선언할 수 없습니다.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">'{0}'은(는) ref struct이므로 비동기 또는 반복기 메서드의 '{0}' 형식 열거자에서 foreach 문을 수행할 수 없습니다.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">'{0}' 보안 특성은 비동기 메서드에 적용할 수 없습니다.</target>
@@ -11208,7 +11223,7 @@ You should consider suppressing the warning only if you're sure that you don't w
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">'{0}' 형식의 인스턴스는 중첩된 함수, 쿼리 식, 반복기 블록 또는 비동기 메서드 내에서 사용할 수 없습니다.</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="translated">무명 메서드, 람다 식 또는 쿼리 식에는 참조 로컬 '{0}'을(를) 사용할 수 없습니다.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">반복기에 by-reference 로컬을 사용할 수 없습니다.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">비동기 메서드에 by-reference 로컬을 사용할 수 없습니다.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">'{0}'에 대한 호출로 반환된 참조는 'await' 또는 'yield' 경계에서 보존할 수 없습니다.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf
index 740896f6f0ab0..871593fc5cad5 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf
@@ -32,6 +32,11 @@
         <target state="translated">„{0}”: zdarzenie abstrakcyjne nie może używać składni metody dostępu zdarzenia</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">Znak „&amp;” dla grup metod nie może być używany w drzewach wyrażeń</target>
@@ -282,14 +287,14 @@
         <target state="translated">Składowa rekordu "{0}" musi być możliwą do odczytu właściwością wystąpienia typu "{1}", aby dopasować parametr pozycyjny "{2}".</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Instrukcja blokady na wartości typu „System.Threading.Lock” nie może być używana w metodach asynchronicznych lub asynchronicznych wyrażeniach lambda.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Zasobu instrukcji przy użyciu typu '{0}' nie można używać w metodach asynchronicznych ani asynchronicznych wyrażeniach lambda.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">Typ „{0}” jest nieprawidłowy dla „using static”. Można używać tylko klasy, struktury, interfejsu, wyliczenia, delegata lub przestrzeni nazw.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">Atrybut AsyncMethodBuilder jest niedozwolony w metodach anonimowych bez jawnego zwracanego typu.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Pole referencyjne można zadeklarować tylko w strukturze referencyjnej.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">Lewa strona przypisania referencyjnego musi być zmienną referencyjną.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">operator swobodnej zmiany</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">Modyfikator „ref” dla argumentu odpowiadającego parametrowi „in” jest równoważny parametrowi „in”. Zamiast tego rozważ użycie parametru „in”.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">Należy określić parametr atrybutu „SizeConst”.</target>
@@ -9221,11 +9251,6 @@ Blok catch() po bloku catch (System.Exception e) może przechwytywać wyjątki n
         <target state="translated">Nie można użyć parametru ref, out ani in „{0}” wewnątrz metody anonimowej, wyrażenia lambda, wyrażenia zapytania lub funkcji lokalnej</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Niebezpieczny kod nie może występować w iteratorach.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">Nie można użyć instrukcji yield z wartością w treści klauzuli catch.</target>
@@ -10610,8 +10635,8 @@ Musisz umożliwić kompilatorowi rozróżnienie metod. Możesz na przykład nada
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'Operatora „await” nie można użyć w wyrażeniu zawierającym typ „{0}”</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'Operatora „await” nie można użyć w wyrażeniu zawierającym typ „{0}”</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Musisz umożliwić kompilatorowi rozróżnienie metod. Możesz na przykład nada
         <target state="translated">Modyfikatora „async” można używać tylko w metodach mających treść.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Parametrów ani elementów lokalnych typu „{0}” nie można deklarować w metodach asynchronicznych ani wyrażeniach lambda.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">Instrukcja foreach nie może działać na modułach wyliczających typu „{0}” w metodach asynchronicznych lub iteratora, ponieważ element „{0}” jest strukturą ref.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">Atrybutu zabezpieczeń „{0}” nie można zastosować dla metody asynchronicznej.</target>
@@ -11208,7 +11223,7 @@ Pominięcie ostrzeżenia należy wziąć pod uwagę tylko w sytuacji, gdy na pew
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">Wystąpienia typu „{0}” nie można użyć wewnątrz funkcji zagnieżdżonej, wyrażenia zapytania, bloku iteratora ani metody asynchronicznej</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ Aby usunąć ostrzeżenie, możesz zamiast tego użyć opcji /reference (ustaw w
         <target state="translated">Nie można użyć zmiennej lokalnej typu ref „{0}” wewnątrz metody anonimowej, wyrażenia lambda ani wyrażenia zapytania</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Iteratory nie mogą mieć zmiennych lokalnych dostępnych przez odwołanie</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">Metody asynchroniczne nie mogą mieć zmiennych lokalnych dostępnych przez odwołanie</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">Odwołanie zwrócone przez wywołanie „{0}” nie może zostać zachowane w granicach „await” lub „yield”.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf
index beac0aa629b3f..a4f21ff34b491 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf
@@ -32,6 +32,11 @@
         <target state="translated">'{0}': o evento abstrato não pode usar a sintaxe do acessador de eventos</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">'&amp;' nos grupos de métodos não pode ser usado em árvores de expressão</target>
@@ -282,14 +287,14 @@
         <target state="translated">O membro do registro '{0}' precisa ser uma propriedade de instância legível ou campo do tipo '{1}' para corresponder ao parâmetro posicional '{2}'.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Uma instrução lock em um valor do tipo "System.Threading.Lock" não pode ser usada em métodos assíncronos ou em expressões lambda assíncronas.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Um recurso de instrução using do tipo "{0}" não pode ser usado em métodos assíncronos ou expressões lambda assíncronas.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">'{0}' tipo não é válido para 'using static'. Somente uma classe, struct, interface, enumeração, delegado ou namespace podem ser usados.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">O atributo AsyncMethodBuilder não é permitido em métodos anônimos sem um tipo de retorno explícito.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Um campo ref só pode ser declarado em uma estrutura ref.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">O lado esquerdo de uma atribuição ref deve ser uma variável ref.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">operador de deslocamento flexível</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">O modificador 'ref' do argumento correspondente ao parâmetro 'in' é equivalente a 'in'. Considere usar 'in' em vez disso.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">O parâmetro de atribuição 'SizeConst' deve ser especificado.</target>
@@ -9221,11 +9251,6 @@ Um bloco catch() depois de um bloco catch (System.Exception e) poderá capturar
         <target state="translated">Não é possível usar os parâmetro ref, out ou in '{0}' dentro de um método anônimo, de uma expressão lambda de uma expressão de consulta ou de uma função local</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Código sem segurança só pode aparecer em iteradores</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">Não é possível usar a instrução yield no corpo de uma cláusula catch</target>
@@ -10610,8 +10635,8 @@ Forneça ao compilador alguma forma de diferenciar os métodos. Por exemplo, voc
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'aguardar' não pode ser usado em uma expressão que contém o tipo '{0}'</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'aguardar' não pode ser usado em uma expressão que contém o tipo '{0}'</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Forneça ao compilador alguma forma de diferenciar os métodos. Por exemplo, voc
         <target state="translated">O modificador 'async' só pode ser usado em métodos que têm um corpo.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Os parâmetros ou locais do tipo '{0}' não podem ser declarados nos métodos async ou expressões async lambda.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">a instrução foreach não pode operar em enumeradores do tipo '{0}' em métodos assíncronos ou iteradores porque '{0}' é uma struct de referência.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">Atributo de segurança "{0}" não pode ser aplicado a um método Assíncrono.</target>
@@ -11208,7 +11223,7 @@ Você pode suprimir o aviso se tiver certeza de que não vai querer aguardar a c
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">A instância do tipo '{0}' não pode ser usada dentro de uma função aninhada, expressão de consulta, bloco de iteradores ou método assíncrono</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ Para incorporar informações de tipo de interoperabilidade para os dois assembl
         <target state="translated">Não é possível usar a referência local '{0}' em um método anônimo, expressão lambda ou expressão de consulta</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Os iteradores não podem ter locais por referência</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">Os métodos assíncronos não podem ter locais por referência</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">Uma referência retornada por uma chamada para '{0}' não pode ser preservada no limite 'await' ou 'yield'.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf
index 9fe64263348de..7bc750767181d 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf
@@ -32,6 +32,11 @@
         <target state="translated">"{0}": абстрактное событие не может использовать синтаксис метода доступа к событиям.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">"&amp;" в группах методов не может использоваться в деревьях выражений</target>
@@ -282,14 +287,14 @@
         <target state="translated">Элемент записи "{0}" должен быть доступным для чтения свойством экземпляра или полем типа "{1}", чтобы соответствовать позиционному параметру "{2}".</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Оператор блокировки над значением типа "System.Threading.Lock" нельзя использовать в асинхронных методах и в асинхронных лямбда-выражениях.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">Ресурс оператора использования типа "{0}" нельзя применять в асинхронных методах или асинхронных лямбда-выражениях.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">' {0} ' недопустим для 'использования статики'. Можно использовать только класс, структуру, интерфейс, перечисление, делегат или пространство имен.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">Атрибут AsyncMethodBuilder запрещен для анонимных методов без явного типа возвращаемого значения.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Поле ссылки может быть объявлено только в структуре ссылки.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">Левая сторона назначения ref должна быть переменной ref.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">нестрогий оператор сдвига</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">Модификатор "ref" для аргумента, соответствующего параметру "in", эквивалентен "in". Попробуйте вместо этого использовать "in".</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">Должен быть указан параметр атрибута "SizeConst".</target>
@@ -9222,11 +9252,6 @@ A catch() block after a catch (System.Exception e) block can catch non-CLS excep
         <target state="translated">Недопустимо использовать параметр "{0}" с модификаторами ref, out или in внутри анонимного метода, лямбда-выражения, выражения запроса или локальной функции</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Небезопасный код не может использоваться в итераторах.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">Нельзя использовать оператор yield в теле предложения catch.</target>
@@ -10611,8 +10636,8 @@ Give the compiler some way to differentiate the methods. For example, you can gi
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'"await" нельзя использовать в выражении, содержащем тип "{0}"</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'"await" нельзя использовать в выражении, содержащем тип "{0}"</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10680,16 +10705,6 @@ Give the compiler some way to differentiate the methods. For example, you can gi
         <target state="translated">Модификатор "async" можно использовать только в методах, имеющих тело.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Параметры или локальные переменные типа "{0}" не могут объявляться в асинхронных методах и в асинхронных лямбда-выражениях.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">Оператор foreach нельзя использовать с перечислителями типа "{0}" в методах с модификатором Async или Iterator, так как "{0}" является ссылочной структурой.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">Атрибут безопасности "{0}" нельзя применить к асинхронному методу.</target>
@@ -11209,7 +11224,7 @@ You should consider suppressing the warning only if you're sure that you don't w
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">Экземпляр типа "{0}" нельзя использовать внутри вложенной функции, выражения запроса, блока итератора или асинхронного метода.</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12520,16 +12535,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="translated">Невозможно использовать локальную переменную ref "{0}" внутри анонимного метода, лямбда-выражения или выражения запроса</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Итераторы не могут иметь локальных переменных по ссылке</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">Асинхронные методы не могут иметь локальных переменных по ссылке</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">Ссылка, возвращенная вызовом ' {0} ', не может быть сохранена за границей 'wait' или 'yield'.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf
index 12d5367482f69..61a78dded475d 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf
@@ -32,6 +32,11 @@
         <target state="translated">'{0}': soyut olay, olay erişeni söz dizimini kullanamaz</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">Metot gruplarındaki '&amp;', ifade ağaçlarında kullanılamaz</target>
@@ -282,14 +287,14 @@
         <target state="translated">{0} kayıt üyesi, {1} konumsal parametresi ile eşleşmesi için {2} türünde okunabilir bir örnek özelliği veya alan olmalıdır.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">'System.Threading.Lock' türündeki bir değere ilişkin lock deyimi, asenkron yöntemlerde veya asenkron lambda ifadelerinde kullanılamaz.</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">'{0}' türündeki bir using deyimi kaynağı, asenkron yöntemlerde veya asenkron lambda ifadelerinde kullanılamaz.</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">'{0}' türü 'using static' için geçerli değil. Yalnızca bir sınıf, yapı, arabirim, sabit liste, temsilci veya ad alanı kullanılabilir.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">Açık dönüş türü olmadan, anonim yöntemlerde AsyncMethodBuilder özniteliğine izin verilmez.</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">Başvuru alanı yalnızca başvuru yapısında bildirilebilir.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">ref atamasının sol tarafı, ref değişkeni olmalıdır.</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">esnek kaydırma işleci</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">'in' parametresine karşılık gelen bir bağımsız değişken için 'ref' değiştiricisi 'in' ile eşdeğerdir. Bunun yerine 'in' kullanmayı düşünün.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">'SizeConst' öznitelik parametresi belirtilmelidir.</target>
@@ -9221,11 +9251,6 @@ RuntimeCompatibilityAttribute AssemblyInfo.cs dosyasında false olarak ayarlanm
         <target state="translated">Anonim metot, lambda ifadesi, sorgu ifadesi veya yerel işlev içinde '{0}' ref, out veya in parametresi kullanılamaz</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Güvenli olmayan kod yineleyicilerde görünmeyebilir</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">Catch yan tümcesinin gövdesinde yield ile bir değer döndürülemez</target>
@@ -10610,8 +10635,8 @@ Derleyiciye yöntemleri ayrıştırma yolu verin. Örneğin, bunlara farklı adl
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'await', '{0}' türünü içeren bir ifadede kullanılamaz</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'await', '{0}' türünü içeren bir ifadede kullanılamaz</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Derleyiciye yöntemleri ayrıştırma yolu verin. Örneğin, bunlara farklı adl
         <target state="translated">Async' değiştiricisi yalnızca gövdesi olan metotlarda kullanılabilir.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">Zaman uyumsuz yöntemlerde veya zaman uyumsuz lambda ifadelerinde '{0}' türündeki parametreler veya yerel öğeler bildirilemez.</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">'{0}' bir başvuru yapısı olduğundan foreach deyimi, async veya iterator metotlarındaki '{0}' türü numaralandırıcılar üzerinde çalışamaz.</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">'{0}' güvenlik özniteliği bir Async yöntemine uygulanamaz.</target>
@@ -11208,7 +11223,7 @@ Yalnızca asenkron çağrının tamamlanmasını beklemek istemediğinizden ve 
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">'{0}' türünün örneği iç içe geçmiş bir işlevde, sorgu ifadesinde, yineleyici bloğunda veya zaman uyumsuz bir metotta kullanılamaz</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ Uyarıyı kaldırmak için, /reference kullanabilirsiniz (Birlikte Çalışma T
         <target state="translated">'{0}' ref yerel değeri bir anonim metotta, lambda ifadesinde veya sorgu ifadesinde kullanılamaz</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Yineleyiciler başvuruya göre yerel değerlere sahip olamaz</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">Zaman uyumsuz metotlar başvuruya göre yerel değerlere sahip olamaz</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">'{0}' hedefine bir çağrı tarafından döndürülen başvuru, 'await' veya 'yield' sınırında korunamıyor.</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf
index 8474baffa7512..b0175426fa302 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf
@@ -32,6 +32,11 @@
         <target state="translated">“{0}”: 抽象事件不可使用事件访问器语法</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">不可在表达式树中使用方法组上的 "&amp;"</target>
@@ -282,14 +287,14 @@
         <target state="translated">记录成员 '{0}' 必须为类型 '{1}' 的可读实例属性或字段，以匹配位置参数 '{2}'。</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">类型“System.Threading.Lock”的值的 lock 语句不能用于异步方法或异步 lambda 表达式。</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">无法在异步方法或异步 lambda 表达式中使用类型为“{0}”的 using 语句资源。</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">“{0}”类型对于 "using static" 无效。只能使用类、结构、接口、枚举、委托或命名空间。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">没有显式返回类型的匿名方法不允许使用 AsyncMethodBuilder 属性。</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">ref 字段只能在 ref 结构中声明。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">ref 赋值的左侧必须为 ref 变量。</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">移位运算符</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">与 “in” 参数对应的参数的 “ref” 修饰符等效于 “in”。请考虑改用 “in”。</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">必须指定属性参数 “SizeConst”。</target>
@@ -9221,11 +9251,6 @@ A catch() block after a catch (System.Exception e) block can catch non-CLS excep
         <target state="translated">不能在匿名方法、lambda 表达式、查询表达式或本地函数中使用 ref、out 或 in 参数“{0}”</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">迭代器中不能出现不安全的代码</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">无法在 catch 子句体中生成值</target>
@@ -10610,8 +10635,8 @@ Give the compiler some way to differentiate the methods. For example, you can gi
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'“等待”不能在包含“{0}”类型的表达式中使用</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'“等待”不能在包含“{0}”类型的表达式中使用</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Give the compiler some way to differentiate the methods. For example, you can gi
         <target state="translated">只能在具有正文的方法中使用 "async" 修饰符。</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">不能在异步方法或异步 lambda 表达式中声明类型“{0}”的参数或局部变量。</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">foreach 语句无法在类型“{0}”的枚举器上使用异步或迭代器方法操作，因为“{0}”是 ref 结构。</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">安全特性“{0}”不可应用于异步方法。</target>
@@ -11208,7 +11223,7 @@ You should consider suppressing the warning only if you're sure that you don't w
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">“{0}”类型的实例不能在嵌套函数、查询表达式、迭代器块或异步方法中使用</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="translated">不能在匿名方法、lambda 表达式或查询表达式内使用 ref 局部变量“{0}”</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">迭代器不能有按引用局部变量</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">异步方法不能有按引用局部变量</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">调用“{0}”所返回的引用不能跨 "await" 或 "yield" 边界保留。</target>
diff --git a/src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf b/src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf
index 2f85585c6f3b6..59932f5945760 100644
--- a/src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf
+++ b/src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf
@@ -32,6 +32,11 @@
         <target state="translated">'{0}' 抽象事件無法使用事件存取子語法</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_AddressOfInIterator">
+        <source>The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</source>
+        <target state="new">The '&amp;' operator cannot be used on parameters or local variables in iterator methods.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_AddressOfMethodGroupInExpressionTree">
         <source>'&amp;' on method groups cannot be used in expression trees</source>
         <target state="translated">不得在運算式樹狀架構中對方法群組使用 '&amp;'</target>
@@ -282,14 +287,14 @@
         <target state="translated">記錄成員 '{0}' 必須是類型 '{1}' 的可讀取執行個體屬性或欄位，才能符合位置參數 '{2}'。</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLock">
-        <source>A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">型別 'System.Threading.Lock' 的值上的 lock 陳述式不能用在非同步方法或非同步 Lambda 運算式中。</target>
+      <trans-unit id="ERR_BadSpecialByRefIterator">
+        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
+        <target state="new">foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefUsing">
-        <source>A using statement resource of type '{0}' cannot be used in async methods or async lambda expressions.</source>
-        <target state="translated">類型 '{0}' 的 using 陳述式資源不能用於非同步方法或非同步 Lambda 運算式。</target>
+      <trans-unit id="ERR_BadSpecialByRefParameter">
+        <source>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
+        <target state="new">Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_BadRefInUsingAlias">
@@ -317,6 +322,11 @@
         <target state="translated">'{0}' 類型對 'using static' 無效。只能使用類別、結構、介面、列舉、委派或命名空間。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_BadYieldInUnsafe">
+        <source>Cannot use 'yield return' in an 'unsafe' block</source>
+        <target state="new">Cannot use 'yield return' in an 'unsafe' block</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_BuilderAttributeDisallowed">
         <source>The AsyncMethodBuilder attribute is disallowed on anonymous methods without an explicit return type.</source>
         <target state="translated">沒有明確傳回型別的匿名方法上不允許 AsyncMethodBuilder 屬性。</target>
@@ -1767,6 +1777,11 @@
         <target state="translated">ref 欄位只能在 ref 結構中宣告。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_RefLocalAcrossAwait">
+        <source>A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="new">A 'ref' local cannot be preserved across 'await' or 'yield' boundary.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefLocalOrParamExpected">
         <source>The left-hand side of a ref assignment must be a ref variable.</source>
         <target state="translated">參考指派的左側必須為 ref 變數。</target>
@@ -2422,6 +2437,11 @@
         <target state="new">ref struct interfaces</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefUnsafeInIteratorAsync">
+        <source>ref and unsafe in async and iterator methods</source>
+        <target state="new">ref and unsafe in async and iterator methods</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRelaxedShiftOperator">
         <source>relaxed shift operator</source>
         <target state="translated">寬鬆移位 (Shift) 運算子</target>
@@ -2597,6 +2617,16 @@
         <target state="translated">對應至 'in' 參數之引數的 'ref' 修飾元相當於 'in'。請考慮改為使用 'in'。</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_BadYieldInLock_Title">
+        <source>'yield return' should not be used in the body of a lock statement</source>
+        <target state="new">'yield return' should not be used in the body of a lock statement</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_ByValArraySizeConstRequired">
         <source>Attribute parameter 'SizeConst' must be specified.</source>
         <target state="translated">必須指定屬性參數 'SizeConst'。</target>
@@ -9221,11 +9251,6 @@ A catch() block after a catch (System.Exception e) block can catch non-CLS excep
         <target state="translated">無法在匿名方法、Lambda 運算式、查詢運算式或區域函式中使用 ref、out 或 in 參數 '{0}'</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_IllegalInnerUnsafe">
-        <source>Unsafe code may not appear in iterators</source>
-        <target state="translated">Unsafe 程式碼不可出現在迭代器中</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_BadYieldInCatch">
         <source>Cannot yield a value in the body of a catch clause</source>
         <target state="translated">無法在 catch 子句主體中使用 yield 產生值</target>
@@ -10610,8 +10635,8 @@ Give the compiler some way to differentiate the methods. For example, you can gi
         <note />
       </trans-unit>
       <trans-unit id="ERR_ByRefTypeAndAwait">
-        <source>'await' cannot be used in an expression containing the type '{0}'</source>
-        <target state="translated">'await' 不得用於包含類型 '{0}' 的運算式中</target>
+        <source>Instance of type '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
+        <target state="needs-review-translation">'await' 不得用於包含類型 '{0}' 的運算式中</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_UnsafeAsyncArgType">
@@ -10679,16 +10704,6 @@ Give the compiler some way to differentiate the methods. For example, you can gi
         <target state="translated">async' 修飾元只可用於具有主體的方法。</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefLocal">
-        <source>Parameters or locals of type '{0}' cannot be declared in async methods or async lambda expressions.</source>
-        <target state="translated">類型 '{0}' 的參數或區域變數，不可在非同步方法或非同步 Lambda 運算式中宣告。</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadSpecialByRefIterator">
-        <source>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</source>
-        <target state="needs-review-translation">foreach 陳述式無法對 async 或 iterator 方法中類型 '{0}' 的列舉值進行操作，因為 '{0}' 為 ref struct。</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_SecurityCriticalOrSecuritySafeCriticalOnAsync">
         <source>Security attribute '{0}' cannot be applied to an Async method.</source>
         <target state="translated">安全屬性 '{0}' 無法套用至非同步方法。</target>
@@ -11208,7 +11223,7 @@ You should consider suppressing the warning only if you're sure that you don't w
       </trans-unit>
       <trans-unit id="ERR_SpecialByRefInLambda">
         <source>Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</source>
-        <target state="translated">類型 '{0}' 的執行個體不可用於巢狀函式、查詢運算式、迭代區塊或非同步方法中</target>
+        <target state="new">Instance of type '{0}' cannot be used inside a nested function, query expression, iterator block or async method</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SecurityAttributeMissingAction">
@@ -12519,16 +12534,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="translated">無法在匿名方法、Lambda 運算式或查詢運算式中使用參考本機 '{0}'</target>
         <note />
       </trans-unit>
-      <trans-unit id="ERR_BadIteratorLocalType">
-        <source>Iterators cannot have by-reference locals</source>
-        <target state="translated">Iterator 不可有 by-reference local</target>
-        <note />
-      </trans-unit>
-      <trans-unit id="ERR_BadAsyncLocalType">
-        <source>Async methods cannot have by-reference locals</source>
-        <target state="translated">非同步方法不可有 by-reference local</target>
-        <note />
-      </trans-unit>
       <trans-unit id="ERR_RefReturningCallAndAwait">
         <source>A reference returned by a call to '{0}' cannot be preserved across 'await' or 'yield' boundary.</source>
         <target state="translated">對 '{0}' 之呼叫所傳回的參考無法在 'await' 或 'yield' 界限間保留。</target>
diff --git a/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTests.cs b/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTests.cs
index 9842e30b2a710..2ea7b234d8da8 100644
--- a/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTests.cs
+++ b/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTests.cs
@@ -133,7 +133,7 @@ private static void VerifyMissingType(string source, WellKnownType type, params
         // Instrumentation to investigate CI failure: https://github.com/dotnet/roslyn/issues/34207
         private CSharpCompilation CreateCompilationWithAsyncIterator(string source, CSharpCompilationOptions options = null, CSharpParseOptions parseOptions = null)
             => CreateCompilationWithTasksExtensions(new[] { (CSharpTestSource)CSharpTestBase.Parse(source, filename: "source", parseOptions), CSharpTestBase.Parse(AsyncStreamsTypes, filename: "AsyncStreamsTypes", parseOptions) },
-                options: options, parseOptions: parseOptions);
+                options: options);
 
         private CSharpCompilation CreateCompilationWithAsyncIterator(CSharpTestSource source, CSharpCompilationOptions options = null, CSharpParseOptions parseOptions = null)
             => CreateCompilationWithTasksExtensions(new[] { source, AsyncStreamsTypes }, options: options, parseOptions: parseOptions);
@@ -631,15 +631,288 @@ static async System.Threading.Tasks.Task Main()
 ref struct S
 {
 }";
+
+            var expectedDiagnostics = new[]
+            {
+                // source(4,65): error CS9504: The type 'S' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'T' in the generic type or method 'IAsyncEnumerable<T>'
+                //     static async System.Collections.Generic.IAsyncEnumerable<S> M()
+                Diagnostic(ErrorCode.ERR_NotRefStructConstraintNotSatisfied, "M").WithArguments("System.Collections.Generic.IAsyncEnumerable<T>", "T", "S").WithLocation(4, 65)
+            };
+
             var comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe);
+            comp.VerifyDiagnostics(expectedDiagnostics);
+
+            comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe, parseOptions: TestOptions.RegularNext);
+            comp.VerifyDiagnostics(expectedDiagnostics);
+
+            comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe, parseOptions: TestOptions.Regular12);
             comp.VerifyDiagnostics(
                 // source(4,65): error CS9504: The type 'S' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'T' in the generic type or method 'IAsyncEnumerable<T>'
                 //     static async System.Collections.Generic.IAsyncEnumerable<S> M()
                 Diagnostic(ErrorCode.ERR_NotRefStructConstraintNotSatisfied, "M").WithArguments("System.Collections.Generic.IAsyncEnumerable<T>", "T", "S").WithLocation(4, 65),
-                // source(11,24): error CS4012: Parameters or locals of type 'S' cannot be declared in async methods or async lambda expressions.
+                // source(11,24): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         await foreach (var s in M())
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("S").WithLocation(11, 24)
-                );
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(11, 24));
+        }
+
+        [Fact]
+        public void RefStructElementType_NonGeneric()
+        {
+            string source = """
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    public E GetAsyncEnumerator() => new E();
+                    static async Task Main()
+                    {
+                        await foreach (var s in new C())
+                        {
+                            System.Console.Write(s.F);
+                        }
+                    }
+                }
+                class E
+                {
+                    bool _done;
+                    public S Current => new S { F = 123 };
+                    public async Task<bool> MoveNextAsync()
+                    {
+                        await Task.Yield();
+                        return !_done ? (_done = true) : false;
+                    }
+                }
+                ref struct S
+                {
+                    public int F;
+                }
+                """;
+
+            var comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe, parseOptions: TestOptions.Regular12);
+            comp.VerifyDiagnostics(
+                // source(8,24): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 24));
+
+            comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe, parseOptions: TestOptions.RegularNext);
+            comp.VerifyEmitDiagnostics();
+
+            comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe);
+            var verifier = CompileAndVerify(comp, expectedOutput: "123", verify: Verification.FailsILVerify);
+            verifier.VerifyDiagnostics();
+            verifier.VerifyIL("C.<Main>d__1.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+                {
+                  // Code size      238 (0xee)
+                  .maxstack  3
+                  .locals init (int V_0,
+                                S V_1, //s
+                                System.Runtime.CompilerServices.TaskAwaiter<bool> V_2,
+                                C.<Main>d__1 V_3,
+                                System.Exception V_4)
+                  IL_0000:  ldarg.0
+                  IL_0001:  ldfld      "int C.<Main>d__1.<>1__state"
+                  IL_0006:  stloc.0
+                  .try
+                  {
+                    IL_0007:  ldloc.0
+                    IL_0008:  brfalse.s  IL_0012
+                    IL_000a:  br.s       IL_000c
+                    IL_000c:  ldloc.0
+                    IL_000d:  ldc.i4.1
+                    IL_000e:  beq.s      IL_0014
+                    IL_0010:  br.s       IL_0016
+                    IL_0012:  br.s       IL_0082
+                    IL_0014:  br.s       IL_0082
+                    IL_0016:  nop
+                    IL_0017:  nop
+                    IL_0018:  ldarg.0
+                    IL_0019:  newobj     "C..ctor()"
+                    IL_001e:  call       "E C.GetAsyncEnumerator()"
+                    IL_0023:  stfld      "E C.<Main>d__1.<>s__1"
+                    IL_0028:  br.s       IL_0044
+                    IL_002a:  ldarg.0
+                    IL_002b:  ldfld      "E C.<Main>d__1.<>s__1"
+                    IL_0030:  callvirt   "S E.Current.get"
+                    IL_0035:  stloc.1
+                    IL_0036:  nop
+                    IL_0037:  ldloc.1
+                    IL_0038:  ldfld      "int S.F"
+                    IL_003d:  call       "void System.Console.Write(int)"
+                    IL_0042:  nop
+                    IL_0043:  nop
+                    IL_0044:  ldarg.0
+                    IL_0045:  ldfld      "E C.<Main>d__1.<>s__1"
+                    IL_004a:  callvirt   "System.Threading.Tasks.Task<bool> E.MoveNextAsync()"
+                    IL_004f:  callvirt   "System.Runtime.CompilerServices.TaskAwaiter<bool> System.Threading.Tasks.Task<bool>.GetAwaiter()"
+                    IL_0054:  stloc.2
+                    IL_0055:  ldloca.s   V_2
+                    IL_0057:  call       "bool System.Runtime.CompilerServices.TaskAwaiter<bool>.IsCompleted.get"
+                    IL_005c:  brtrue.s   IL_009e
+                    IL_005e:  ldarg.0
+                    IL_005f:  ldc.i4.0
+                    IL_0060:  dup
+                    IL_0061:  stloc.0
+                    IL_0062:  stfld      "int C.<Main>d__1.<>1__state"
+                    IL_0067:  ldarg.0
+                    IL_0068:  ldloc.2
+                    IL_0069:  stfld      "System.Runtime.CompilerServices.TaskAwaiter<bool> C.<Main>d__1.<>u__1"
+                    IL_006e:  ldarg.0
+                    IL_006f:  stloc.3
+                    IL_0070:  ldarg.0
+                    IL_0071:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__1.<>t__builder"
+                    IL_0076:  ldloca.s   V_2
+                    IL_0078:  ldloca.s   V_3
+                    IL_007a:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.TaskAwaiter<bool>, C.<Main>d__1>(ref System.Runtime.CompilerServices.TaskAwaiter<bool>, ref C.<Main>d__1)"
+                    IL_007f:  nop
+                    IL_0080:  leave.s    IL_00ed
+                    IL_0082:  ldarg.0
+                    IL_0083:  ldfld      "System.Runtime.CompilerServices.TaskAwaiter<bool> C.<Main>d__1.<>u__1"
+                    IL_0088:  stloc.2
+                    IL_0089:  ldarg.0
+                    IL_008a:  ldflda     "System.Runtime.CompilerServices.TaskAwaiter<bool> C.<Main>d__1.<>u__1"
+                    IL_008f:  initobj    "System.Runtime.CompilerServices.TaskAwaiter<bool>"
+                    IL_0095:  ldarg.0
+                    IL_0096:  ldc.i4.m1
+                    IL_0097:  dup
+                    IL_0098:  stloc.0
+                    IL_0099:  stfld      "int C.<Main>d__1.<>1__state"
+                    IL_009e:  ldarg.0
+                    IL_009f:  ldloca.s   V_2
+                    IL_00a1:  call       "bool System.Runtime.CompilerServices.TaskAwaiter<bool>.GetResult()"
+                    IL_00a6:  stfld      "bool C.<Main>d__1.<>s__2"
+                    IL_00ab:  ldarg.0
+                    IL_00ac:  ldfld      "bool C.<Main>d__1.<>s__2"
+                    IL_00b1:  brtrue     IL_002a
+                    IL_00b6:  ldarg.0
+                    IL_00b7:  ldnull
+                    IL_00b8:  stfld      "E C.<Main>d__1.<>s__1"
+                    IL_00bd:  leave.s    IL_00d9
+                  }
+                  catch System.Exception
+                  {
+                    IL_00bf:  stloc.s    V_4
+                    IL_00c1:  ldarg.0
+                    IL_00c2:  ldc.i4.s   -2
+                    IL_00c4:  stfld      "int C.<Main>d__1.<>1__state"
+                    IL_00c9:  ldarg.0
+                    IL_00ca:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__1.<>t__builder"
+                    IL_00cf:  ldloc.s    V_4
+                    IL_00d1:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                    IL_00d6:  nop
+                    IL_00d7:  leave.s    IL_00ed
+                  }
+                  IL_00d9:  ldarg.0
+                  IL_00da:  ldc.i4.s   -2
+                  IL_00dc:  stfld      "int C.<Main>d__1.<>1__state"
+                  IL_00e1:  ldarg.0
+                  IL_00e2:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__1.<>t__builder"
+                  IL_00e7:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+                  IL_00ec:  nop
+                  IL_00ed:  ret
+                }
+                """);
+        }
+
+        [Fact]
+        public void RefStructElementType_NonGeneric_AwaitAfter()
+        {
+            string source = """
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    public E GetAsyncEnumerator() => new E();
+                    static async Task Main()
+                    {
+                        await foreach (var s in new C())
+                        {
+                            System.Console.Write(s.F);
+                            await Task.Yield();
+                        }
+                    }
+                }
+                class E
+                {
+                    bool _done;
+                    public S Current => new S { F = 123 };
+                    public async Task<bool> MoveNextAsync()
+                    {
+                        await Task.Yield();
+                        return !_done ? (_done = true) : false;
+                    }
+                }
+                ref struct S
+                {
+                    public int F;
+                }
+                """;
+
+            var comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe, parseOptions: TestOptions.Regular12);
+            comp.VerifyDiagnostics(
+                // source(8,24): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 24));
+
+            comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe, parseOptions: TestOptions.RegularNext);
+            comp.VerifyEmitDiagnostics();
+
+            comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe);
+            var verifier = CompileAndVerify(comp, expectedOutput: "123", verify: Verification.FailsILVerify);
+            verifier.VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefStructElementType_NonGeneric_AwaitBefore()
+        {
+            string source = """
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    public E GetAsyncEnumerator() => new E();
+                    static async Task Main()
+                    {
+                        await foreach (var s in new C())
+                        {
+                            await Task.Yield();
+                            System.Console.Write(s.F);
+                        }
+                    }
+                }
+                class E
+                {
+                    bool _done;
+                    public S Current => new S { F = 123 };
+                    public async Task<bool> MoveNextAsync()
+                    {
+                        await Task.Yield();
+                        return !_done ? (_done = true) : false;
+                    }
+                }
+                ref struct S
+                {
+                    public int F;
+                }
+                """;
+
+            var comp = CreateCompilationWithAsyncIterator(source, parseOptions: TestOptions.Regular12);
+            comp.VerifyDiagnostics(
+                // source(8,24): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 24));
+
+            var expectedDiagnostics = new[]
+            {
+                // source(11,34): error CS4007: Instance of type 'S' cannot be preserved across 'await' or 'yield' boundary.
+                //             System.Console.Write(s.F);
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s.F").WithArguments("S").WithLocation(11, 34)
+            };
+
+            comp = CreateCompilationWithAsyncIterator(source, parseOptions: TestOptions.RegularNext);
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
+
+            comp = CreateCompilationWithAsyncIterator(source);
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncSpillTests.cs b/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncSpillTests.cs
index 0a7f649ff7d64..999bf6667331d 100644
--- a/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncSpillTests.cs
+++ b/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncSpillTests.cs
@@ -3789,9 +3789,11 @@ static async Task Main()
                 var comp = CreateCompilationWithMscorlibAndSpan(source, options: options);
                 comp.VerifyDiagnostics();
                 comp.VerifyEmitDiagnostics(
-                    // (9,66): error CS4007: 'await' cannot be used in an expression containing the type 'System.Span<int>'
-                    //         await Async1(F1(), G(F2(), stackalloc int[] { 1, 2, 3 }, await F3()));
-                    Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "await F3()").WithArguments("System.Span<int>").WithLocation(9, 66)
+                    // (8,5): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                    //     {
+                    Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, @"{
+        await Async1(F1(), G(F2(), stackalloc int[] { 1, 2, 3 }, await F3()));
+    }").WithArguments("System.Span<int>").WithLocation(8, 5)
                     );
             }
         }
@@ -3897,16 +3899,16 @@ public ref struct S
     public bool P2 => true;
 }
 ";
-            CreateCompilation(source, options: TestOptions.DebugDll).VerifyDiagnostics().VerifyEmitDiagnostics(
-                // (9,17): error CS4013: Instance of type 'S' cannot be used inside a nested function, query expression, iterator block or async method
-                //             Q { F: { P1: true } } when await c => r, // error: cached Q.F is alive
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "F").WithArguments("S").WithLocation(9, 17)
-                );
-            CreateCompilation(source, options: TestOptions.ReleaseDll).VerifyDiagnostics().VerifyEmitDiagnostics(
-                // (9,17): error CS4013: Instance of type 'S' cannot be used inside a nested function, query expression, iterator block or async method
+
+            var expectedDiagnostics = new[]
+            {
+                // (9,17): error CS4007: Instance of type 'S' cannot be preserved across 'await' or 'yield' boundary.
                 //             Q { F: { P1: true } } when await c => r, // error: cached Q.F is alive
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "F").WithArguments("S").WithLocation(9, 17)
-                );
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "F").WithArguments("S").WithLocation(9, 17)
+            };
+
+            CreateCompilation(source, options: TestOptions.DebugDll).VerifyDiagnostics().VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(source, options: TestOptions.ReleaseDll).VerifyDiagnostics().VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitForeachTests.cs b/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitForeachTests.cs
index bce2519fce4d0..2dcb0b215b19d 100644
--- a/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitForeachTests.cs
+++ b/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitForeachTests.cs
@@ -1695,12 +1695,26 @@ public System.Threading.Tasks.Task<bool> MoveNextAsync()
             => throw null;
         public int Current { get => throw null; }
     }
-}";
-            var comp = CreateCompilationWithTasksExtensions(source + s_IAsyncEnumerable);
+}" + s_IAsyncEnumerable;
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12);
             comp.VerifyDiagnostics(
-                // (6,32): error CS8177: Async methods cannot have by-reference locals
+                // (6,32): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         await foreach (ref var i in new C())
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "i").WithLocation(6, 32));
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "i").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 32));
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,37): error CS1510: A ref or out value must be an assignable variable
+                //         await foreach (ref var i in new C())
+                Diagnostic(ErrorCode.ERR_RefLvalueExpected, "new C()").WithLocation(6, 37)
+            };
+
+            comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext);
+            comp.VerifyDiagnostics(expectedDiagnostics);
+
+            comp = CreateCompilationWithTasksExtensions(source);
+            comp.VerifyDiagnostics(expectedDiagnostics);
 
             var tree = comp.SyntaxTrees.Single();
             var model = (SyntaxTreeSemanticModel)comp.GetSemanticModel(tree, ignoreAccessibility: false);
@@ -1708,6 +1722,119 @@ public System.Threading.Tasks.Task<bool> MoveNextAsync()
             Assert.Equal(default, model.GetForEachStatementInfo(foreachSyntax));
         }
 
+        [Theory, CombinatorialData]
+        public void TestWithPattern_Ref_Iterator([CombinatorialValues("        ", "readonly")] string modifier)
+        {
+            var source = $$"""
+                using System;
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    static async Task Main()
+                    {
+                        await foreach (int i in F())
+                        {
+                            Console.Write(i);
+                        }
+                    }
+
+                    static async IAsyncEnumerable<int> F()
+                    {
+                        await foreach (ref {{modifier}} var i in new C())
+                        {
+                            yield return i;
+                        }
+                    }
+
+                    public Enumerator GetAsyncEnumerator(System.Threading.CancellationToken token = default) => new();
+
+                    public sealed class Enumerator
+                    {
+                        private readonly int[] _array = [1, 2, 3];
+                        private int _index = -1;
+                        public Task<bool> MoveNextAsync()
+                        {
+                            if (_index < _array.Length) _index++;
+                            return Task.FromResult(_index < _array.Length);
+                        }       
+                        public ref int Current => ref _array[_index];
+                    }
+                }
+                """ + AsyncStreamsTypes;
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12);
+            comp.VerifyDiagnostics(
+                // (17,41): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (ref readonly var i in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "i").WithArguments("ref and unsafe in async and iterator methods").WithLocation(17, 41));
+
+            var expectedOutput = "123";
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe, parseOptions: TestOptions.RegularNext);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_Ref_Iterator_Used()
+        {
+            var source = """
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    static async IAsyncEnumerable<int> F()
+                    {
+                        await foreach (ref var i in new C())
+                        {
+                            yield return i;
+                            M(ref i);
+                        }
+                    }
+
+                    static void M(ref int i) { }
+
+                    public Enumerator GetAsyncEnumerator(System.Threading.CancellationToken token = default) => new();
+
+                    public sealed class Enumerator
+                    {
+                        private readonly int[] _array = [1, 2, 3];
+                        private int _index = -1;
+                        public Task<bool> MoveNextAsync()
+                        {
+                            if (_index < _array.Length) _index++;
+                            return Task.FromResult(_index < _array.Length);
+                        }       
+                        public ref int Current => ref _array[_index];
+                    }
+                }
+                """ + AsyncStreamsTypes;
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12);
+            comp.VerifyDiagnostics(
+                // (8,32): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (ref var i in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "i").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 32));
+
+            var expectedDiagnostics = new[]
+            {
+                // (11,19): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             M(ref i);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "i").WithLocation(11, 19)
+            };
+
+            comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext);
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
+
+            comp = CreateCompilationWithTasksExtensions(source);
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
         [Fact]
         public void TestWithPattern_PointerType()
         {
@@ -1935,7 +2062,427 @@ public S(int i)
         }
 
         [Fact]
-        public void TestWithPattern_RefReturningCurrent()
+        public void TestWithPattern_RefStructEnumerator_Async()
+        {
+            var source = """
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async Task Main()
+                    {
+                        await foreach (var s in new C())
+                        {
+                        }
+                    }
+                    public Enumerator GetAsyncEnumerator() => new Enumerator();
+                    public ref struct Enumerator
+                    {
+                        public int Current => 0;
+                        public Task<bool> MoveNextAsync() => throw null;
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,15): error CS8344: foreach statement cannot operate on enumerators of type 'C.Enumerator' in async or iterator methods because 'C.Enumerator' is a ref struct.
+                //         await foreach (var s in new C())
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefIterator, "foreach").WithArguments("C.Enumerator").WithLocation(6, 15)
+            };
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(source).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void TestWithPattern_RefStructEnumerator_AsyncIterator()
+        {
+            var source = """
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async IAsyncEnumerable<int> M()
+                    {
+                        await foreach (var x in new C())
+                        {
+                            yield return x;
+                        }
+                    }
+                    public Enumerator GetAsyncEnumerator() => new Enumerator();
+                    public ref struct Enumerator
+                    {
+                        public int Current => 0;
+                        public Task<bool> MoveNextAsync() => throw null;
+                    }
+                }
+                """ + s_IAsyncEnumerable;
+
+            var expectedDiagnostics = new[]
+            {
+                // (7,15): error CS8344: foreach statement cannot operate on enumerators of type 'C.Enumerator' in async or iterator methods because 'C.Enumerator' is a ref struct.
+                //         await foreach (var x in new C())
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefIterator, "foreach").WithArguments("C.Enumerator").WithLocation(7, 15)
+            };
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilationWithTasksExtensions(source).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void TestWithPattern_RefStructEnumerator_Iterator()
+        {
+            var source = """
+                using System.Collections.Generic;
+                public class C
+                {
+                    public static IEnumerable<int> M()
+                    {
+                        foreach (var x in new C())
+                        {
+                            yield return x;
+                        }
+                    }
+                    public Enumerator GetEnumerator() => new Enumerator();
+                    public ref struct Enumerator
+                    {
+                        public int Current => 0;
+                        public bool MoveNext() => throw null;
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,9): error CS8344: foreach statement cannot operate on enumerators of type 'C.Enumerator' in async or iterator methods because 'C.Enumerator' is a ref struct.
+                //         foreach (var x in new C())
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefIterator, "foreach").WithArguments("C.Enumerator").WithLocation(6, 9)
+            };
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(source).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void TestWithPattern_RefStructEnumerable_Async()
+        {
+            var source = """
+                using System;
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async Task Main()
+                    {
+                        await foreach (var x in new Enumerable())
+                        {
+                            await Task.Yield();
+                            Console.Write($"{x} ");
+                        }
+                        Console.Write("Done");
+                    }
+                    public ref struct Enumerable
+                    {
+                        public Enumerator GetAsyncEnumerator() => new();
+                    }
+                    public class Enumerator
+                    {
+                        int i = 0;
+                        public int Current => i;
+                        public async Task<bool> MoveNextAsync()
+                        {
+                            i++;
+                            await Task.Yield();
+                            return i < 3;
+                        }
+                    }
+                }
+                """ + s_IAsyncEnumerable;
+            var comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: "1 2 Done").VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefStructEnumerable_AsyncIterator()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async Task Main()
+                    {
+                        await foreach (var i in M())
+                        {
+                            Console.Write($"{i} ");
+                        }
+                        Console.Write("Done");
+                    }
+                    public static async IAsyncEnumerable<int> M()
+                    {
+                        await foreach (var x in new Enumerable())
+                        {
+                            await Task.Yield();
+                            yield return x * 2;
+                        }
+                        yield return -1;
+                    }
+                    public ref struct Enumerable
+                    {
+                        public Enumerator GetAsyncEnumerator() => new();
+                    }
+                    public class Enumerator
+                    {
+                        int i = 0;
+                        public int Current => i;
+                        public async Task<bool> MoveNextAsync()
+                        {
+                            i++;
+                            await Task.Yield();
+                            return i < 3;
+                        }
+                    }
+                }
+                """ + AsyncStreamsTypes;
+            var comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: "2 4 -1 Done").VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefStructEnumerable_Iterator()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                public class C
+                {
+                    public static void Main()
+                    {
+                        foreach (var i in M())
+                        {
+                            Console.Write($"{i} ");
+                        }
+                        Console.Write("Done");
+                    }
+                    public static IEnumerable<int> M()
+                    {
+                        foreach (var x in new Enumerable())
+                        {
+                            yield return x * 2;
+                        }
+                        yield return -1;
+                    }
+                    public ref struct Enumerable
+                    {
+                        public Enumerator GetEnumerator() => new();
+                    }
+                    public class Enumerator
+                    {
+                        int i = 0;
+                        public int Current => i;
+                        public bool MoveNext()
+                        {
+                            i++;
+                            return i < 3;
+                        }
+                    }
+                }
+                """;
+            CompileAndVerify(source, expectedOutput: "2 4 -1 Done").VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefStructCurrent_Async()
+        {
+            var source = """
+                using System;
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async Task Main()
+                    {
+                        await foreach (var s in new C())
+                        {
+                            Console.Write($"{s.ToString()} ");
+                        }
+                        Console.Write("Done");
+                    }
+                    public Enumerator GetAsyncEnumerator() => new Enumerator();
+                    public sealed class Enumerator : IAsyncDisposable
+                    {
+                        int i = 0;
+                        public S Current => new S(i);
+                        public async Task<bool> MoveNextAsync()
+                        {
+                            i++;
+                            await Task.Yield();
+                            return i < 3;
+                        }
+                        public async ValueTask DisposeAsync()
+                        {
+                            await Task.Yield();
+                        }
+                    }
+                }
+                public ref struct S
+                {
+                    int i;
+                    public S(int i)
+                    {
+                        this.i = i;
+                    }
+                    public override string ToString() => i.ToString();
+                }
+                """ + s_IAsyncEnumerable;
+
+            var expectedDiagnostics = new[]
+            {
+                // (7,24): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 24)
+            };
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(expectedDiagnostics);
+
+            var expectedOutput = "1 2 Done";
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.FailsILVerify).VerifyDiagnostics();
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.FailsILVerify).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefStructCurrent_AsyncIterator()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async Task Main()
+                    {
+                        await foreach (var s in M())
+                        {
+                            Console.Write($"M:{s} ");
+                        }
+                        Console.Write("MainDone");
+                    }
+                    public static async IAsyncEnumerable<string> M()
+                    {
+                        await foreach (var s in new C())
+                        {
+                            yield return s.ToString();
+                        }
+                        yield return "Done";
+                    }
+                    public Enumerator GetAsyncEnumerator() => new Enumerator();
+                    public sealed class Enumerator : IAsyncDisposable
+                    {
+                        int i = 0;
+                        public S Current => new S(i);
+                        public async Task<bool> MoveNextAsync()
+                        {
+                            i++;
+                            await Task.Yield();
+                            return i < 3;
+                        }
+                        public async ValueTask DisposeAsync()
+                        {
+                            await Task.Yield();
+                        }
+                    }
+                }
+                public ref struct S
+                {
+                    int i;
+                    public S(int i)
+                    {
+                        this.i = i;
+                    }
+                    public override string ToString() => i.ToString();
+                }
+                """ + AsyncStreamsTypes;
+
+            var expectedDiagnostics = new[]
+            {
+                // (16,24): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(16, 24)
+            };
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(expectedDiagnostics);
+
+            var expectedOutput = "M:1 M:2 M:Done MainDone";
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.FailsILVerify).VerifyDiagnostics();
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.FailsILVerify).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefStructCurrent_Iterator()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                public class C
+                {
+                    public static void Main()
+                    {
+                        foreach (var s in M())
+                        {
+                            Console.Write($"M:{s} ");
+                        }
+                        Console.Write("MainDone");
+                    }
+                    public static IEnumerable<string> M()
+                    {
+                        foreach (var s in new C())
+                        {
+                            yield return s.ToString();
+                        }
+                        yield return "Done";
+                    }
+                    public Enumerator GetEnumerator() => new Enumerator();
+                    public sealed class Enumerator
+                    {
+                        int i = 0;
+                        public S Current => new S(i);
+                        public bool MoveNext()
+                        {
+                            i++;
+                            return i < 3;
+                        }
+                    }
+                }
+                public ref struct S
+                {
+                    int i;
+                    public S(int i)
+                    {
+                        this.i = i;
+                    }
+                    public override string ToString() => i.ToString();
+                }
+                """;
+
+            var expectedOutput = "M:1 M:2 M:Done MainDone";
+
+            CompileAndVerify(source, parseOptions: TestOptions.Regular12, expectedOutput: expectedOutput, verify: Verification.FailsILVerify).VerifyDiagnostics();
+            CompileAndVerify(source, parseOptions: TestOptions.RegularNext, expectedOutput: expectedOutput, verify: Verification.FailsILVerify).VerifyDiagnostics();
+            CompileAndVerify(source, expectedOutput: expectedOutput, verify: Verification.FailsILVerify).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefReturningCurrent_Async()
         {
             string source = @"
 using static System.Console;
@@ -1986,6 +2533,275 @@ public override string ToString()
             CompileAndVerify(comp, expectedOutput: "1 2 3 Done", verify: Verification.Fails);
         }
 
+        [Fact]
+        public void TestWithPattern_RefReturningCurrent_Async_RefVariable()
+        {
+            string source = """
+                using System;
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async Task Main()
+                    {
+                        await foreach (ref var s in new C())
+                        {
+                            Console.Write($"{s} ");
+                            s.F++;
+                        }
+                        Console.Write("Done");
+                    }
+                    public Enumerator GetAsyncEnumerator() => new Enumerator();
+                    public sealed class Enumerator
+                    {
+                        S _current;
+                        public ref S Current => ref _current;
+                        public async Task<bool> MoveNextAsync()
+                        {
+                            Current = new S(Current.F + 1);
+                            await Task.Yield();
+                            return Current.F < 4;
+                        }
+                    }
+                }
+                public struct S
+                {
+                    public int F;
+                    public S(int i)
+                    {
+                        this.F = i;
+                    }
+                    public override string ToString() => F.ToString();
+                }
+                """ + s_IAsyncEnumerable;
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (7,32): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (ref var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "s").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 32));
+
+            var expectedOutput = "1 3 Done";
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefReturningCurrent_AsyncIterator_RefVariable_01()
+        {
+            string source = """
+                using System;
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async Task Main()
+                    {
+                        await foreach (var s in M())
+                        {
+                            Console.Write($"M:{s} ");
+                        }
+                        Console.Write("MainDone");
+                    }
+                    public static async IAsyncEnumerable<string> M()
+                    {
+                        await foreach (ref var s in new C())
+                        {
+                            s.F++;
+                            yield return s.ToString();
+                        }
+                        yield return "Done";
+                    }
+                    public Enumerator GetAsyncEnumerator() => new Enumerator();
+                    public sealed class Enumerator
+                    {
+                        S _current;
+                        public ref S Current => ref _current;
+                        public async Task<bool> MoveNextAsync()
+                        {
+                            Current = new S(Current.F + 1);
+                            await Task.Yield();
+                            return Current.F < 4;
+                        }
+                    }
+                }
+                public struct S
+                {
+                    public int F;
+                    public S(int i)
+                    {
+                        this.F = i;
+                    }
+                    public override string ToString() => F.ToString();
+                }
+                """ + AsyncStreamsTypes;
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (16,32): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (ref var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "s").WithArguments("ref and unsafe in async and iterator methods").WithLocation(16, 32));
+
+            var expectedOutput = "M:2 M:4 M:Done MainDone";
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefReturningCurrent_AsyncIterator_RefVariable_02()
+        {
+            string source = """
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                public class C
+                {
+                    public static async IAsyncEnumerable<string> M()
+                    {
+                        await foreach (ref var s in new C())
+                        {
+                            yield return s.ToString();
+                            s.F++;
+                        }
+                        yield return "Done";
+                    }
+                    public Enumerator GetAsyncEnumerator() => new Enumerator();
+                    public sealed class Enumerator
+                    {
+                        public ref S Current => throw null;
+                        public Task<bool> MoveNextAsync() => throw null;
+                    }
+                }
+                public struct S
+                {
+                    public int F;
+                }
+                """ + AsyncStreamsTypes;
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (7,32): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await foreach (ref var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "s").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 32));
+
+            var expectedDiagnostics = new[]
+            {
+                // (10,13): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             s.F++;
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "s.F").WithLocation(10, 13)
+            };
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilationWithTasksExtensions(source).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void TestWithPattern_RefReturningCurrent_Iterator_RefVariable_01()
+        {
+            string source = """
+                using System;
+                using System.Collections.Generic;
+                public class C
+                {
+                    public static void Main()
+                    {
+                        foreach (var s in M())
+                        {
+                            Console.Write($"M:{s} ");
+                        }
+                        Console.Write("MainDone");
+                    }
+                    public static IEnumerable<string> M()
+                    {
+                        foreach (ref var s in new C())
+                        {
+                            s.F++;
+                            yield return s.ToString();
+                        }
+                        yield return "Done";
+                    }
+                    public Enumerator GetEnumerator() => new Enumerator();
+                    public sealed class Enumerator
+                    {
+                        S _current;
+                        public ref S Current => ref _current;
+                        public bool MoveNext()
+                        {
+                            Current = new S(Current.F + 1);
+                            return Current.F < 4;
+                        }
+                    }
+                }
+                public struct S
+                {
+                    public int F;
+                    public S(int i)
+                    {
+                        this.F = i;
+                    }
+                    public override string ToString() => F.ToString();
+                }
+                """;
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (15,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "s").WithArguments("ref and unsafe in async and iterator methods").WithLocation(15, 26));
+
+            var expectedOutput = "M:2 M:4 M:Done MainDone";
+
+            CompileAndVerify(source, parseOptions: TestOptions.RegularNext, expectedOutput: expectedOutput).VerifyDiagnostics();
+            CompileAndVerify(source, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void TestWithPattern_RefReturningCurrent_Iterator_RefVariable_02()
+        {
+            string source = """
+                using System.Collections.Generic;
+                public class C
+                {
+                    public static IEnumerable<string> M()
+                    {
+                        foreach (ref var s in new C())
+                        {
+                            yield return s.ToString();
+                            s.F++;
+                        }
+                        yield return "Done";
+                    }
+                    public Enumerator GetEnumerator() => new Enumerator();
+                    public sealed class Enumerator
+                    {
+                        public ref S Current => throw null;
+                        public bool MoveNext() => throw null;
+                    }
+                }
+                public struct S
+                {
+                    public int F;
+                }
+                """;
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (6,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref var s in new C())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "s").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 26));
+
+            var expectedDiagnostics = new[]
+            {
+                // (9,13): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             s.F++;
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "s.F").WithLocation(9, 13)
+            };
+
+            CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(source).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
         [Fact]
         public void TestWithPattern_IterationVariableIsReadOnly()
         {
diff --git a/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitUsingTests.cs b/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitUsingTests.cs
index c388178fb6771..7282e99ed475f 100644
--- a/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitUsingTests.cs
+++ b/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitUsingTests.cs
@@ -3084,5 +3084,440 @@ public void Dispose()
             comp.VerifyDiagnostics();
             CompileAndVerify(comp, expectedOutput: "DISPOSED");
         }
+
+        [Fact]
+        public void RefStruct_AwaitInside()
+        {
+            var source = """
+                using System.Threading.Tasks;
+                class C
+                {
+                    async Task M()
+                    {
+                        using (new R())
+                        {
+                            await Task.Yield();
+                        }
+                    }
+                }
+                ref struct R
+                {
+                    public void Dispose() { }
+                }
+                """;
+            // https://github.com/dotnet/roslyn/issues/73280 - should not be a langversion error since this remains an error in C# 13
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (6,16): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         using (new R())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "new R()").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 16));
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,16): error CS4007: Instance of type 'R' cannot be preserved across 'await' or 'yield' boundary.
+                //         using (new R())
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "new R()").WithArguments("R").WithLocation(6, 16)
+            };
+
+            CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(source).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void RefStruct_YieldReturnInside()
+        {
+            var source = """
+                using System.Collections.Generic;
+                class C
+                {
+                    IEnumerable<int> M()
+                    {
+                        using (new R())
+                        {
+                            yield return 1;
+                        }
+                    }
+                }
+                ref struct R
+                {
+                    public void Dispose() { }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,16): error CS4007: Instance of type 'R' cannot be preserved across 'await' or 'yield' boundary.
+                //         using (new R())
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "new R()").WithArguments("R").WithLocation(6, 16)
+            };
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(source).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void RefStruct_YieldBreakInside()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                class C
+                {
+                    static void Main()
+                    {
+                        foreach (var x in M(true)) { Console.Write(x); }
+                        Console.Write(" ");
+                        foreach (var x in M(false)) { Console.Write(x); }
+                    }
+                    static IEnumerable<int> M(bool b)
+                    {
+                        yield return 123;
+                        using (new R())
+                        {
+                            if (b) { yield break; }
+                        }
+                        yield return 456;
+                    }
+                }
+                ref struct R
+                {
+                    public R() => Console.Write("C");
+                    public void Dispose() => Console.Write("D");
+                }
+                """;
+
+            var expectedOutput = "123CD 123CD456";
+
+            CompileAndVerify(source, expectedOutput: expectedOutput, parseOptions: TestOptions.Regular12).VerifyDiagnostics();
+            CompileAndVerify(source, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext).VerifyDiagnostics();
+            CompileAndVerify(source, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefStruct_AwaitResource()
+        {
+            var source = """
+                using System;
+                using System.Threading.Tasks;
+                class C
+                {
+                    static async Task Main()
+                    {
+                        Console.Write("1");
+                        using ((await GetC()).GetR())
+                        {
+                            Console.Write("2");
+                        }
+                        Console.Write("3");
+                    }
+                    static async Task<C> GetC()
+                    {
+                        Console.Write("Ga");
+                        await Task.Yield();
+                        Console.Write("Gb");
+                        return new C();
+                    }
+                    R GetR() => new R();
+                }
+                ref struct R
+                {
+                    public R() => Console.Write("C");
+                    public void Dispose() => Console.Write("D");
+                }
+                """;
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (8,16): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         using ((await GetC()).GetR())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "(await GetC()).GetR()").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 16));
+
+            var expectedOutput = "1GaGbC2D3";
+
+            CompileAndVerify(source, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext, verify: Verification.FailsILVerify).VerifyDiagnostics();
+            CompileAndVerify(source, expectedOutput: expectedOutput, verify: Verification.FailsILVerify).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefStruct_AwaitOutside()
+        {
+            var source = """
+                using System;
+                using System.Threading.Tasks;
+                class C
+                {
+                    static async Task Main()
+                    {
+                        Console.Write("1");
+                        await Task.Yield();
+                        Console.Write("2");
+                        using (new R())
+                        {
+                            Console.Write("3");
+                        }
+                        Console.Write("4");
+                        await Task.Yield();
+                        Console.Write("5");
+                    }
+                }
+                ref struct R
+                {
+                    public R() => Console.Write("C");
+                    public void Dispose() => Console.Write("D");
+                }
+                """;
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (10,16): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         using (new R())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "new R()").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 16));
+
+            var expectedOutput = "12C3D45";
+
+            CompileAndVerify(source, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext).VerifyDiagnostics();
+            CompileAndVerify(source, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefStruct_YieldReturnOutside()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                class C
+                {
+                    static void Main()
+                    {
+                        foreach (var x in M())
+                        {
+                            Console.Write(x);
+                        }
+                    }
+                    static IEnumerable<string> M()
+                    {
+                        Console.Write("1");
+                        yield return "a";
+                        Console.Write("2");
+                        using (new R())
+                        {
+                            Console.Write("3");
+                        }
+                        Console.Write("4");
+                        yield return "b";
+                        Console.Write("5");
+                    }
+                }
+                ref struct R
+                {
+                    public R() => Console.Write("C");
+                    public void Dispose() => Console.Write("D");
+                }
+                """;
+
+            var expectedOutput = "1a2C3D4b5";
+
+            CompileAndVerify(source, expectedOutput: expectedOutput, parseOptions: TestOptions.Regular12).VerifyDiagnostics();
+            CompileAndVerify(source, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext).VerifyDiagnostics();
+            CompileAndVerify(source, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefStruct_AwaitUsing()
+        {
+            var source = """
+                using System;
+                using System.Threading.Tasks;
+                class C
+                {
+                    static async Task Main()
+                    {
+                        Console.Write("1");
+                        await using (new R())
+                        {
+                            Console.Write("2");
+                        }
+                        Console.Write("3");
+                    }
+                }
+                ref struct R
+                {
+                    public R() => Console.Write("C");
+                    public ValueTask DisposeAsync()
+                    {
+                        Console.Write("D");
+                        return default;
+                    }
+                }
+                """;
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (8,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await using (new R())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "new R()").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 22));
+
+            var expectedOutput = "1C2D3";
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefStruct_AwaitUsing_AwaitInside()
+        {
+            var source = """
+                using System.Threading.Tasks;
+                class C
+                {
+                    async Task M()
+                    {
+                        await using (new R())
+                        {
+                            await Task.Yield();
+                        }
+                    }
+                }
+                ref struct R
+                {
+                    public ValueTask DisposeAsync() => default;
+                }
+                """;
+            // https://github.com/dotnet/roslyn/issues/73280 - should not be a langversion error since this remains an error in C# 13
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (6,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await using (new R())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "new R()").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 22));
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,22): error CS4007: Instance of type 'R' cannot be preserved across 'await' or 'yield' boundary.
+                //         await using (new R())
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "new R()").WithArguments("R").WithLocation(6, 22)
+            };
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilationWithTasksExtensions(source).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void RefStruct_AwaitUsing_YieldReturnInside()
+        {
+            var source = """
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                class C
+                {
+                    async IAsyncEnumerable<int> M()
+                    {
+                        await using (new R())
+                        {
+                            yield return 123;
+                        }
+                    }
+                }
+                ref struct R
+                {
+                    public ValueTask DisposeAsync() => default;
+                }
+                """ + AsyncStreamsTypes;
+            // https://github.com/dotnet/roslyn/issues/73280 - should not be a langversion error since this remains an error in C# 13
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (7,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await using (new R())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "new R()").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 22));
+
+            var expectedDiagnostics = new[]
+            {
+                // (7,22): error CS4007: Instance of type 'R' cannot be preserved across 'await' or 'yield' boundary.
+                //         await using (new R())
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "new R()").WithArguments("R").WithLocation(7, 22)
+            };
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilationWithTasksExtensions(source).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void RefStruct_AwaitUsing_YieldReturnInside_Var()
+        {
+            var source = """
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                class C
+                {
+                    async IAsyncEnumerable<int> M()
+                    {
+                        await using var _ = new R();
+                        yield return 123;
+                    }
+                }
+                ref struct R
+                {
+                    public ValueTask DisposeAsync() => default;
+                }
+                """ + AsyncStreamsTypes;
+            // https://github.com/dotnet/roslyn/issues/73280 - should not be a langversion error since this remains an error in C# 13
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (7,21): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await using var _ = new R();
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 21));
+
+            var expectedDiagnostics = new[]
+            {
+                // (7,25): error CS4007: Instance of type 'R' cannot be preserved across 'await' or 'yield' boundary.
+                //         await using var _ = new R();
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "_ = new R()").WithArguments("R").WithLocation(7, 25)
+            };
+
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilationWithTasksExtensions(source).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void RefStruct_AwaitUsing_YieldBreakInside()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                class C
+                {
+                    static async Task Main()
+                    {
+                        await foreach (var x in M(true)) { Console.Write(x); }
+                        Console.Write(" ");
+                        await foreach (var x in M(false)) { Console.Write(x); }
+                    }
+                    static async IAsyncEnumerable<int> M(bool b)
+                    {
+                        yield return 1;
+                        await using (new R())
+                        {
+                            if (b) { yield break; }
+                        }
+                        yield return 2;
+                    }
+                }
+                ref struct R
+                {
+                    public R() => Console.Write("C");
+                    public ValueTask DisposeAsync()
+                    {
+                        Console.Write("D");
+                        return default;
+                    }
+                }
+                """ + AsyncStreamsTypes;
+            CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (15,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         await using (new R())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "new R()").WithArguments("ref and unsafe in async and iterator methods").WithLocation(15, 22));
+
+            var expectedOutput = "1CD 1CD2";
+
+            var comp = CreateCompilationWithTasksExtensions(source, parseOptions: TestOptions.RegularNext, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
     }
 }
diff --git a/src/Compilers/CSharp/Test/Emit2/Emit/EditAndContinue/EditAndContinueStateMachineTests.cs b/src/Compilers/CSharp/Test/Emit2/Emit/EditAndContinue/EditAndContinueStateMachineTests.cs
index 028ff1e8ff7d4..d2f3bf455c812 100644
--- a/src/Compilers/CSharp/Test/Emit2/Emit/EditAndContinue/EditAndContinueStateMachineTests.cs
+++ b/src/Compilers/CSharp/Test/Emit2/Emit/EditAndContinue/EditAndContinueStateMachineTests.cs
@@ -5463,7 +5463,10 @@ static IEnumerable<int> F()
             var compilation1 = compilation0.WithSource(source1.Tree);
 
             var v0 = CompileAndVerify(compilation0);
-            v0.VerifyDiagnostics();
+            v0.VerifyDiagnostics(
+                // (17,34): warning CS9237: 'yield return' should not be used in the body of a lock statement
+                //                                  yield return 1;      
+                Diagnostic(ErrorCode.WRN_BadYieldInLock, "yield").WithLocation(17, 34));
             var md0 = ModuleMetadata.CreateFromImage(v0.EmittedAssemblyData);
 
             var f0 = compilation0.GetMember<MethodSymbol>("C.F");
diff --git a/src/Compilers/CSharp/Test/Emit2/Emit/NumericIntPtrTests.cs b/src/Compilers/CSharp/Test/Emit2/Emit/NumericIntPtrTests.cs
index 4760ca1c2c254..458de9f14813a 100644
--- a/src/Compilers/CSharp/Test/Emit2/Emit/NumericIntPtrTests.cs
+++ b/src/Compilers/CSharp/Test/Emit2/Emit/NumericIntPtrTests.cs
@@ -1359,20 +1359,44 @@ static IEnumerable<int> F()
         yield return sizeof(System.UIntPtr);
     }
 }";
-            var comp = CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular9, targetFramework: TargetFramework.Net70);
-            comp.VerifyDiagnostics(
-                // (6,22): error CS1629: Unsafe code may not appear in iterators
+            // https://github.com/dotnet/roslyn/issues/73280 - should not be a langversion error since this remains an error in C# 13
+            var expectedDiagnostics = new[]
+            {
+                // (6,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         yield return sizeof(nint);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "sizeof(nint)").WithLocation(6, 22),
-                // (7,22): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "sizeof(nint)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 22),
+                // (7,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         yield return sizeof(nuint);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "sizeof(nuint)").WithLocation(7, 22),
-                // (8,22): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "sizeof(nuint)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 22),
+                // (8,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         yield return sizeof(System.IntPtr);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "sizeof(System.IntPtr)").WithLocation(8, 22),
-                // (9,22): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "sizeof(System.IntPtr)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 22),
+                // (9,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         yield return sizeof(System.UIntPtr);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "sizeof(System.UIntPtr)").WithLocation(9, 22));
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "sizeof(System.UIntPtr)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(9, 22)
+            };
+
+            CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular9, targetFramework: TargetFramework.Net70).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net70).VerifyDiagnostics(expectedDiagnostics);
+
+            expectedDiagnostics = new[]
+            {
+                // (6,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(6, 22),
+                // (7,22): error CS0233: 'nuint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nuint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nuint)").WithArguments("nuint").WithLocation(7, 22),
+                // (8,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(System.IntPtr);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(System.IntPtr)").WithArguments("nint").WithLocation(8, 22),
+                // (9,22): error CS0233: 'nuint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(System.UIntPtr);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(System.UIntPtr)").WithArguments("nuint").WithLocation(9, 22)
+            };
+
+            CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net70).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, targetFramework: TargetFramework.Net70).VerifyDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Emit2/Semantics/InlineArrayTests.cs b/src/Compilers/CSharp/Test/Emit2/Semantics/InlineArrayTests.cs
index 9b88f1fb7d741..0dd97b08e44a8 100644
--- a/src/Compilers/CSharp/Test/Emit2/Semantics/InlineArrayTests.cs
+++ b/src/Compilers/CSharp/Test/Emit2/Semantics/InlineArrayTests.cs
@@ -4691,9 +4691,12 @@ static async Task<T> FromResult<T>(T r)
 ";
             var comp = CreateCompilation(src + Buffer10Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
             comp.VerifyEmitDiagnostics(
-                // (24,22): error CS4007: 'await' cannot be used in an expression containing the type 'System.ReadOnlySpan<Buffer10<int>>'
-                //            [Get01()][await FromResult(Get02(x))];
-                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "await FromResult(Get02(x))").WithArguments("System.ReadOnlySpan<Buffer10<int>>").WithLocation(24, 22)
+                // (20,12): error CS4007: Instance of type 'System.ReadOnlySpan<Buffer10<int>>' cannot be preserved across 'await' or 'yield' boundary.
+                //         => MemoryMarshal.CreateReadOnlySpan(
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, @"MemoryMarshal.CreateReadOnlySpan(
+                ref Unsafe.As<Buffer10<Buffer10<int>>, Buffer10<int>>(
+                        ref Unsafe.AsRef(in GetC(x).F)),
+                10)").WithArguments("System.ReadOnlySpan<Buffer10<int>>").WithLocation(20, 12)
                 );
         }
 
@@ -4743,9 +4746,12 @@ static async Task<T> FromResult<T>(T r)
 ";
             var comp = CreateCompilation(src + Buffer10Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
             comp.VerifyEmitDiagnostics(
-                    // (24,13): error CS4007: 'await' cannot be used in an expression containing the type 'System.ReadOnlySpan<int>'
-                    //            [await FromResult(Get02(x))];
-                    Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "await FromResult(Get02(x))").WithArguments("System.ReadOnlySpan<int>").WithLocation(24, 13)
+                // (20,12): error CS4007: Instance of type 'System.ReadOnlySpan<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         => MemoryMarshal.CreateReadOnlySpan(
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, @"MemoryMarshal.CreateReadOnlySpan(
+                ref Unsafe.As<Buffer10<int>, int>(
+                        ref Unsafe.AsRef(in GetC(x).F[Get01()])),
+                10)").WithArguments("System.ReadOnlySpan<int>").WithLocation(20, 12)
                 );
         }
 
@@ -20136,30 +20142,45 @@ .locals init (int V_0,
             CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
         }
 
-        [Fact]
+        [ConditionalFact(typeof(CoreClrOnly))]
         public void Foreach_InAsync_03()
         {
             var src = @"
 class Program
 {
-    static async void Test()
+    static Buffer4<int> s_buffer;
+
+    static async System.Threading.Tasks.Task Main()
     {
+        s_buffer[1] = 3;
+
         foreach (ref int y in GetBuffer())
         {
+            y *= y;
+            System.Console.Write(y);
         }
 
         await System.Threading.Tasks.Task.Yield();
+
+        System.Console.Write(s_buffer[1]);
     }
 
-    static ref Buffer4<int> GetBuffer() => throw null;
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseDll);
-            comp.VerifyDiagnostics(
-                // (6,26): error CS8177: Async methods cannot have by-reference locals
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (10,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         foreach (ref int y in GetBuffer())
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "y").WithLocation(6, 26)
-                );
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 26));
+
+            var expectedOutput = "09009";
+
+            CompileAndVerify(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            CompileAndVerify(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                expectedOutput: expectedOutput).VerifyDiagnostics();
         }
 
         [Fact]
@@ -20580,30 +20601,48 @@ .locals init (int V_0,
             CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
         }
 
-        [Fact]
+        [ConditionalFact(typeof(CoreClrOnly))]
         public void Foreach_InAsync_07()
         {
             var src = @"
 class Program
 {
-    static async void Test()
+    static Buffer4<int> s_buffer;
+
+    static async System.Threading.Tasks.Task Main()
     {
+        s_buffer[1] = 3;
+
+        int i = 0;
         foreach (ref readonly int y in GetBuffer())
         {
+            System.Console.Write(y);
+            s_buffer[i++]++;
+            System.Console.Write(y);
+            System.Console.Write(' ');
         }
 
         await System.Threading.Tasks.Task.Yield();
+
+        System.Console.Write(s_buffer[1]);
     }
 
-    static ref readonly Buffer4<int> GetBuffer() => throw null;
+    static ref readonly Buffer4<int> GetBuffer() => ref s_buffer;
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseDll);
-            comp.VerifyDiagnostics(
-                // (6,35): error CS8177: Async methods cannot have by-reference locals
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (11,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         foreach (ref readonly int y in GetBuffer())
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "y").WithLocation(6, 35)
-                );
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(11, 35));
+
+            var expectedOutput = "01 34 01 01 4";
+
+            CompileAndVerify(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                verify: Verification.FailsILVerify, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            CompileAndVerify(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                verify: Verification.FailsILVerify, expectedOutput: expectedOutput).VerifyDiagnostics();
         }
 
         [Fact]
@@ -20780,132 +20819,209 @@ .locals init (int V_0,
         }
 
         [ConditionalFact(typeof(CoreClrOnly))]
-        public void Foreach_InIterator_01()
+        public void Foreach_InAsync_10()
         {
             var src = @"
 class Program
 {
-    static private Buffer4<int> F = default;
-    private static int index = 0;
+    static Buffer4<int> s_buffer;
 
-    static void Main()
+    static async System.Threading.Tasks.Task Main()
     {
-        foreach (var a in Test())
-        {}
+        s_buffer[1] = 3;
+
+        ref Buffer4<int> buffer = ref GetBuffer();
+        foreach (ref int y in buffer)
+        {
+            y *= y;
+            System.Console.Write(y);
+        }
+
+        await System.Threading.Tasks.Task.Yield();
+
+        System.Console.Write(s_buffer[1]);
     }
 
-    static System.Collections.Generic.IEnumerable<int> Test()
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (10,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref Buffer4<int> buffer = ref GetBuffer();
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "buffer").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 26),
+                // (11,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in buffer)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(11, 26));
+
+            var expectedOutput = "09009";
+
+            CompileAndVerify(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            CompileAndVerify(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void Foreach_InAsync_11()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static async System.Threading.Tasks.Task Main()
     {
-        yield return -1;
+        s_buffer[1] = 3;
 
-        foreach (var y in GetBuffer())
+        foreach (ref int y in GetBuffer())
         {
-            Increment();
-            System.Console.Write(' ');
+            await System.Threading.Tasks.Task.Yield();
+            y *= y;
             System.Console.Write(y);
         }
 
-        yield return -2;
-    }
+        await System.Threading.Tasks.Task.Yield();
 
-    static ref Buffer4<int> GetBuffer()
-    {
-        System.Console.Write(-1);
-        return ref F;
+        System.Console.Write(s_buffer[1]);
     }
 
-    static void Increment()
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (10,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 26));
+
+            var expectedDiagnostics = new[]
+            {
+                // (13,13): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             y *= y;
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(13, 13),
+                // (13,18): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             y *= y;
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(13, 18),
+                // (14,34): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             System.Console.Write(y);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(14, 34)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InAsync_12()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static async System.Threading.Tasks.Task Main()
     {
-        index++;
+        s_buffer[1] = 3;
 
-        if (index < 4)
+        foreach (ref int y in GetBuffer())
         {
-            F[index] = index;
+            y *= y;
+            System.Console.Write(y);
+            await System.Threading.Tasks.Task.Yield();
         }
+
+        await System.Threading.Tasks.Task.Yield();
+
+        System.Console.Write(s_buffer[1]);
     }
+
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
-            var verifier = CompileAndVerify(comp, expectedOutput: "-1 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+" + Buffer4Definition;
 
-            verifier.VerifyIL("Program.<Test>d__3.System.Collections.IEnumerator.MoveNext",
-@"
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (10,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 26));
+
+            var expectedDiagnostics = new[]
+            {
+                // (10,9): error CS8178: A reference returned by a call to 'Program.GetBuffer()' cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (ref int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait, @"foreach (ref int y in GetBuffer())
+        {
+            y *= y;
+            System.Console.Write(y);
+            await System.Threading.Tasks.Task.Yield();
+        }").WithArguments("Program.GetBuffer()").WithLocation(10, 9)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InAsync_13()
+        {
+            var src = @"
+class Program
 {
-  // Code size      126 (0x7e)
-  .maxstack  2
-  .locals init (int V_0,
-                Buffer4<int>& V_1,
-                int V_2)
-  IL_0000:  ldarg.0
-  IL_0001:  ldfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0006:  stloc.0
-  IL_0007:  ldloc.0
-  IL_0008:  switch    (
-        IL_001b,
-        IL_0032,
-        IL_0075)
-  IL_0019:  ldc.i4.0
-  IL_001a:  ret
-  IL_001b:  ldarg.0
-  IL_001c:  ldc.i4.m1
-  IL_001d:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0022:  ldarg.0
-  IL_0023:  ldc.i4.m1
-  IL_0024:  stfld      ""int Program.<Test>d__3.<>2__current""
-  IL_0029:  ldarg.0
-  IL_002a:  ldc.i4.1
-  IL_002b:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0030:  ldc.i4.1
-  IL_0031:  ret
-  IL_0032:  ldarg.0
-  IL_0033:  ldc.i4.m1
-  IL_0034:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0039:  call       ""ref Buffer4<int> Program.GetBuffer()""
-  IL_003e:  stloc.1
-  IL_003f:  ldc.i4.0
-  IL_0040:  stloc.2
-  IL_0041:  br.s       IL_0060
-  IL_0043:  ldloc.1
-  IL_0044:  ldloc.2
-  IL_0045:  call       ""ref int <PrivateImplementationDetails>.InlineArrayElementRef<Buffer4<int>, int>(ref Buffer4<int>, int)""
-  IL_004a:  ldind.i4
-  IL_004b:  call       ""void Program.Increment()""
-  IL_0050:  ldc.i4.s   32
-  IL_0052:  call       ""void System.Console.Write(char)""
-  IL_0057:  call       ""void System.Console.Write(int)""
-  IL_005c:  ldloc.2
-  IL_005d:  ldc.i4.1
-  IL_005e:  add
-  IL_005f:  stloc.2
-  IL_0060:  ldloc.2
-  IL_0061:  ldc.i4.4
-  IL_0062:  blt.s      IL_0043
-  IL_0064:  ldarg.0
-  IL_0065:  ldc.i4.s   -2
-  IL_0067:  stfld      ""int Program.<Test>d__3.<>2__current""
-  IL_006c:  ldarg.0
-  IL_006d:  ldc.i4.2
-  IL_006e:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0073:  ldc.i4.1
-  IL_0074:  ret
-  IL_0075:  ldarg.0
-  IL_0076:  ldc.i4.m1
-  IL_0077:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_007c:  ldc.i4.0
-  IL_007d:  ret
+    static Buffer4<int> s_buffer;
+
+    static async System.Threading.Tasks.Task Main()
+    {
+        s_buffer[1] = 3;
+
+        ref Buffer4<int> buffer = ref GetBuffer();
+        foreach (ref int y in buffer)
+        {
+            y *= y;
+            System.Console.Write(y);
+            await System.Threading.Tasks.Task.Yield();
+        }
+
+        await System.Threading.Tasks.Task.Yield();
+
+        System.Console.Write(s_buffer[1]);
+    }
+
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
 }
-");
-            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
-            CompileAndVerify(comp, expectedOutput: "-1 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (10,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref Buffer4<int> buffer = ref GetBuffer();
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "buffer").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 26),
+                // (11,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in buffer)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(11, 26));
+
+            var expectedDiagnostics = new[]
+            {
+                // (11,31): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (ref int y in buffer)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "buffer").WithLocation(11, 31)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
-        [ConditionalFact(typeof(CoreClrOnly))]
-        public void Foreach_InIterator_02()
+        [Fact]
+        public void Foreach_InAsync_14()
         {
             var src = @"
+using System.Threading.Tasks;
+
 class C
 {
-    public Buffer4<int> F = default;
+    public readonly Buffer4<int> F = default;
 }
 
 class Program
@@ -20915,19 +21031,20 @@ class Program
 
     static void Main()
     {
-        foreach (var a in Test(c))
-        {}
+        Test(c).Wait();
     }
 
-    static System.Collections.Generic.IEnumerable<int> Test(C x)
+    static async Task Test(C x)
     {
-        foreach (var y in x.F)
+        ref readonly Buffer4<int> f = ref x.F;
+        foreach (var y in f)
         {
             Increment();    
             System.Console.Write(' ');
             System.Console.Write(y);
 
-            yield return -1;
+            await Task.Yield();
+            await Task.Delay(2);
         }
     }
 
@@ -20937,146 +21054,266 @@ static void Increment()
 
         if (index < 4)
         {
-            c.F[index] = index;
+            System.Runtime.CompilerServices.Unsafe.AsRef(in c.F)[index] = index;
         }
     }
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
-            var verifier = CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+" + Buffer4Definition;
 
-            verifier.VerifyIL("Program.<Test>d__3.System.Collections.IEnumerator.MoveNext",
-@"
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (21,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref readonly Buffer4<int> f = ref x.F;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "f").WithArguments("ref and unsafe in async and iterator methods").WithLocation(21, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (22,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var y in f)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "f").WithLocation(22, 27)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InAsync_15()
+        {
+            var src = @"
+class Program
 {
-  // Code size      151 (0x97)
-  .maxstack  3
-  .locals init (int V_0)
-  IL_0000:  ldarg.0
-  IL_0001:  ldfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0006:  stloc.0
-  IL_0007:  ldloc.0
-  IL_0008:  brfalse.s  IL_0010
-  IL_000a:  ldloc.0
-  IL_000b:  ldc.i4.1
-  IL_000c:  beq.s      IL_0070
-  IL_000e:  ldc.i4.0
-  IL_000f:  ret
-  IL_0010:  ldarg.0
-  IL_0011:  ldc.i4.m1
-  IL_0012:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0017:  ldarg.0
-  IL_0018:  ldarg.0
-  IL_0019:  ldfld      ""C Program.<Test>d__3.x""
-  IL_001e:  stfld      ""C Program.<Test>d__3.<>7__wrap2""
-  IL_0023:  ldarg.0
-  IL_0024:  ldfld      ""C Program.<Test>d__3.<>7__wrap2""
-  IL_0029:  ldfld      ""Buffer4<int> C.F""
-  IL_002e:  pop
-  IL_002f:  ldarg.0
-  IL_0030:  ldc.i4.0
-  IL_0031:  stfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_0036:  br.s       IL_0085
-  IL_0038:  ldarg.0
-  IL_0039:  ldfld      ""C Program.<Test>d__3.<>7__wrap2""
-  IL_003e:  ldflda     ""Buffer4<int> C.F""
-  IL_0043:  ldarg.0
-  IL_0044:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_0049:  call       ""ref int <PrivateImplementationDetails>.InlineArrayElementRef<Buffer4<int>, int>(ref Buffer4<int>, int)""
-  IL_004e:  ldind.i4
-  IL_004f:  call       ""void Program.Increment()""
-  IL_0054:  ldc.i4.s   32
-  IL_0056:  call       ""void System.Console.Write(char)""
-  IL_005b:  call       ""void System.Console.Write(int)""
-  IL_0060:  ldarg.0
-  IL_0061:  ldc.i4.m1
-  IL_0062:  stfld      ""int Program.<Test>d__3.<>2__current""
-  IL_0067:  ldarg.0
-  IL_0068:  ldc.i4.1
-  IL_0069:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_006e:  ldc.i4.1
-  IL_006f:  ret
-  IL_0070:  ldarg.0
-  IL_0071:  ldc.i4.m1
-  IL_0072:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0077:  ldarg.0
-  IL_0078:  ldarg.0
-  IL_0079:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_007e:  ldc.i4.1
-  IL_007f:  add
-  IL_0080:  stfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_0085:  ldarg.0
-  IL_0086:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_008b:  ldc.i4.4
-  IL_008c:  blt.s      IL_0038
-  IL_008e:  ldarg.0
-  IL_008f:  ldnull
-  IL_0090:  stfld      ""C Program.<Test>d__3.<>7__wrap2""
-  IL_0095:  ldc.i4.0
-  IL_0096:  ret
+    static Buffer4<int> s_buffer;
+
+    static async System.Threading.Tasks.Task Main()
+    {
+        foreach (ref readonly int y in GetBuffer())
+        {
+            System.Console.Write(y);
+            await System.Threading.Tasks.Task.Yield();
+        }
+
+        await System.Threading.Tasks.Task.Yield();
+    }
+
+    static ref readonly Buffer4<int> GetBuffer() => ref s_buffer;
 }
-");
-            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
-            CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (8,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref readonly int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (8,9): error CS8178: A reference returned by a call to 'Program.GetBuffer()' cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (ref readonly int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait, @"foreach (ref readonly int y in GetBuffer())
+        {
+            System.Console.Write(y);
+            await System.Threading.Tasks.Task.Yield();
+        }").WithArguments("Program.GetBuffer()").WithLocation(8, 9)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
-        public void Foreach_InIterator_03()
+        public void Foreach_InAsync_16()
         {
             var src = @"
 class Program
 {
-    static System.Collections.Generic.IEnumerable<int> Test()
+    static Buffer4<int> s_buffer;
+
+    static async System.Threading.Tasks.Task Main()
     {
-        foreach (ref int y in GetBuffer())
+        foreach (ref readonly int y in GetBuffer())
         {
+            await System.Threading.Tasks.Task.Yield();
+            System.Console.Write(y);
         }
 
-        yield return -1;
+        await System.Threading.Tasks.Task.Yield();
     }
 
-    static ref Buffer4<int> GetBuffer() => throw null;
+    static ref readonly Buffer4<int> GetBuffer() => ref s_buffer;
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseDll);
-            comp.VerifyDiagnostics(
-                // (6,26): error CS8176: Iterators cannot have by-reference locals
-                //         foreach (ref int y in GetBuffer())
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "y").WithLocation(6, 26)
-                );
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (8,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref readonly int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (11,34): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             System.Console.Write(y);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(11, 34)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [ConditionalFact(typeof(CoreClrOnly))]
+        public void Foreach_InAsync_17()
+        {
+            var src = @"
+using System.Threading.Tasks;
+
+class C
+{
+    public readonly Buffer4<int> F = default;
+}
+
+class Program
+{
+    private static C c = new C();
+    private static int index = 0;
+
+    static void Main()
+    {
+        Test(c).Wait();
+    }
+
+    static async Task Test(C x)
+    {
+        foreach (ref readonly int y in x.F)
+        {
+            Increment();    
+            System.Console.Write(' ');
+            System.Console.Write(y);
+
+            await Task.Yield();
+            await Task.Delay(2);
+        }
+    }
+
+    static void Increment()
+    {
+        index++;
+
+        if (index < 4)
+        {
+            System.Runtime.CompilerServices.Unsafe.AsRef(in c.F)[index] = index;
+        }
+    }
+}
+" + Buffer4Definition;
+            var expectedOutput = " 0 1 2 3";
+            var comp = CreateCompilation(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
+            var verifier = CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.Fails).VerifyDiagnostics();
+            comp = CreateCompilation(src, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.Fails).VerifyDiagnostics();
         }
 
         [Fact]
-        public void Foreach_InIterator_04()
+        public void Foreach_InAsync_18()
         {
             var src = @"
+using System.Threading.Tasks;
+
+class C
+{
+    public readonly Buffer4<int> F = default;
+}
+
 class Program
 {
-    static System.Collections.Generic.IEnumerable<int> Test()
+    static async Task Test(C x)
     {
-        foreach (int y in GetBuffer())
+        foreach (ref readonly int y in x.F)
         {
-            yield return -1;
+            await Task.Yield();
+            System.Console.Write(y);
         }
     }
+}
+" + Buffer4Definition;
 
-    static ref Buffer4<int> GetBuffer() => throw null;
+            CreateCompilation(src, targetFramework: TargetFramework.Net80, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (13,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref readonly int y in x.F)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(13, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (16,34): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             System.Console.Write(y);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(16, 34)
+            };
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InAsync_19()
+        {
+            var src = @"
+using System.Threading.Tasks;
+
+class C
+{
+    public readonly Buffer4<int> F = default;
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseDll);
 
-            comp.VerifyEmitDiagnostics(
-                // (6,9): error CS8178: A reference returned by a call to 'Program.GetBuffer()' cannot be preserved across 'await' or 'yield' boundary.
-                //         foreach (int y in GetBuffer())
-                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait,
-        @"foreach (int y in GetBuffer())
+class Program
+{
+    static async Task Test(C x)
+    {
+        ref readonly Buffer4<int> f = ref x.F;
+
+        foreach (var i in f) System.Console.Write(i);
+
+        foreach (var y in f)
         {
-            yield return -1;
-        }").WithArguments("Program.GetBuffer()").WithLocation(6, 9)
-                );
+            System.Console.Write(y);
+            await Task.Yield();
+        }
+
+        foreach (var j in f) System.Console.Write(j);
+
+        foreach (var z in f)
+        {
+            System.Console.Write(z);
+            await Task.Yield();
+        }
+    }
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (13,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref readonly Buffer4<int> f = ref x.F;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "f").WithArguments("ref and unsafe in async and iterator methods").WithLocation(13, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (17,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var y in f)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "f").WithLocation(17, 27),
+                // (23,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var j in f) System.Console.Write(j);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "f").WithLocation(23, 27),
+                // (25,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var z in f)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "f").WithLocation(25, 27)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [ConditionalFact(typeof(CoreClrOnly))]
-        public void Foreach_InIterator_05()
+        public void Foreach_InIterator_01()
         {
             var src = @"
 class Program
@@ -21104,7 +21341,7 @@ static System.Collections.Generic.IEnumerable<int> Test()
         yield return -2;
     }
 
-    static ref readonly Buffer4<int> GetBuffer()
+    static ref Buffer4<int> GetBuffer()
     {
         System.Console.Write(-1);
         return ref F;
@@ -21156,14 +21393,14 @@ .locals init (int V_0,
   IL_0032:  ldarg.0
   IL_0033:  ldc.i4.m1
   IL_0034:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0039:  call       ""ref readonly Buffer4<int> Program.GetBuffer()""
+  IL_0039:  call       ""ref Buffer4<int> Program.GetBuffer()""
   IL_003e:  stloc.1
   IL_003f:  ldc.i4.0
   IL_0040:  stloc.2
   IL_0041:  br.s       IL_0060
   IL_0043:  ldloc.1
   IL_0044:  ldloc.2
-  IL_0045:  call       ""ref readonly int <PrivateImplementationDetails>.InlineArrayElementRefReadOnly<Buffer4<int>, int>(in Buffer4<int>, int)""
+  IL_0045:  call       ""ref int <PrivateImplementationDetails>.InlineArrayElementRef<Buffer4<int>, int>(ref Buffer4<int>, int)""
   IL_004a:  ldind.i4
   IL_004b:  call       ""void Program.Increment()""
   IL_0050:  ldc.i4.s   32
@@ -21190,13 +21427,1009 @@ .locals init (int V_0,
   IL_007c:  ldc.i4.0
   IL_007d:  ret
 }
-");
-            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
-            CompileAndVerify(comp, expectedOutput: "-1 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+");
+            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
+            CompileAndVerify(comp, expectedOutput: "-1 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+        }
+
+        [ConditionalFact(typeof(CoreClrOnly))]
+        public void Foreach_InIterator_02()
+        {
+            var src = @"
+class C
+{
+    public Buffer4<int> F = default;
+}
+
+class Program
+{
+    private static C c = new C();
+    private static int index = 0;
+
+    static void Main()
+    {
+        foreach (var a in Test(c))
+        {}
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test(C x)
+    {
+        foreach (var y in x.F)
+        {
+            Increment();    
+            System.Console.Write(' ');
+            System.Console.Write(y);
+
+            yield return -1;
+        }
+    }
+
+    static void Increment()
+    {
+        index++;
+
+        if (index < 4)
+        {
+            c.F[index] = index;
+        }
+    }
+}
+";
+            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
+            var verifier = CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+
+            verifier.VerifyIL("Program.<Test>d__3.System.Collections.IEnumerator.MoveNext",
+@"
+{
+  // Code size      151 (0x97)
+  .maxstack  3
+  .locals init (int V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  ldfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0006:  stloc.0
+  IL_0007:  ldloc.0
+  IL_0008:  brfalse.s  IL_0010
+  IL_000a:  ldloc.0
+  IL_000b:  ldc.i4.1
+  IL_000c:  beq.s      IL_0070
+  IL_000e:  ldc.i4.0
+  IL_000f:  ret
+  IL_0010:  ldarg.0
+  IL_0011:  ldc.i4.m1
+  IL_0012:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0017:  ldarg.0
+  IL_0018:  ldarg.0
+  IL_0019:  ldfld      ""C Program.<Test>d__3.x""
+  IL_001e:  stfld      ""C Program.<Test>d__3.<>7__wrap2""
+  IL_0023:  ldarg.0
+  IL_0024:  ldfld      ""C Program.<Test>d__3.<>7__wrap2""
+  IL_0029:  ldfld      ""Buffer4<int> C.F""
+  IL_002e:  pop
+  IL_002f:  ldarg.0
+  IL_0030:  ldc.i4.0
+  IL_0031:  stfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_0036:  br.s       IL_0085
+  IL_0038:  ldarg.0
+  IL_0039:  ldfld      ""C Program.<Test>d__3.<>7__wrap2""
+  IL_003e:  ldflda     ""Buffer4<int> C.F""
+  IL_0043:  ldarg.0
+  IL_0044:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_0049:  call       ""ref int <PrivateImplementationDetails>.InlineArrayElementRef<Buffer4<int>, int>(ref Buffer4<int>, int)""
+  IL_004e:  ldind.i4
+  IL_004f:  call       ""void Program.Increment()""
+  IL_0054:  ldc.i4.s   32
+  IL_0056:  call       ""void System.Console.Write(char)""
+  IL_005b:  call       ""void System.Console.Write(int)""
+  IL_0060:  ldarg.0
+  IL_0061:  ldc.i4.m1
+  IL_0062:  stfld      ""int Program.<Test>d__3.<>2__current""
+  IL_0067:  ldarg.0
+  IL_0068:  ldc.i4.1
+  IL_0069:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_006e:  ldc.i4.1
+  IL_006f:  ret
+  IL_0070:  ldarg.0
+  IL_0071:  ldc.i4.m1
+  IL_0072:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0077:  ldarg.0
+  IL_0078:  ldarg.0
+  IL_0079:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_007e:  ldc.i4.1
+  IL_007f:  add
+  IL_0080:  stfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_0085:  ldarg.0
+  IL_0086:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_008b:  ldc.i4.4
+  IL_008c:  blt.s      IL_0038
+  IL_008e:  ldarg.0
+  IL_008f:  ldnull
+  IL_0090:  stfld      ""C Program.<Test>d__3.<>7__wrap2""
+  IL_0095:  ldc.i4.0
+  IL_0096:  ret
+}
+");
+            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
+            CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+        }
+
+        [ConditionalFact(typeof(CoreClrOnly))]
+        public void Foreach_InIterator_03()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static void Main()
+    {
+        s_buffer[2] = 3;
+
+        foreach (int x in Test())
+        {
+            System.Console.Write(x);
+        }
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        foreach (ref int y in GetBuffer())
+        {
+            y *= y;
+            System.Console.Write(y);
+        }
+
+        yield return -1;
+
+        System.Console.Write(s_buffer[2]);
+    }
+
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (18,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(18, 26));
+
+            var expectedOutput = "0090-19";
+
+            CompileAndVerify(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            CompileAndVerify(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void Foreach_InIterator_04()
+        {
+            var src = @"
+class Program
+{
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        foreach (int y in GetBuffer())
+        {
+            yield return -1;
+        }
+    }
+
+    static ref Buffer4<int> GetBuffer() => throw null;
+}
+";
+            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseDll);
+
+            comp.VerifyEmitDiagnostics(
+                // (6,9): error CS8178: A reference returned by a call to 'Program.GetBuffer()' cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait,
+        @"foreach (int y in GetBuffer())
+        {
+            yield return -1;
+        }").WithArguments("Program.GetBuffer()").WithLocation(6, 9)
+                );
+        }
+
+        [ConditionalFact(typeof(CoreClrOnly))]
+        public void Foreach_InIterator_05()
+        {
+            var src = @"
+class Program
+{
+    static private Buffer4<int> F = default;
+    private static int index = 0;
+
+    static void Main()
+    {
+        foreach (var a in Test())
+        {}
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        yield return -1;
+
+        foreach (var y in GetBuffer())
+        {
+            Increment();
+            System.Console.Write(' ');
+            System.Console.Write(y);
+        }
+
+        yield return -2;
+    }
+
+    static ref readonly Buffer4<int> GetBuffer()
+    {
+        System.Console.Write(-1);
+        return ref F;
+    }
+
+    static void Increment()
+    {
+        index++;
+
+        if (index < 4)
+        {
+            F[index] = index;
+        }
+    }
+}
+";
+            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
+            var verifier = CompileAndVerify(comp, expectedOutput: "-1 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+
+            verifier.VerifyIL("Program.<Test>d__3.System.Collections.IEnumerator.MoveNext",
+@"
+{
+  // Code size      126 (0x7e)
+  .maxstack  2
+  .locals init (int V_0,
+                Buffer4<int>& V_1,
+                int V_2)
+  IL_0000:  ldarg.0
+  IL_0001:  ldfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0006:  stloc.0
+  IL_0007:  ldloc.0
+  IL_0008:  switch    (
+        IL_001b,
+        IL_0032,
+        IL_0075)
+  IL_0019:  ldc.i4.0
+  IL_001a:  ret
+  IL_001b:  ldarg.0
+  IL_001c:  ldc.i4.m1
+  IL_001d:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0022:  ldarg.0
+  IL_0023:  ldc.i4.m1
+  IL_0024:  stfld      ""int Program.<Test>d__3.<>2__current""
+  IL_0029:  ldarg.0
+  IL_002a:  ldc.i4.1
+  IL_002b:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0030:  ldc.i4.1
+  IL_0031:  ret
+  IL_0032:  ldarg.0
+  IL_0033:  ldc.i4.m1
+  IL_0034:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0039:  call       ""ref readonly Buffer4<int> Program.GetBuffer()""
+  IL_003e:  stloc.1
+  IL_003f:  ldc.i4.0
+  IL_0040:  stloc.2
+  IL_0041:  br.s       IL_0060
+  IL_0043:  ldloc.1
+  IL_0044:  ldloc.2
+  IL_0045:  call       ""ref readonly int <PrivateImplementationDetails>.InlineArrayElementRefReadOnly<Buffer4<int>, int>(in Buffer4<int>, int)""
+  IL_004a:  ldind.i4
+  IL_004b:  call       ""void Program.Increment()""
+  IL_0050:  ldc.i4.s   32
+  IL_0052:  call       ""void System.Console.Write(char)""
+  IL_0057:  call       ""void System.Console.Write(int)""
+  IL_005c:  ldloc.2
+  IL_005d:  ldc.i4.1
+  IL_005e:  add
+  IL_005f:  stloc.2
+  IL_0060:  ldloc.2
+  IL_0061:  ldc.i4.4
+  IL_0062:  blt.s      IL_0043
+  IL_0064:  ldarg.0
+  IL_0065:  ldc.i4.s   -2
+  IL_0067:  stfld      ""int Program.<Test>d__3.<>2__current""
+  IL_006c:  ldarg.0
+  IL_006d:  ldc.i4.2
+  IL_006e:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0073:  ldc.i4.1
+  IL_0074:  ret
+  IL_0075:  ldarg.0
+  IL_0076:  ldc.i4.m1
+  IL_0077:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_007c:  ldc.i4.0
+  IL_007d:  ret
+}
+");
+            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
+            CompileAndVerify(comp, expectedOutput: "-1 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+        }
+
+        [ConditionalFact(typeof(CoreClrOnly))]
+        public void Foreach_InIterator_06()
+        {
+            var src = @"
+class C
+{
+    public readonly Buffer4<int> F = default;
+}
+
+class Program
+{
+    private static C c = new C();
+    private static int index = 0;
+
+    static void Main()
+    {
+        foreach (var a in Test(c))
+        {}
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test(C x)
+    {
+        foreach (var y in x.F)
+        {
+            Increment();    
+            System.Console.Write(' ');
+            System.Console.Write(y);
+
+            yield return -1;
+        }
+    }
+
+    static void Increment()
+    {
+        index++;
+
+        if (index < 4)
+        {
+            System.Runtime.CompilerServices.Unsafe.AsRef(in c.F)[index] = index;
+        }
+    }
+}
+";
+            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
+            var verifier = CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+
+            verifier.VerifyIL("Program.<Test>d__3.System.Collections.IEnumerator.MoveNext",
+@"
+{
+  // Code size      151 (0x97)
+  .maxstack  3
+  .locals init (int V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  ldfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0006:  stloc.0
+  IL_0007:  ldloc.0
+  IL_0008:  brfalse.s  IL_0010
+  IL_000a:  ldloc.0
+  IL_000b:  ldc.i4.1
+  IL_000c:  beq.s      IL_0070
+  IL_000e:  ldc.i4.0
+  IL_000f:  ret
+  IL_0010:  ldarg.0
+  IL_0011:  ldc.i4.m1
+  IL_0012:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0017:  ldarg.0
+  IL_0018:  ldarg.0
+  IL_0019:  ldfld      ""C Program.<Test>d__3.x""
+  IL_001e:  stfld      ""C Program.<Test>d__3.<>7__wrap2""
+  IL_0023:  ldarg.0
+  IL_0024:  ldfld      ""C Program.<Test>d__3.<>7__wrap2""
+  IL_0029:  ldfld      ""Buffer4<int> C.F""
+  IL_002e:  pop
+  IL_002f:  ldarg.0
+  IL_0030:  ldc.i4.0
+  IL_0031:  stfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_0036:  br.s       IL_0085
+  IL_0038:  ldarg.0
+  IL_0039:  ldfld      ""C Program.<Test>d__3.<>7__wrap2""
+  IL_003e:  ldflda     ""Buffer4<int> C.F""
+  IL_0043:  ldarg.0
+  IL_0044:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_0049:  call       ""ref readonly int <PrivateImplementationDetails>.InlineArrayElementRefReadOnly<Buffer4<int>, int>(in Buffer4<int>, int)""
+  IL_004e:  ldind.i4
+  IL_004f:  call       ""void Program.Increment()""
+  IL_0054:  ldc.i4.s   32
+  IL_0056:  call       ""void System.Console.Write(char)""
+  IL_005b:  call       ""void System.Console.Write(int)""
+  IL_0060:  ldarg.0
+  IL_0061:  ldc.i4.m1
+  IL_0062:  stfld      ""int Program.<Test>d__3.<>2__current""
+  IL_0067:  ldarg.0
+  IL_0068:  ldc.i4.1
+  IL_0069:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_006e:  ldc.i4.1
+  IL_006f:  ret
+  IL_0070:  ldarg.0
+  IL_0071:  ldc.i4.m1
+  IL_0072:  stfld      ""int Program.<Test>d__3.<>1__state""
+  IL_0077:  ldarg.0
+  IL_0078:  ldarg.0
+  IL_0079:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_007e:  ldc.i4.1
+  IL_007f:  add
+  IL_0080:  stfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_0085:  ldarg.0
+  IL_0086:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
+  IL_008b:  ldc.i4.4
+  IL_008c:  blt.s      IL_0038
+  IL_008e:  ldarg.0
+  IL_008f:  ldnull
+  IL_0090:  stfld      ""C Program.<Test>d__3.<>7__wrap2""
+  IL_0095:  ldc.i4.0
+  IL_0096:  ret
+}
+");
+            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
+            CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+        }
+
+        [ConditionalFact(typeof(CoreClrOnly))]
+        public void Foreach_InIterator_07()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static void Main()
+    {
+        s_buffer[2] = 3;
+
+        foreach (int x in Test())
+        {
+            System.Console.Write(x);
+        }
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        int i = 0;
+        foreach (ref readonly int y in GetBuffer())
+        {
+            System.Console.Write(y);
+            s_buffer[i++]++;
+            System.Console.Write(y);
+            System.Console.Write(' ');
+        }
+
+        yield return -1;
+
+        System.Console.Write(s_buffer[2]);
+    }
+
+    static ref readonly Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (19,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref readonly int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(19, 35));
+
+            var expectedOutput = "01 01 34 01 -14";
+
+            CompileAndVerify(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                verify: Verification.FailsILVerify, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            CompileAndVerify(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                verify: Verification.FailsILVerify, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void Foreach_InIterator_08()
+        {
+            var src = @"
+class Program
+{
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        foreach (int y in GetBuffer())
+        {
+            yield return -1;
+        }
+    }
+
+    static ref readonly Buffer4<int> GetBuffer() => throw null;
+}
+";
+            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseDll);
+
+            comp.VerifyEmitDiagnostics(
+                // (6,9): error CS8178: A reference returned by a call to 'Program.GetBuffer()' cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait,
+        @"foreach (int y in GetBuffer())
+        {
+            yield return -1;
+        }").WithArguments("Program.GetBuffer()").WithLocation(6, 9)
+                );
+        }
+
+        [ConditionalFact(typeof(CoreClrOnly))]
+        public void Foreach_InIterator_09()
+        {
+            var src = @"
+class Program
+{
+    static void Main()
+    {
+        foreach (var a in Test())
+        {}
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        foreach (var y in GetBuffer())
+        {
+            System.Console.Write(' ');
+            System.Console.Write(y);
+            yield return -1;
+        }
+    }
+
+    static Buffer4<int> GetBuffer()
+    {
+        Buffer4<int> x = default;
+        x[0] = 111;
+        x[1] = 112;
+        x[2] = 113;
+        x[3] = 114;
+ 
+        System.Console.Write(-1);
+        return x;
+    }
+}
+";
+            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
+            var verifier = CompileAndVerify(comp, expectedOutput: "-1 111 112 113 114").VerifyDiagnostics();
+
+            verifier.VerifyIL("Program.<Test>d__1.System.Collections.IEnumerator.MoveNext",
+@"
+{
+  // Code size      121 (0x79)
+  .maxstack  3
+  .locals init (int V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  ldfld      ""int Program.<Test>d__1.<>1__state""
+  IL_0006:  stloc.0
+  IL_0007:  ldloc.0
+  IL_0008:  brfalse.s  IL_0010
+  IL_000a:  ldloc.0
+  IL_000b:  ldc.i4.1
+  IL_000c:  beq.s      IL_0059
+  IL_000e:  ldc.i4.0
+  IL_000f:  ret
+  IL_0010:  ldarg.0
+  IL_0011:  ldc.i4.m1
+  IL_0012:  stfld      ""int Program.<Test>d__1.<>1__state""
+  IL_0017:  ldarg.0
+  IL_0018:  call       ""Buffer4<int> Program.GetBuffer()""
+  IL_001d:  stfld      ""Buffer4<int> Program.<Test>d__1.<>7__wrap1""
+  IL_0022:  ldarg.0
+  IL_0023:  ldc.i4.0
+  IL_0024:  stfld      ""int Program.<Test>d__1.<>7__wrap2""
+  IL_0029:  br.s       IL_006e
+  IL_002b:  ldarg.0
+  IL_002c:  ldflda     ""Buffer4<int> Program.<Test>d__1.<>7__wrap1""
+  IL_0031:  ldarg.0
+  IL_0032:  ldfld      ""int Program.<Test>d__1.<>7__wrap2""
+  IL_0037:  call       ""ref readonly int <PrivateImplementationDetails>.InlineArrayElementRefReadOnly<Buffer4<int>, int>(in Buffer4<int>, int)""
+  IL_003c:  ldind.i4
+  IL_003d:  ldc.i4.s   32
+  IL_003f:  call       ""void System.Console.Write(char)""
+  IL_0044:  call       ""void System.Console.Write(int)""
+  IL_0049:  ldarg.0
+  IL_004a:  ldc.i4.m1
+  IL_004b:  stfld      ""int Program.<Test>d__1.<>2__current""
+  IL_0050:  ldarg.0
+  IL_0051:  ldc.i4.1
+  IL_0052:  stfld      ""int Program.<Test>d__1.<>1__state""
+  IL_0057:  ldc.i4.1
+  IL_0058:  ret
+  IL_0059:  ldarg.0
+  IL_005a:  ldc.i4.m1
+  IL_005b:  stfld      ""int Program.<Test>d__1.<>1__state""
+  IL_0060:  ldarg.0
+  IL_0061:  ldarg.0
+  IL_0062:  ldfld      ""int Program.<Test>d__1.<>7__wrap2""
+  IL_0067:  ldc.i4.1
+  IL_0068:  add
+  IL_0069:  stfld      ""int Program.<Test>d__1.<>7__wrap2""
+  IL_006e:  ldarg.0
+  IL_006f:  ldfld      ""int Program.<Test>d__1.<>7__wrap2""
+  IL_0074:  ldc.i4.4
+  IL_0075:  blt.s      IL_002b
+  IL_0077:  ldc.i4.0
+  IL_0078:  ret
+}
+");
+            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
+            CompileAndVerify(comp, expectedOutput: "-1 111 112 113 114").VerifyDiagnostics();
+        }
+
+        [ConditionalFact(typeof(CoreClrOnly))]
+        public void Foreach_InIterator_10()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static void Main()
+    {
+        s_buffer[2] = 3;
+
+        foreach (int x in Test())
+        {
+            System.Console.Write(x);
+        }
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        ref Buffer4<int> buffer = ref GetBuffer();
+        foreach (ref int y in buffer)
+        {
+            y *= y;
+            System.Console.Write(y);
+        }
+
+        yield return -1;
+
+        System.Console.Write(s_buffer[2]);
+    }
+
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (18,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref Buffer4<int> buffer = ref GetBuffer();
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "buffer").WithArguments("ref and unsafe in async and iterator methods").WithLocation(18, 26),
+                // (19,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in buffer)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(19, 26));
+
+            var expectedOutput = "0090-19";
+
+            CompileAndVerify(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            CompileAndVerify(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe,
+                expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void Foreach_InIterator_11()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static void Main()
+    {
+        s_buffer[2] = 3;
+
+        foreach (int x in Test())
+        {
+            System.Console.Write(x);
+        }
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        foreach (ref int y in GetBuffer())
+        {
+            yield return 1;
+            y *= y;
+            System.Console.Write(y);
+        }
+
+        yield return -1;
+
+        System.Console.Write(s_buffer[2]);
+    }
+
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (18,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(18, 26));
+
+            var expectedDiagnostics = new[]
+            {
+                // (21,13): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             y *= y;
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(21, 13),
+                // (21,18): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             y *= y;
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(21, 18),
+                // (22,34): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             System.Console.Write(y);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(22, 34)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InIterator_12()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static void Main()
+    {
+        s_buffer[2] = 3;
+
+        foreach (int x in Test())
+        {
+            System.Console.Write(x);
+        }
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        foreach (ref int y in GetBuffer())
+        {
+            y *= y;
+            System.Console.Write(y);
+            yield return 1;
+        }
+
+        yield return -1;
+
+        System.Console.Write(s_buffer[2]);
+    }
+
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (18,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(18, 26));
+
+            var expectedDiagnostics = new[]
+            {
+                // (18,9): error CS8178: A reference returned by a call to 'Program.GetBuffer()' cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (ref int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait, @"foreach (ref int y in GetBuffer())
+        {
+            y *= y;
+            System.Console.Write(y);
+            yield return 1;
+        }").WithArguments("Program.GetBuffer()").WithLocation(18, 9)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InIterator_13()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static void Main()
+    {
+        s_buffer[2] = 3;
+
+        foreach (int x in Test())
+        {
+            System.Console.Write(x);
+        }
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        ref Buffer4<int> buffer = ref GetBuffer();
+        foreach (ref int y in buffer)
+        {
+            y *= y;
+            System.Console.Write(y);
+            yield return 1;
+        }
+
+        yield return -1;
+
+        System.Console.Write(s_buffer[2]);
+    }
+
+    static ref Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (18,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref Buffer4<int> buffer = ref GetBuffer();
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "buffer").WithArguments("ref and unsafe in async and iterator methods").WithLocation(18, 26),
+                // (19,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref int y in buffer)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(19, 26));
+
+            var expectedDiagnostics = new[]
+            {
+                // (19,31): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (ref int y in buffer)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "buffer").WithLocation(19, 31)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InIterator_14()
+        {
+            var src = @"
+class C
+{
+    public readonly Buffer4<int> F = default;
+}
+
+class Program
+{
+    private static C c = new C();
+    private static int index = 0;
+
+    static void Main()
+    {
+        foreach (var a in Test(c))
+        {}
+    }
+
+    static System.Collections.Generic.IEnumerable<int> Test(C x)
+    {
+        ref readonly Buffer4<int> f = ref x.F;
+        foreach (var y in f)
+        {
+            Increment();    
+            System.Console.Write(' ');
+            System.Console.Write(y);
+
+            yield return -1;
+        }
+    }
+
+    static void Increment()
+    {
+        index++;
+
+        if (index < 4)
+        {
+            System.Runtime.CompilerServices.Unsafe.AsRef(in c.F)[index] = index;
+        }
+    }
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (20,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref readonly Buffer4<int> f = ref x.F;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "f").WithArguments("ref and unsafe in async and iterator methods").WithLocation(20, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (21,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var y in f)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "f").WithLocation(21, 27)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InIterator_15()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        foreach (ref readonly int y in GetBuffer())
+        {
+            System.Console.Write(y);
+            yield return 1;
+        }
+
+        yield return -1;
+    }
+
+    static ref readonly Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (8,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref readonly int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (8,9): error CS8178: A reference returned by a call to 'Program.GetBuffer()' cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (ref readonly int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait, @"foreach (ref readonly int y in GetBuffer())
+        {
+            System.Console.Write(y);
+            yield return 1;
+        }").WithArguments("Program.GetBuffer()").WithLocation(8, 9)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Foreach_InIterator_16()
+        {
+            var src = @"
+class Program
+{
+    static Buffer4<int> s_buffer;
+
+    static System.Collections.Generic.IEnumerable<int> Test()
+    {
+        foreach (ref readonly int y in GetBuffer())
+        {
+            yield return 1;
+            System.Console.Write(y);
+        }
+
+        yield return -1;
+    }
+
+    static ref readonly Buffer4<int> GetBuffer() => ref s_buffer;
+}
+" + Buffer4Definition;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (8,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref readonly int y in GetBuffer())
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (11,34): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             System.Console.Write(y);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(11, 34)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [ConditionalFact(typeof(CoreClrOnly))]
-        public void Foreach_InIterator_06()
+        public void Foreach_InIterator_17()
         {
             var src = @"
 class C
@@ -21217,7 +22450,7 @@ static void Main()
 
     static System.Collections.Generic.IEnumerable<int> Test(C x)
     {
-        foreach (var y in x.F)
+        foreach (ref readonly int y in x.F)
         {
             Increment();    
             System.Console.Write(' ');
@@ -21237,240 +22470,107 @@ static void Increment()
         }
     }
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
-            var verifier = CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
-
-            verifier.VerifyIL("Program.<Test>d__3.System.Collections.IEnumerator.MoveNext",
-@"
-{
-  // Code size      151 (0x97)
-  .maxstack  3
-  .locals init (int V_0)
-  IL_0000:  ldarg.0
-  IL_0001:  ldfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0006:  stloc.0
-  IL_0007:  ldloc.0
-  IL_0008:  brfalse.s  IL_0010
-  IL_000a:  ldloc.0
-  IL_000b:  ldc.i4.1
-  IL_000c:  beq.s      IL_0070
-  IL_000e:  ldc.i4.0
-  IL_000f:  ret
-  IL_0010:  ldarg.0
-  IL_0011:  ldc.i4.m1
-  IL_0012:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0017:  ldarg.0
-  IL_0018:  ldarg.0
-  IL_0019:  ldfld      ""C Program.<Test>d__3.x""
-  IL_001e:  stfld      ""C Program.<Test>d__3.<>7__wrap2""
-  IL_0023:  ldarg.0
-  IL_0024:  ldfld      ""C Program.<Test>d__3.<>7__wrap2""
-  IL_0029:  ldfld      ""Buffer4<int> C.F""
-  IL_002e:  pop
-  IL_002f:  ldarg.0
-  IL_0030:  ldc.i4.0
-  IL_0031:  stfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_0036:  br.s       IL_0085
-  IL_0038:  ldarg.0
-  IL_0039:  ldfld      ""C Program.<Test>d__3.<>7__wrap2""
-  IL_003e:  ldflda     ""Buffer4<int> C.F""
-  IL_0043:  ldarg.0
-  IL_0044:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_0049:  call       ""ref readonly int <PrivateImplementationDetails>.InlineArrayElementRefReadOnly<Buffer4<int>, int>(in Buffer4<int>, int)""
-  IL_004e:  ldind.i4
-  IL_004f:  call       ""void Program.Increment()""
-  IL_0054:  ldc.i4.s   32
-  IL_0056:  call       ""void System.Console.Write(char)""
-  IL_005b:  call       ""void System.Console.Write(int)""
-  IL_0060:  ldarg.0
-  IL_0061:  ldc.i4.m1
-  IL_0062:  stfld      ""int Program.<Test>d__3.<>2__current""
-  IL_0067:  ldarg.0
-  IL_0068:  ldc.i4.1
-  IL_0069:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_006e:  ldc.i4.1
-  IL_006f:  ret
-  IL_0070:  ldarg.0
-  IL_0071:  ldc.i4.m1
-  IL_0072:  stfld      ""int Program.<Test>d__3.<>1__state""
-  IL_0077:  ldarg.0
-  IL_0078:  ldarg.0
-  IL_0079:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_007e:  ldc.i4.1
-  IL_007f:  add
-  IL_0080:  stfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_0085:  ldarg.0
-  IL_0086:  ldfld      ""int Program.<Test>d__3.<>7__wrap1""
-  IL_008b:  ldc.i4.4
-  IL_008c:  blt.s      IL_0038
-  IL_008e:  ldarg.0
-  IL_008f:  ldnull
-  IL_0090:  stfld      ""C Program.<Test>d__3.<>7__wrap2""
-  IL_0095:  ldc.i4.0
-  IL_0096:  ret
-}
-");
-            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
-            CompileAndVerify(comp, expectedOutput: " 0 1 2 3", verify: Verification.Fails).VerifyDiagnostics();
+" + Buffer4Definition;
+            var expectedOutput = " 0 1 2 3";
+            var comp = CreateCompilation(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
+            var verifier = CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.Fails).VerifyDiagnostics();
+            comp = CreateCompilation(src, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
+            CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.Fails).VerifyDiagnostics();
         }
 
         [Fact]
-        public void Foreach_InIterator_07()
+        public void Foreach_InIterator_18()
         {
             var src = @"
-class Program
+class C
 {
-    static System.Collections.Generic.IEnumerable<int> Test()
-    {
-        foreach (ref readonly int y in GetBuffer())
-        {
-        }
-
-        yield return -1;
-    }
-
-    static ref readonly Buffer4<int> GetBuffer() => throw null;
+    public readonly Buffer4<int> F = default;
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseDll);
-            comp.VerifyDiagnostics(
-                // (6,35): error CS8176: Iterators cannot have by-reference locals
-                //         foreach (ref readonly int y in GetBuffer())
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "y").WithLocation(6, 35)
-                );
-        }
 
-        [Fact]
-        public void Foreach_InIterator_08()
-        {
-            var src = @"
 class Program
 {
-    static System.Collections.Generic.IEnumerable<int> Test()
+    static System.Collections.Generic.IEnumerable<int> Test(C x)
     {
-        foreach (int y in GetBuffer())
+        foreach (ref readonly int y in x.F)
         {
             yield return -1;
+            System.Console.Write(y);
         }
     }
-
-    static ref readonly Buffer4<int> GetBuffer() => throw null;
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseDll);
+" + Buffer4Definition;
 
-            comp.VerifyEmitDiagnostics(
-                // (6,9): error CS8178: A reference returned by a call to 'Program.GetBuffer()' cannot be preserved across 'await' or 'yield' boundary.
-                //         foreach (int y in GetBuffer())
-                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait,
-        @"foreach (int y in GetBuffer())
-        {
-            yield return -1;
-        }").WithArguments("Program.GetBuffer()").WithLocation(6, 9)
-                );
+            CreateCompilation(src, targetFramework: TargetFramework.Net80, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (11,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         foreach (ref readonly int y in x.F)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(11, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (14,34): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //             System.Console.Write(y);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(14, 34)
+            };
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
-        [ConditionalFact(typeof(CoreClrOnly))]
-        public void Foreach_InIterator_09()
+        [Fact]
+        public void Foreach_InIterator_19()
         {
             var src = @"
+class C
+{
+    public readonly Buffer4<int> F = default;
+}
+
 class Program
 {
-    static void Main()
+    static System.Collections.Generic.IEnumerable<int> Test(C x)
     {
-        foreach (var a in Test())
-        {}
-    }
+        ref readonly Buffer4<int> f = ref x.F;
 
-    static System.Collections.Generic.IEnumerable<int> Test()
-    {
-        foreach (var y in GetBuffer())
+        foreach (var i in f) System.Console.Write(i);
+
+        foreach (var y in f)
         {
-            System.Console.Write(' ');
             System.Console.Write(y);
             yield return -1;
         }
-    }
 
-    static Buffer4<int> GetBuffer()
-    {
-        Buffer4<int> x = default;
-        x[0] = 111;
-        x[1] = 112;
-        x[2] = 113;
-        x[3] = 114;
- 
-        System.Console.Write(-1);
-        return x;
+        foreach (var j in f) System.Console.Write(j);
+
+        foreach (var z in f)
+        {
+            System.Console.Write(z);
+            yield return -2;
+        }
     }
 }
-";
-            var comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
-            var verifier = CompileAndVerify(comp, expectedOutput: "-1 111 112 113 114").VerifyDiagnostics();
+" + Buffer4Definition;
 
-            verifier.VerifyIL("Program.<Test>d__1.System.Collections.IEnumerator.MoveNext",
-@"
-{
-  // Code size      121 (0x79)
-  .maxstack  3
-  .locals init (int V_0)
-  IL_0000:  ldarg.0
-  IL_0001:  ldfld      ""int Program.<Test>d__1.<>1__state""
-  IL_0006:  stloc.0
-  IL_0007:  ldloc.0
-  IL_0008:  brfalse.s  IL_0010
-  IL_000a:  ldloc.0
-  IL_000b:  ldc.i4.1
-  IL_000c:  beq.s      IL_0059
-  IL_000e:  ldc.i4.0
-  IL_000f:  ret
-  IL_0010:  ldarg.0
-  IL_0011:  ldc.i4.m1
-  IL_0012:  stfld      ""int Program.<Test>d__1.<>1__state""
-  IL_0017:  ldarg.0
-  IL_0018:  call       ""Buffer4<int> Program.GetBuffer()""
-  IL_001d:  stfld      ""Buffer4<int> Program.<Test>d__1.<>7__wrap1""
-  IL_0022:  ldarg.0
-  IL_0023:  ldc.i4.0
-  IL_0024:  stfld      ""int Program.<Test>d__1.<>7__wrap2""
-  IL_0029:  br.s       IL_006e
-  IL_002b:  ldarg.0
-  IL_002c:  ldflda     ""Buffer4<int> Program.<Test>d__1.<>7__wrap1""
-  IL_0031:  ldarg.0
-  IL_0032:  ldfld      ""int Program.<Test>d__1.<>7__wrap2""
-  IL_0037:  call       ""ref readonly int <PrivateImplementationDetails>.InlineArrayElementRefReadOnly<Buffer4<int>, int>(in Buffer4<int>, int)""
-  IL_003c:  ldind.i4
-  IL_003d:  ldc.i4.s   32
-  IL_003f:  call       ""void System.Console.Write(char)""
-  IL_0044:  call       ""void System.Console.Write(int)""
-  IL_0049:  ldarg.0
-  IL_004a:  ldc.i4.m1
-  IL_004b:  stfld      ""int Program.<Test>d__1.<>2__current""
-  IL_0050:  ldarg.0
-  IL_0051:  ldc.i4.1
-  IL_0052:  stfld      ""int Program.<Test>d__1.<>1__state""
-  IL_0057:  ldc.i4.1
-  IL_0058:  ret
-  IL_0059:  ldarg.0
-  IL_005a:  ldc.i4.m1
-  IL_005b:  stfld      ""int Program.<Test>d__1.<>1__state""
-  IL_0060:  ldarg.0
-  IL_0061:  ldarg.0
-  IL_0062:  ldfld      ""int Program.<Test>d__1.<>7__wrap2""
-  IL_0067:  ldc.i4.1
-  IL_0068:  add
-  IL_0069:  stfld      ""int Program.<Test>d__1.<>7__wrap2""
-  IL_006e:  ldarg.0
-  IL_006f:  ldfld      ""int Program.<Test>d__1.<>7__wrap2""
-  IL_0074:  ldc.i4.4
-  IL_0075:  blt.s      IL_002b
-  IL_0077:  ldc.i4.0
-  IL_0078:  ret
-}
-");
-            comp = CreateCompilation(src + Buffer4Definition, targetFramework: TargetFramework.Net80, options: TestOptions.DebugExe);
-            CompileAndVerify(comp, expectedOutput: "-1 111 112 113 114").VerifyDiagnostics();
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (11,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref readonly Buffer4<int> f = ref x.F;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "f").WithArguments("ref and unsafe in async and iterator methods").WithLocation(11, 35));
+
+            var expectedDiagnostics = new[]
+            {
+                // (15,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var y in f)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "f").WithLocation(15, 27),
+                // (21,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var j in f) System.Console.Write(j);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "f").WithLocation(21, 27),
+                // (23,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var z in f)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "f").WithLocation(23, 27)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [ConditionalFact(typeof(CoreClrOnly))]
@@ -21988,5 +23088,218 @@ struct ThreeStringBuffer {
             var comp = CreateCompilation(src, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe);
             var verifier = CompileAndVerify(comp, expectedOutput: "123124").VerifyDiagnostics();
         }
+
+        [Fact]
+        public void Initialization_Await_RefStruct()
+        {
+            var src = """
+                using System.Threading.Tasks;
+
+                var b = new Buffer();
+                b[0] = await GetInt();
+                b[1] = await GetInt();
+
+                static Task<int> GetInt() => Task.FromResult(42);
+                
+                [System.Runtime.CompilerServices.InlineArray(4)]
+                ref struct Buffer
+                {
+                    private int _element0;
+                }
+                """;
+
+            CreateCompilation(src, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net80).VerifyDiagnostics(
+                // (3,1): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                // var b = new Buffer();
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(3, 1),
+                // (4,1): error CS0306: The type 'Buffer' may not be used as a type argument
+                // b[0] = await GetInt();
+                Diagnostic(ErrorCode.ERR_BadTypeArgument, "b[0]").WithArguments("Buffer").WithLocation(4, 1),
+                // (5,1): error CS0306: The type 'Buffer' may not be used as a type argument
+                // b[1] = await GetInt();
+                Diagnostic(ErrorCode.ERR_BadTypeArgument, "b[1]").WithArguments("Buffer").WithLocation(5, 1),
+                // (10,12): warning CS9184: 'Inline arrays' language feature is not supported for an inline array type that is not valid as a type argument, or has element type that is not valid as a type argument.
+                // ref struct Buffer
+                Diagnostic(ErrorCode.WRN_InlineArrayNotSupportedByLanguage, "Buffer").WithLocation(10, 12));
+
+            var expectedDiagnostics = new[]
+            {
+                // (4,1): error CS0306: The type 'Buffer' may not be used as a type argument
+                // b[0] = await GetInt();
+                Diagnostic(ErrorCode.ERR_BadTypeArgument, "b[0]").WithArguments("Buffer").WithLocation(4, 1),
+                // (5,1): error CS0306: The type 'Buffer' may not be used as a type argument
+                // b[1] = await GetInt();
+                Diagnostic(ErrorCode.ERR_BadTypeArgument, "b[1]").WithArguments("Buffer").WithLocation(5, 1),
+                // (10,12): warning CS9184: 'Inline arrays' language feature is not supported for an inline array type that is not valid as a type argument, or has element type that is not valid as a type argument.
+                // ref struct Buffer
+                Diagnostic(ErrorCode.WRN_InlineArrayNotSupportedByLanguage, "Buffer").WithLocation(10, 12)
+            };
+
+            CreateCompilation(src, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net80).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(src, targetFramework: TargetFramework.Net80).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Initialization_Await()
+        {
+            var src = """
+                using System.Threading.Tasks;
+
+                var b = new Buffer();
+                b[0] = await GetInt();
+                System.Console.Write(b[1]);
+                b[1] = await GetInt();
+                System.Console.Write(b[1]);
+
+                static Task<int> GetInt() => Task.FromResult(42);
+                
+                [System.Runtime.CompilerServices.InlineArray(4)]
+                struct Buffer
+                {
+                    private int _element0;
+                }
+                """;
+            foreach (var parseOptions in new[] { TestOptions.Regular12, TestOptions.RegularNext, TestOptions.RegularPreview })
+            {
+                var verifier = CompileAndVerify(src, expectedOutput: ExecutionConditionUtil.IsDesktop ? null : "042",
+                    parseOptions: parseOptions, targetFramework: TargetFramework.Net80, verify: Verification.FailsPEVerify);
+                verifier.VerifyDiagnostics();
+                verifier.VerifyIL("Program.<<Main>$>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext()", """
+                    {
+                      // Code size      316 (0x13c)
+                      .maxstack  3
+                      .locals init (int V_0,
+                                    int V_1,
+                                    System.Runtime.CompilerServices.TaskAwaiter<int> V_2,
+                                    System.Exception V_3)
+                      IL_0000:  ldarg.0
+                      IL_0001:  ldfld      "int Program.<<Main>$>d__0.<>1__state"
+                      IL_0006:  stloc.0
+                      .try
+                      {
+                        IL_0007:  ldloc.0
+                        IL_0008:  brfalse.s  IL_0054
+                        IL_000a:  ldloc.0
+                        IL_000b:  ldc.i4.1
+                        IL_000c:  beq        IL_00cb
+                        IL_0011:  ldarg.0
+                        IL_0012:  ldflda     "Buffer Program.<<Main>$>d__0.<b>5__2"
+                        IL_0017:  initobj    "Buffer"
+                        IL_001d:  call       "System.Threading.Tasks.Task<int> Program.<<Main>$>g__GetInt|0_0()"
+                        IL_0022:  callvirt   "System.Runtime.CompilerServices.TaskAwaiter<int> System.Threading.Tasks.Task<int>.GetAwaiter()"
+                        IL_0027:  stloc.2
+                        IL_0028:  ldloca.s   V_2
+                        IL_002a:  call       "bool System.Runtime.CompilerServices.TaskAwaiter<int>.IsCompleted.get"
+                        IL_002f:  brtrue.s   IL_0070
+                        IL_0031:  ldarg.0
+                        IL_0032:  ldc.i4.0
+                        IL_0033:  dup
+                        IL_0034:  stloc.0
+                        IL_0035:  stfld      "int Program.<<Main>$>d__0.<>1__state"
+                        IL_003a:  ldarg.0
+                        IL_003b:  ldloc.2
+                        IL_003c:  stfld      "System.Runtime.CompilerServices.TaskAwaiter<int> Program.<<Main>$>d__0.<>u__1"
+                        IL_0041:  ldarg.0
+                        IL_0042:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<Main>$>d__0.<>t__builder"
+                        IL_0047:  ldloca.s   V_2
+                        IL_0049:  ldarg.0
+                        IL_004a:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.TaskAwaiter<int>, Program.<<Main>$>d__0>(ref System.Runtime.CompilerServices.TaskAwaiter<int>, ref Program.<<Main>$>d__0)"
+                        IL_004f:  leave      IL_013b
+                        IL_0054:  ldarg.0
+                        IL_0055:  ldfld      "System.Runtime.CompilerServices.TaskAwaiter<int> Program.<<Main>$>d__0.<>u__1"
+                        IL_005a:  stloc.2
+                        IL_005b:  ldarg.0
+                        IL_005c:  ldflda     "System.Runtime.CompilerServices.TaskAwaiter<int> Program.<<Main>$>d__0.<>u__1"
+                        IL_0061:  initobj    "System.Runtime.CompilerServices.TaskAwaiter<int>"
+                        IL_0067:  ldarg.0
+                        IL_0068:  ldc.i4.m1
+                        IL_0069:  dup
+                        IL_006a:  stloc.0
+                        IL_006b:  stfld      "int Program.<<Main>$>d__0.<>1__state"
+                        IL_0070:  ldloca.s   V_2
+                        IL_0072:  call       "int System.Runtime.CompilerServices.TaskAwaiter<int>.GetResult()"
+                        IL_0077:  stloc.1
+                        IL_0078:  ldarg.0
+                        IL_0079:  ldflda     "Buffer Program.<<Main>$>d__0.<b>5__2"
+                        IL_007e:  call       "ref int <PrivateImplementationDetails>.InlineArrayFirstElementRef<Buffer, int>(ref Buffer)"
+                        IL_0083:  ldloc.1
+                        IL_0084:  stind.i4
+                        IL_0085:  ldarg.0
+                        IL_0086:  ldflda     "Buffer Program.<<Main>$>d__0.<b>5__2"
+                        IL_008b:  ldc.i4.1
+                        IL_008c:  call       "ref int <PrivateImplementationDetails>.InlineArrayElementRef<Buffer, int>(ref Buffer, int)"
+                        IL_0091:  ldind.i4
+                        IL_0092:  call       "void System.Console.Write(int)"
+                        IL_0097:  call       "System.Threading.Tasks.Task<int> Program.<<Main>$>g__GetInt|0_0()"
+                        IL_009c:  callvirt   "System.Runtime.CompilerServices.TaskAwaiter<int> System.Threading.Tasks.Task<int>.GetAwaiter()"
+                        IL_00a1:  stloc.2
+                        IL_00a2:  ldloca.s   V_2
+                        IL_00a4:  call       "bool System.Runtime.CompilerServices.TaskAwaiter<int>.IsCompleted.get"
+                        IL_00a9:  brtrue.s   IL_00e7
+                        IL_00ab:  ldarg.0
+                        IL_00ac:  ldc.i4.1
+                        IL_00ad:  dup
+                        IL_00ae:  stloc.0
+                        IL_00af:  stfld      "int Program.<<Main>$>d__0.<>1__state"
+                        IL_00b4:  ldarg.0
+                        IL_00b5:  ldloc.2
+                        IL_00b6:  stfld      "System.Runtime.CompilerServices.TaskAwaiter<int> Program.<<Main>$>d__0.<>u__1"
+                        IL_00bb:  ldarg.0
+                        IL_00bc:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<Main>$>d__0.<>t__builder"
+                        IL_00c1:  ldloca.s   V_2
+                        IL_00c3:  ldarg.0
+                        IL_00c4:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.TaskAwaiter<int>, Program.<<Main>$>d__0>(ref System.Runtime.CompilerServices.TaskAwaiter<int>, ref Program.<<Main>$>d__0)"
+                        IL_00c9:  leave.s    IL_013b
+                        IL_00cb:  ldarg.0
+                        IL_00cc:  ldfld      "System.Runtime.CompilerServices.TaskAwaiter<int> Program.<<Main>$>d__0.<>u__1"
+                        IL_00d1:  stloc.2
+                        IL_00d2:  ldarg.0
+                        IL_00d3:  ldflda     "System.Runtime.CompilerServices.TaskAwaiter<int> Program.<<Main>$>d__0.<>u__1"
+                        IL_00d8:  initobj    "System.Runtime.CompilerServices.TaskAwaiter<int>"
+                        IL_00de:  ldarg.0
+                        IL_00df:  ldc.i4.m1
+                        IL_00e0:  dup
+                        IL_00e1:  stloc.0
+                        IL_00e2:  stfld      "int Program.<<Main>$>d__0.<>1__state"
+                        IL_00e7:  ldloca.s   V_2
+                        IL_00e9:  call       "int System.Runtime.CompilerServices.TaskAwaiter<int>.GetResult()"
+                        IL_00ee:  stloc.1
+                        IL_00ef:  ldarg.0
+                        IL_00f0:  ldflda     "Buffer Program.<<Main>$>d__0.<b>5__2"
+                        IL_00f5:  ldc.i4.1
+                        IL_00f6:  call       "ref int <PrivateImplementationDetails>.InlineArrayElementRef<Buffer, int>(ref Buffer, int)"
+                        IL_00fb:  ldloc.1
+                        IL_00fc:  stind.i4
+                        IL_00fd:  ldarg.0
+                        IL_00fe:  ldflda     "Buffer Program.<<Main>$>d__0.<b>5__2"
+                        IL_0103:  ldc.i4.1
+                        IL_0104:  call       "ref int <PrivateImplementationDetails>.InlineArrayElementRef<Buffer, int>(ref Buffer, int)"
+                        IL_0109:  ldind.i4
+                        IL_010a:  call       "void System.Console.Write(int)"
+                        IL_010f:  leave.s    IL_0128
+                      }
+                      catch System.Exception
+                      {
+                        IL_0111:  stloc.3
+                        IL_0112:  ldarg.0
+                        IL_0113:  ldc.i4.s   -2
+                        IL_0115:  stfld      "int Program.<<Main>$>d__0.<>1__state"
+                        IL_011a:  ldarg.0
+                        IL_011b:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<Main>$>d__0.<>t__builder"
+                        IL_0120:  ldloc.3
+                        IL_0121:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                        IL_0126:  leave.s    IL_013b
+                      }
+                      IL_0128:  ldarg.0
+                      IL_0129:  ldc.i4.s   -2
+                      IL_012b:  stfld      "int Program.<<Main>$>d__0.<>1__state"
+                      IL_0130:  ldarg.0
+                      IL_0131:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<Main>$>d__0.<>t__builder"
+                      IL_0136:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+                      IL_013b:  ret
+                    }
+                    """);
+            }
+        }
     }
 }
diff --git a/src/Compilers/CSharp/Test/Emit2/Semantics/LockTests.cs b/src/Compilers/CSharp/Test/Emit2/Semantics/LockTests.cs
index 61b08bdf399e5..61614ef54b64e 100644
--- a/src/Compilers/CSharp/Test/Emit2/Semantics/LockTests.cs
+++ b/src/Compilers/CSharp/Test/Emit2/Semantics/LockTests.cs
@@ -1861,9 +1861,6 @@ public void Await()
             }
             """;
         CreateCompilation([source, LockTypeDefinition]).VerifyDiagnostics(
-            // (4,7): error CS9217: A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.
-            // lock (new Lock())
-            Diagnostic(ErrorCode.ERR_BadSpecialByRefLock, "new Lock()").WithLocation(4, 7),
             // (6,5): error CS1996: Cannot await in the body of a lock statement
             //     await Task.Yield();
             Diagnostic(ErrorCode.ERR_BadAwaitInLock, "await Task.Yield()").WithLocation(6, 5));
@@ -1875,19 +1872,137 @@ public void AsyncMethod()
         var source = """
             #pragma warning disable 1998 // async method lacks 'await' operators
             using System.Threading;
+            using System.Threading.Tasks;
 
             class C
             {
-                async void M()
+                static async Task Main()
                 {
-                    lock (new Lock()) { }
+                    lock (new Lock()) { System.Console.Write("L"); }
                 }
             }
             """;
-        CreateCompilation([source, LockTypeDefinition]).VerifyDiagnostics(
-            // (8,15): error CS9217: A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.
-            //         lock (new Lock()) { }
-            Diagnostic(ErrorCode.ERR_BadSpecialByRefLock, "new Lock()").WithLocation(8, 15));
+        var expectedOutput = "ELD";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("C.<Main>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size       94 (0x5e)
+              .maxstack  2
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Exception V_2)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int C.<Main>d__0.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  nop
+                IL_0008:  newobj     "System.Threading.Lock..ctor()"
+                IL_000d:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0012:  stloc.1
+                .try
+                {
+                  IL_0013:  nop
+                  IL_0014:  ldstr      "L"
+                  IL_0019:  call       "void System.Console.Write(string)"
+                  IL_001e:  nop
+                  IL_001f:  nop
+                  IL_0020:  leave.s    IL_002f
+                }
+                finally
+                {
+                  IL_0022:  ldloc.0
+                  IL_0023:  ldc.i4.0
+                  IL_0024:  bge.s      IL_002e
+                  IL_0026:  ldloca.s   V_1
+                  IL_0028:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_002d:  nop
+                  IL_002e:  endfinally
+                }
+                IL_002f:  leave.s    IL_0049
+              }
+              catch System.Exception
+              {
+                IL_0031:  stloc.2
+                IL_0032:  ldarg.0
+                IL_0033:  ldc.i4.s   -2
+                IL_0035:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_003a:  ldarg.0
+                IL_003b:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_0040:  ldloc.2
+                IL_0041:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_0046:  nop
+                IL_0047:  leave.s    IL_005d
+              }
+              IL_0049:  ldarg.0
+              IL_004a:  ldc.i4.s   -2
+              IL_004c:  stfld      "int C.<Main>d__0.<>1__state"
+              IL_0051:  ldarg.0
+              IL_0052:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+              IL_0057:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_005c:  nop
+              IL_005d:  ret
+            }
+            """);
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("C.<Main>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size       87 (0x57)
+              .maxstack  2
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Exception V_2)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int C.<Main>d__0.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  newobj     "System.Threading.Lock..ctor()"
+                IL_000c:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0011:  stloc.1
+                .try
+                {
+                  IL_0012:  ldstr      "L"
+                  IL_0017:  call       "void System.Console.Write(string)"
+                  IL_001c:  leave.s    IL_002a
+                }
+                finally
+                {
+                  IL_001e:  ldloc.0
+                  IL_001f:  ldc.i4.0
+                  IL_0020:  bge.s      IL_0029
+                  IL_0022:  ldloca.s   V_1
+                  IL_0024:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_0029:  endfinally
+                }
+                IL_002a:  leave.s    IL_0043
+              }
+              catch System.Exception
+              {
+                IL_002c:  stloc.2
+                IL_002d:  ldarg.0
+                IL_002e:  ldc.i4.s   -2
+                IL_0030:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_0035:  ldarg.0
+                IL_0036:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_003b:  ldloc.2
+                IL_003c:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_0041:  leave.s    IL_0056
+              }
+              IL_0043:  ldarg.0
+              IL_0044:  ldc.i4.s   -2
+              IL_0046:  stfld      "int C.<Main>d__0.<>1__state"
+              IL_004b:  ldarg.0
+              IL_004c:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+              IL_0051:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_0056:  ret
+            }
+            """);
     }
 
     [Fact]
@@ -1899,17 +2014,527 @@ public void AsyncMethod_WithAwait()
 
             class C
             {
-                async void M()
+                static async Task Main()
                 {
                     await Task.Yield();
-                    lock (new Lock()) { }
+                    lock (new Lock()) { System.Console.Write("L"); }
+                    await Task.Yield();
                 }
             }
             """;
-        CreateCompilation([source, LockTypeDefinition]).VerifyDiagnostics(
-            // (9,15): error CS9217: A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.
-            //         lock (new Lock()) { }
-            Diagnostic(ErrorCode.ERR_BadSpecialByRefLock, "new Lock()").WithLocation(9, 15));
+        var expectedOutput = "ELD";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("C.<Main>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      311 (0x137)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_1,
+                            System.Runtime.CompilerServices.YieldAwaitable V_2,
+                            C.<Main>d__0 V_3,
+                            System.Threading.Lock.Scope V_4,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_5,
+                            System.Exception V_6)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int C.<Main>d__0.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  brfalse.s  IL_0012
+                IL_000a:  br.s       IL_000c
+                IL_000c:  ldloc.0
+                IL_000d:  ldc.i4.1
+                IL_000e:  beq.s      IL_0014
+                IL_0010:  br.s       IL_0019
+                IL_0012:  br.s       IL_0058
+                IL_0014:  br         IL_00e1
+                IL_0019:  nop
+                IL_001a:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_001f:  stloc.2
+                IL_0020:  ldloca.s   V_2
+                IL_0022:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_0027:  stloc.1
+                IL_0028:  ldloca.s   V_1
+                IL_002a:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_002f:  brtrue.s   IL_0074
+                IL_0031:  ldarg.0
+                IL_0032:  ldc.i4.0
+                IL_0033:  dup
+                IL_0034:  stloc.0
+                IL_0035:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_003a:  ldarg.0
+                IL_003b:  ldloc.1
+                IL_003c:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_0041:  ldarg.0
+                IL_0042:  stloc.3
+                IL_0043:  ldarg.0
+                IL_0044:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_0049:  ldloca.s   V_1
+                IL_004b:  ldloca.s   V_3
+                IL_004d:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, C.<Main>d__0>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref C.<Main>d__0)"
+                IL_0052:  nop
+                IL_0053:  leave      IL_0136
+                IL_0058:  ldarg.0
+                IL_0059:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_005e:  stloc.1
+                IL_005f:  ldarg.0
+                IL_0060:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_0065:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_006b:  ldarg.0
+                IL_006c:  ldc.i4.m1
+                IL_006d:  dup
+                IL_006e:  stloc.0
+                IL_006f:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_0074:  ldloca.s   V_1
+                IL_0076:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_007b:  nop
+                IL_007c:  newobj     "System.Threading.Lock..ctor()"
+                IL_0081:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0086:  stloc.s    V_4
+                .try
+                {
+                  IL_0088:  nop
+                  IL_0089:  ldstr      "L"
+                  IL_008e:  call       "void System.Console.Write(string)"
+                  IL_0093:  nop
+                  IL_0094:  nop
+                  IL_0095:  leave.s    IL_00a4
+                }
+                finally
+                {
+                  IL_0097:  ldloc.0
+                  IL_0098:  ldc.i4.0
+                  IL_0099:  bge.s      IL_00a3
+                  IL_009b:  ldloca.s   V_4
+                  IL_009d:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_00a2:  nop
+                  IL_00a3:  endfinally
+                }
+                IL_00a4:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_00a9:  stloc.2
+                IL_00aa:  ldloca.s   V_2
+                IL_00ac:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_00b1:  stloc.s    V_5
+                IL_00b3:  ldloca.s   V_5
+                IL_00b5:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_00ba:  brtrue.s   IL_00fe
+                IL_00bc:  ldarg.0
+                IL_00bd:  ldc.i4.1
+                IL_00be:  dup
+                IL_00bf:  stloc.0
+                IL_00c0:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_00c5:  ldarg.0
+                IL_00c6:  ldloc.s    V_5
+                IL_00c8:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_00cd:  ldarg.0
+                IL_00ce:  stloc.3
+                IL_00cf:  ldarg.0
+                IL_00d0:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_00d5:  ldloca.s   V_5
+                IL_00d7:  ldloca.s   V_3
+                IL_00d9:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, C.<Main>d__0>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref C.<Main>d__0)"
+                IL_00de:  nop
+                IL_00df:  leave.s    IL_0136
+                IL_00e1:  ldarg.0
+                IL_00e2:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_00e7:  stloc.s    V_5
+                IL_00e9:  ldarg.0
+                IL_00ea:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_00ef:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_00f5:  ldarg.0
+                IL_00f6:  ldc.i4.m1
+                IL_00f7:  dup
+                IL_00f8:  stloc.0
+                IL_00f9:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_00fe:  ldloca.s   V_5
+                IL_0100:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_0105:  nop
+                IL_0106:  leave.s    IL_0122
+              }
+              catch System.Exception
+              {
+                IL_0108:  stloc.s    V_6
+                IL_010a:  ldarg.0
+                IL_010b:  ldc.i4.s   -2
+                IL_010d:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_0112:  ldarg.0
+                IL_0113:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_0118:  ldloc.s    V_6
+                IL_011a:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_011f:  nop
+                IL_0120:  leave.s    IL_0136
+              }
+              IL_0122:  ldarg.0
+              IL_0123:  ldc.i4.s   -2
+              IL_0125:  stfld      "int C.<Main>d__0.<>1__state"
+              IL_012a:  ldarg.0
+              IL_012b:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+              IL_0130:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_0135:  nop
+              IL_0136:  ret
+            }
+            """);
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("C.<Main>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      282 (0x11a)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_1,
+                            System.Runtime.CompilerServices.YieldAwaitable V_2,
+                            System.Threading.Lock.Scope V_3,
+                            System.Exception V_4)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int C.<Main>d__0.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  brfalse.s  IL_004b
+                IL_000a:  ldloc.0
+                IL_000b:  ldc.i4.1
+                IL_000c:  beq        IL_00c8
+                IL_0011:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_0016:  stloc.2
+                IL_0017:  ldloca.s   V_2
+                IL_0019:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_001e:  stloc.1
+                IL_001f:  ldloca.s   V_1
+                IL_0021:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_0026:  brtrue.s   IL_0067
+                IL_0028:  ldarg.0
+                IL_0029:  ldc.i4.0
+                IL_002a:  dup
+                IL_002b:  stloc.0
+                IL_002c:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_0031:  ldarg.0
+                IL_0032:  ldloc.1
+                IL_0033:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_0038:  ldarg.0
+                IL_0039:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_003e:  ldloca.s   V_1
+                IL_0040:  ldarg.0
+                IL_0041:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, C.<Main>d__0>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref C.<Main>d__0)"
+                IL_0046:  leave      IL_0119
+                IL_004b:  ldarg.0
+                IL_004c:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_0051:  stloc.1
+                IL_0052:  ldarg.0
+                IL_0053:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_0058:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_005e:  ldarg.0
+                IL_005f:  ldc.i4.m1
+                IL_0060:  dup
+                IL_0061:  stloc.0
+                IL_0062:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_0067:  ldloca.s   V_1
+                IL_0069:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_006e:  newobj     "System.Threading.Lock..ctor()"
+                IL_0073:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0078:  stloc.3
+                .try
+                {
+                  IL_0079:  ldstr      "L"
+                  IL_007e:  call       "void System.Console.Write(string)"
+                  IL_0083:  leave.s    IL_0091
+                }
+                finally
+                {
+                  IL_0085:  ldloc.0
+                  IL_0086:  ldc.i4.0
+                  IL_0087:  bge.s      IL_0090
+                  IL_0089:  ldloca.s   V_3
+                  IL_008b:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_0090:  endfinally
+                }
+                IL_0091:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_0096:  stloc.2
+                IL_0097:  ldloca.s   V_2
+                IL_0099:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_009e:  stloc.1
+                IL_009f:  ldloca.s   V_1
+                IL_00a1:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_00a6:  brtrue.s   IL_00e4
+                IL_00a8:  ldarg.0
+                IL_00a9:  ldc.i4.1
+                IL_00aa:  dup
+                IL_00ab:  stloc.0
+                IL_00ac:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_00b1:  ldarg.0
+                IL_00b2:  ldloc.1
+                IL_00b3:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_00b8:  ldarg.0
+                IL_00b9:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_00be:  ldloca.s   V_1
+                IL_00c0:  ldarg.0
+                IL_00c1:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, C.<Main>d__0>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref C.<Main>d__0)"
+                IL_00c6:  leave.s    IL_0119
+                IL_00c8:  ldarg.0
+                IL_00c9:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_00ce:  stloc.1
+                IL_00cf:  ldarg.0
+                IL_00d0:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter C.<Main>d__0.<>u__1"
+                IL_00d5:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_00db:  ldarg.0
+                IL_00dc:  ldc.i4.m1
+                IL_00dd:  dup
+                IL_00de:  stloc.0
+                IL_00df:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_00e4:  ldloca.s   V_1
+                IL_00e6:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_00eb:  leave.s    IL_0106
+              }
+              catch System.Exception
+              {
+                IL_00ed:  stloc.s    V_4
+                IL_00ef:  ldarg.0
+                IL_00f0:  ldc.i4.s   -2
+                IL_00f2:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_00f7:  ldarg.0
+                IL_00f8:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_00fd:  ldloc.s    V_4
+                IL_00ff:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_0104:  leave.s    IL_0119
+              }
+              IL_0106:  ldarg.0
+              IL_0107:  ldc.i4.s   -2
+              IL_0109:  stfld      "int C.<Main>d__0.<>1__state"
+              IL_010e:  ldarg.0
+              IL_010f:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+              IL_0114:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_0119:  ret
+            }
+            """);
+    }
+
+    [Fact]
+    public void AsyncMethod_AwaitResource()
+    {
+        var source = """
+            #pragma warning disable 1998 // async method lacks 'await' operators
+            using System.Threading;
+            using System.Threading.Tasks;
+
+            class C
+            {
+                static async Task Main()
+                {
+                    lock (await GetLock()) { System.Console.Write("L"); }
+                }
+
+                static async Task<Lock> GetLock() => new Lock();
+            }
+            """;
+        var expectedOutput = "ELD";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("C.<Main>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      211 (0xd3)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> V_2,
+                            C.<Main>d__0 V_3,
+                            System.Exception V_4)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int C.<Main>d__0.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  brfalse.s  IL_000c
+                IL_000a:  br.s       IL_000e
+                IL_000c:  br.s       IL_004a
+                IL_000e:  nop
+                IL_000f:  call       "System.Threading.Tasks.Task<System.Threading.Lock> C.GetLock()"
+                IL_0014:  callvirt   "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> System.Threading.Tasks.Task<System.Threading.Lock>.GetAwaiter()"
+                IL_0019:  stloc.2
+                IL_001a:  ldloca.s   V_2
+                IL_001c:  call       "bool System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>.IsCompleted.get"
+                IL_0021:  brtrue.s   IL_0066
+                IL_0023:  ldarg.0
+                IL_0024:  ldc.i4.0
+                IL_0025:  dup
+                IL_0026:  stloc.0
+                IL_0027:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_002c:  ldarg.0
+                IL_002d:  ldloc.2
+                IL_002e:  stfld      "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> C.<Main>d__0.<>u__1"
+                IL_0033:  ldarg.0
+                IL_0034:  stloc.3
+                IL_0035:  ldarg.0
+                IL_0036:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_003b:  ldloca.s   V_2
+                IL_003d:  ldloca.s   V_3
+                IL_003f:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>, C.<Main>d__0>(ref System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>, ref C.<Main>d__0)"
+                IL_0044:  nop
+                IL_0045:  leave      IL_00d2
+                IL_004a:  ldarg.0
+                IL_004b:  ldfld      "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> C.<Main>d__0.<>u__1"
+                IL_0050:  stloc.2
+                IL_0051:  ldarg.0
+                IL_0052:  ldflda     "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> C.<Main>d__0.<>u__1"
+                IL_0057:  initobj    "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>"
+                IL_005d:  ldarg.0
+                IL_005e:  ldc.i4.m1
+                IL_005f:  dup
+                IL_0060:  stloc.0
+                IL_0061:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_0066:  ldarg.0
+                IL_0067:  ldloca.s   V_2
+                IL_0069:  call       "System.Threading.Lock System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>.GetResult()"
+                IL_006e:  stfld      "System.Threading.Lock C.<Main>d__0.<>s__1"
+                IL_0073:  ldarg.0
+                IL_0074:  ldfld      "System.Threading.Lock C.<Main>d__0.<>s__1"
+                IL_0079:  callvirt   "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_007e:  stloc.1
+                IL_007f:  ldarg.0
+                IL_0080:  ldnull
+                IL_0081:  stfld      "System.Threading.Lock C.<Main>d__0.<>s__1"
+                .try
+                {
+                  IL_0086:  nop
+                  IL_0087:  ldstr      "L"
+                  IL_008c:  call       "void System.Console.Write(string)"
+                  IL_0091:  nop
+                  IL_0092:  nop
+                  IL_0093:  leave.s    IL_00a2
+                }
+                finally
+                {
+                  IL_0095:  ldloc.0
+                  IL_0096:  ldc.i4.0
+                  IL_0097:  bge.s      IL_00a1
+                  IL_0099:  ldloca.s   V_1
+                  IL_009b:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_00a0:  nop
+                  IL_00a1:  endfinally
+                }
+                IL_00a2:  leave.s    IL_00be
+              }
+              catch System.Exception
+              {
+                IL_00a4:  stloc.s    V_4
+                IL_00a6:  ldarg.0
+                IL_00a7:  ldc.i4.s   -2
+                IL_00a9:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_00ae:  ldarg.0
+                IL_00af:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_00b4:  ldloc.s    V_4
+                IL_00b6:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_00bb:  nop
+                IL_00bc:  leave.s    IL_00d2
+              }
+              IL_00be:  ldarg.0
+              IL_00bf:  ldc.i4.s   -2
+              IL_00c1:  stfld      "int C.<Main>d__0.<>1__state"
+              IL_00c6:  ldarg.0
+              IL_00c7:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+              IL_00cc:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_00d1:  nop
+              IL_00d2:  ret
+            }
+            """);
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("C.<Main>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      172 (0xac)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> V_2,
+                            System.Exception V_3)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int C.<Main>d__0.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  brfalse.s  IL_003e
+                IL_000a:  call       "System.Threading.Tasks.Task<System.Threading.Lock> C.GetLock()"
+                IL_000f:  callvirt   "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> System.Threading.Tasks.Task<System.Threading.Lock>.GetAwaiter()"
+                IL_0014:  stloc.2
+                IL_0015:  ldloca.s   V_2
+                IL_0017:  call       "bool System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>.IsCompleted.get"
+                IL_001c:  brtrue.s   IL_005a
+                IL_001e:  ldarg.0
+                IL_001f:  ldc.i4.0
+                IL_0020:  dup
+                IL_0021:  stloc.0
+                IL_0022:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_0027:  ldarg.0
+                IL_0028:  ldloc.2
+                IL_0029:  stfld      "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> C.<Main>d__0.<>u__1"
+                IL_002e:  ldarg.0
+                IL_002f:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_0034:  ldloca.s   V_2
+                IL_0036:  ldarg.0
+                IL_0037:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>, C.<Main>d__0>(ref System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>, ref C.<Main>d__0)"
+                IL_003c:  leave.s    IL_00ab
+                IL_003e:  ldarg.0
+                IL_003f:  ldfld      "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> C.<Main>d__0.<>u__1"
+                IL_0044:  stloc.2
+                IL_0045:  ldarg.0
+                IL_0046:  ldflda     "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock> C.<Main>d__0.<>u__1"
+                IL_004b:  initobj    "System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>"
+                IL_0051:  ldarg.0
+                IL_0052:  ldc.i4.m1
+                IL_0053:  dup
+                IL_0054:  stloc.0
+                IL_0055:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_005a:  ldloca.s   V_2
+                IL_005c:  call       "System.Threading.Lock System.Runtime.CompilerServices.TaskAwaiter<System.Threading.Lock>.GetResult()"
+                IL_0061:  callvirt   "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0066:  stloc.1
+                .try
+                {
+                  IL_0067:  ldstr      "L"
+                  IL_006c:  call       "void System.Console.Write(string)"
+                  IL_0071:  leave.s    IL_007f
+                }
+                finally
+                {
+                  IL_0073:  ldloc.0
+                  IL_0074:  ldc.i4.0
+                  IL_0075:  bge.s      IL_007e
+                  IL_0077:  ldloca.s   V_1
+                  IL_0079:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_007e:  endfinally
+                }
+                IL_007f:  leave.s    IL_0098
+              }
+              catch System.Exception
+              {
+                IL_0081:  stloc.3
+                IL_0082:  ldarg.0
+                IL_0083:  ldc.i4.s   -2
+                IL_0085:  stfld      "int C.<Main>d__0.<>1__state"
+                IL_008a:  ldarg.0
+                IL_008b:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+                IL_0090:  ldloc.3
+                IL_0091:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_0096:  leave.s    IL_00ab
+              }
+              IL_0098:  ldarg.0
+              IL_0099:  ldc.i4.s   -2
+              IL_009b:  stfld      "int C.<Main>d__0.<>1__state"
+              IL_00a0:  ldarg.0
+              IL_00a1:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder"
+              IL_00a6:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_00ab:  ret
+            }
+            """);
     }
 
     [Fact]
@@ -1921,15 +2546,132 @@ public void AsyncLocalFunction()
 
             async void local()
             {
-                lock (new Lock()) { }
+                lock (new Lock()) { System.Console.Write("L"); }
             }
 
             local();
             """;
-        CreateCompilation([source, LockTypeDefinition]).VerifyDiagnostics(
-            // (6,11): error CS9217: A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.
-            //     lock (new Lock()) { }
-            Diagnostic(ErrorCode.ERR_BadSpecialByRefLock, "new Lock()").WithLocation(6, 11));
+        var expectedOutput = "ELD";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<<<Main>$>g__local|0_0>d.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size       94 (0x5e)
+              .maxstack  2
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Exception V_2)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  nop
+                IL_0008:  newobj     "System.Threading.Lock..ctor()"
+                IL_000d:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0012:  stloc.1
+                .try
+                {
+                  IL_0013:  nop
+                  IL_0014:  ldstr      "L"
+                  IL_0019:  call       "void System.Console.Write(string)"
+                  IL_001e:  nop
+                  IL_001f:  nop
+                  IL_0020:  leave.s    IL_002f
+                }
+                finally
+                {
+                  IL_0022:  ldloc.0
+                  IL_0023:  ldc.i4.0
+                  IL_0024:  bge.s      IL_002e
+                  IL_0026:  ldloca.s   V_1
+                  IL_0028:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_002d:  nop
+                  IL_002e:  endfinally
+                }
+                IL_002f:  leave.s    IL_0049
+              }
+              catch System.Exception
+              {
+                IL_0031:  stloc.2
+                IL_0032:  ldarg.0
+                IL_0033:  ldc.i4.s   -2
+                IL_0035:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_003a:  ldarg.0
+                IL_003b:  ldflda     "System.Runtime.CompilerServices.AsyncVoidMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+                IL_0040:  ldloc.2
+                IL_0041:  call       "void System.Runtime.CompilerServices.AsyncVoidMethodBuilder.SetException(System.Exception)"
+                IL_0046:  nop
+                IL_0047:  leave.s    IL_005d
+              }
+              IL_0049:  ldarg.0
+              IL_004a:  ldc.i4.s   -2
+              IL_004c:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+              IL_0051:  ldarg.0
+              IL_0052:  ldflda     "System.Runtime.CompilerServices.AsyncVoidMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+              IL_0057:  call       "void System.Runtime.CompilerServices.AsyncVoidMethodBuilder.SetResult()"
+              IL_005c:  nop
+              IL_005d:  ret
+            }
+            """);
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<<<Main>$>g__local|0_0>d.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size       87 (0x57)
+              .maxstack  2
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Exception V_2)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  newobj     "System.Threading.Lock..ctor()"
+                IL_000c:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0011:  stloc.1
+                .try
+                {
+                  IL_0012:  ldstr      "L"
+                  IL_0017:  call       "void System.Console.Write(string)"
+                  IL_001c:  leave.s    IL_002a
+                }
+                finally
+                {
+                  IL_001e:  ldloc.0
+                  IL_001f:  ldc.i4.0
+                  IL_0020:  bge.s      IL_0029
+                  IL_0022:  ldloca.s   V_1
+                  IL_0024:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_0029:  endfinally
+                }
+                IL_002a:  leave.s    IL_0043
+              }
+              catch System.Exception
+              {
+                IL_002c:  stloc.2
+                IL_002d:  ldarg.0
+                IL_002e:  ldc.i4.s   -2
+                IL_0030:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_0035:  ldarg.0
+                IL_0036:  ldflda     "System.Runtime.CompilerServices.AsyncVoidMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+                IL_003b:  ldloc.2
+                IL_003c:  call       "void System.Runtime.CompilerServices.AsyncVoidMethodBuilder.SetException(System.Exception)"
+                IL_0041:  leave.s    IL_0056
+              }
+              IL_0043:  ldarg.0
+              IL_0044:  ldc.i4.s   -2
+              IL_0046:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+              IL_004b:  ldarg.0
+              IL_004c:  ldflda     "System.Runtime.CompilerServices.AsyncVoidMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+              IL_0051:  call       "void System.Runtime.CompilerServices.AsyncVoidMethodBuilder.SetResult()"
+              IL_0056:  ret
+            }
+            """);
     }
 
     [Fact]
@@ -1939,18 +2681,304 @@ public void AsyncLocalFunction_WithAwait()
             using System.Threading;
             using System.Threading.Tasks;
 
-            async void local()
+            async Task local()
             {
                 await Task.Yield();
-                lock (new Lock()) { }
+                lock (new Lock()) { System.Console.Write("L"); }
+                await Task.Yield();
             }
 
-            local();
+            await local();
             """;
-        CreateCompilation([source, LockTypeDefinition]).VerifyDiagnostics(
-            // (7,11): error CS9217: A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.
-            //     lock (new Lock()) { }
-            Diagnostic(ErrorCode.ERR_BadSpecialByRefLock, "new Lock()").WithLocation(7, 11));
+        var expectedOutput = "ELD";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<<<Main>$>g__local|0_0>d.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      311 (0x137)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_1,
+                            System.Runtime.CompilerServices.YieldAwaitable V_2,
+                            Program.<<<Main>$>g__local|0_0>d V_3,
+                            System.Threading.Lock.Scope V_4,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_5,
+                            System.Exception V_6)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  brfalse.s  IL_0012
+                IL_000a:  br.s       IL_000c
+                IL_000c:  ldloc.0
+                IL_000d:  ldc.i4.1
+                IL_000e:  beq.s      IL_0014
+                IL_0010:  br.s       IL_0019
+                IL_0012:  br.s       IL_0058
+                IL_0014:  br         IL_00e1
+                IL_0019:  nop
+                IL_001a:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_001f:  stloc.2
+                IL_0020:  ldloca.s   V_2
+                IL_0022:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_0027:  stloc.1
+                IL_0028:  ldloca.s   V_1
+                IL_002a:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_002f:  brtrue.s   IL_0074
+                IL_0031:  ldarg.0
+                IL_0032:  ldc.i4.0
+                IL_0033:  dup
+                IL_0034:  stloc.0
+                IL_0035:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_003a:  ldarg.0
+                IL_003b:  ldloc.1
+                IL_003c:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_0041:  ldarg.0
+                IL_0042:  stloc.3
+                IL_0043:  ldarg.0
+                IL_0044:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+                IL_0049:  ldloca.s   V_1
+                IL_004b:  ldloca.s   V_3
+                IL_004d:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<<<Main>$>g__local|0_0>d>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<<<Main>$>g__local|0_0>d)"
+                IL_0052:  nop
+                IL_0053:  leave      IL_0136
+                IL_0058:  ldarg.0
+                IL_0059:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_005e:  stloc.1
+                IL_005f:  ldarg.0
+                IL_0060:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_0065:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_006b:  ldarg.0
+                IL_006c:  ldc.i4.m1
+                IL_006d:  dup
+                IL_006e:  stloc.0
+                IL_006f:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_0074:  ldloca.s   V_1
+                IL_0076:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_007b:  nop
+                IL_007c:  newobj     "System.Threading.Lock..ctor()"
+                IL_0081:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0086:  stloc.s    V_4
+                .try
+                {
+                  IL_0088:  nop
+                  IL_0089:  ldstr      "L"
+                  IL_008e:  call       "void System.Console.Write(string)"
+                  IL_0093:  nop
+                  IL_0094:  nop
+                  IL_0095:  leave.s    IL_00a4
+                }
+                finally
+                {
+                  IL_0097:  ldloc.0
+                  IL_0098:  ldc.i4.0
+                  IL_0099:  bge.s      IL_00a3
+                  IL_009b:  ldloca.s   V_4
+                  IL_009d:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_00a2:  nop
+                  IL_00a3:  endfinally
+                }
+                IL_00a4:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_00a9:  stloc.2
+                IL_00aa:  ldloca.s   V_2
+                IL_00ac:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_00b1:  stloc.s    V_5
+                IL_00b3:  ldloca.s   V_5
+                IL_00b5:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_00ba:  brtrue.s   IL_00fe
+                IL_00bc:  ldarg.0
+                IL_00bd:  ldc.i4.1
+                IL_00be:  dup
+                IL_00bf:  stloc.0
+                IL_00c0:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_00c5:  ldarg.0
+                IL_00c6:  ldloc.s    V_5
+                IL_00c8:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_00cd:  ldarg.0
+                IL_00ce:  stloc.3
+                IL_00cf:  ldarg.0
+                IL_00d0:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+                IL_00d5:  ldloca.s   V_5
+                IL_00d7:  ldloca.s   V_3
+                IL_00d9:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<<<Main>$>g__local|0_0>d>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<<<Main>$>g__local|0_0>d)"
+                IL_00de:  nop
+                IL_00df:  leave.s    IL_0136
+                IL_00e1:  ldarg.0
+                IL_00e2:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_00e7:  stloc.s    V_5
+                IL_00e9:  ldarg.0
+                IL_00ea:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_00ef:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_00f5:  ldarg.0
+                IL_00f6:  ldc.i4.m1
+                IL_00f7:  dup
+                IL_00f8:  stloc.0
+                IL_00f9:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_00fe:  ldloca.s   V_5
+                IL_0100:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_0105:  nop
+                IL_0106:  leave.s    IL_0122
+              }
+              catch System.Exception
+              {
+                IL_0108:  stloc.s    V_6
+                IL_010a:  ldarg.0
+                IL_010b:  ldc.i4.s   -2
+                IL_010d:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_0112:  ldarg.0
+                IL_0113:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+                IL_0118:  ldloc.s    V_6
+                IL_011a:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_011f:  nop
+                IL_0120:  leave.s    IL_0136
+              }
+              IL_0122:  ldarg.0
+              IL_0123:  ldc.i4.s   -2
+              IL_0125:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+              IL_012a:  ldarg.0
+              IL_012b:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+              IL_0130:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_0135:  nop
+              IL_0136:  ret
+            }
+            """);
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<<<Main>$>g__local|0_0>d.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      282 (0x11a)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_1,
+                            System.Runtime.CompilerServices.YieldAwaitable V_2,
+                            System.Threading.Lock.Scope V_3,
+                            System.Exception V_4)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  brfalse.s  IL_004b
+                IL_000a:  ldloc.0
+                IL_000b:  ldc.i4.1
+                IL_000c:  beq        IL_00c8
+                IL_0011:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_0016:  stloc.2
+                IL_0017:  ldloca.s   V_2
+                IL_0019:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_001e:  stloc.1
+                IL_001f:  ldloca.s   V_1
+                IL_0021:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_0026:  brtrue.s   IL_0067
+                IL_0028:  ldarg.0
+                IL_0029:  ldc.i4.0
+                IL_002a:  dup
+                IL_002b:  stloc.0
+                IL_002c:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_0031:  ldarg.0
+                IL_0032:  ldloc.1
+                IL_0033:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_0038:  ldarg.0
+                IL_0039:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+                IL_003e:  ldloca.s   V_1
+                IL_0040:  ldarg.0
+                IL_0041:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<<<Main>$>g__local|0_0>d>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<<<Main>$>g__local|0_0>d)"
+                IL_0046:  leave      IL_0119
+                IL_004b:  ldarg.0
+                IL_004c:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_0051:  stloc.1
+                IL_0052:  ldarg.0
+                IL_0053:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_0058:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_005e:  ldarg.0
+                IL_005f:  ldc.i4.m1
+                IL_0060:  dup
+                IL_0061:  stloc.0
+                IL_0062:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_0067:  ldloca.s   V_1
+                IL_0069:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_006e:  newobj     "System.Threading.Lock..ctor()"
+                IL_0073:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0078:  stloc.3
+                .try
+                {
+                  IL_0079:  ldstr      "L"
+                  IL_007e:  call       "void System.Console.Write(string)"
+                  IL_0083:  leave.s    IL_0091
+                }
+                finally
+                {
+                  IL_0085:  ldloc.0
+                  IL_0086:  ldc.i4.0
+                  IL_0087:  bge.s      IL_0090
+                  IL_0089:  ldloca.s   V_3
+                  IL_008b:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_0090:  endfinally
+                }
+                IL_0091:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_0096:  stloc.2
+                IL_0097:  ldloca.s   V_2
+                IL_0099:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_009e:  stloc.1
+                IL_009f:  ldloca.s   V_1
+                IL_00a1:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_00a6:  brtrue.s   IL_00e4
+                IL_00a8:  ldarg.0
+                IL_00a9:  ldc.i4.1
+                IL_00aa:  dup
+                IL_00ab:  stloc.0
+                IL_00ac:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_00b1:  ldarg.0
+                IL_00b2:  ldloc.1
+                IL_00b3:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_00b8:  ldarg.0
+                IL_00b9:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+                IL_00be:  ldloca.s   V_1
+                IL_00c0:  ldarg.0
+                IL_00c1:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<<<Main>$>g__local|0_0>d>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<<<Main>$>g__local|0_0>d)"
+                IL_00c6:  leave.s    IL_0119
+                IL_00c8:  ldarg.0
+                IL_00c9:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_00ce:  stloc.1
+                IL_00cf:  ldarg.0
+                IL_00d0:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<<<Main>$>g__local|0_0>d.<>u__1"
+                IL_00d5:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_00db:  ldarg.0
+                IL_00dc:  ldc.i4.m1
+                IL_00dd:  dup
+                IL_00de:  stloc.0
+                IL_00df:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_00e4:  ldloca.s   V_1
+                IL_00e6:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_00eb:  leave.s    IL_0106
+              }
+              catch System.Exception
+              {
+                IL_00ed:  stloc.s    V_4
+                IL_00ef:  ldarg.0
+                IL_00f0:  ldc.i4.s   -2
+                IL_00f2:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+                IL_00f7:  ldarg.0
+                IL_00f8:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+                IL_00fd:  ldloc.s    V_4
+                IL_00ff:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_0104:  leave.s    IL_0119
+              }
+              IL_0106:  ldarg.0
+              IL_0107:  ldc.i4.s   -2
+              IL_0109:  stfld      "int Program.<<<Main>$>g__local|0_0>d.<>1__state"
+              IL_010e:  ldarg.0
+              IL_010f:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<<<Main>$>g__local|0_0>d.<>t__builder"
+              IL_0114:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_0119:  ret
+            }
+            """);
     }
 
     [Fact]
@@ -1962,13 +2990,132 @@ public void AsyncLambda()
 
             var lam = async () =>
             {
-                lock (new Lock()) { }
+                lock (new Lock()) { System.Console.Write("L"); }
             };
+
+            await lam();
             """;
-        CreateCompilation([source, LockTypeDefinition]).VerifyDiagnostics(
-            // (6,11): error CS9217: A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.
-            //     lock (new Lock()) { }
-            Diagnostic(ErrorCode.ERR_BadSpecialByRefLock, "new Lock()").WithLocation(6, 11));
+        var expectedOutput = "ELD";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<>c.<<<Main>$>b__0_0>d.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size       94 (0x5e)
+              .maxstack  2
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Exception V_2)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  nop
+                IL_0008:  newobj     "System.Threading.Lock..ctor()"
+                IL_000d:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0012:  stloc.1
+                .try
+                {
+                  IL_0013:  nop
+                  IL_0014:  ldstr      "L"
+                  IL_0019:  call       "void System.Console.Write(string)"
+                  IL_001e:  nop
+                  IL_001f:  nop
+                  IL_0020:  leave.s    IL_002f
+                }
+                finally
+                {
+                  IL_0022:  ldloc.0
+                  IL_0023:  ldc.i4.0
+                  IL_0024:  bge.s      IL_002e
+                  IL_0026:  ldloca.s   V_1
+                  IL_0028:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_002d:  nop
+                  IL_002e:  endfinally
+                }
+                IL_002f:  leave.s    IL_0049
+              }
+              catch System.Exception
+              {
+                IL_0031:  stloc.2
+                IL_0032:  ldarg.0
+                IL_0033:  ldc.i4.s   -2
+                IL_0035:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_003a:  ldarg.0
+                IL_003b:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+                IL_0040:  ldloc.2
+                IL_0041:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_0046:  nop
+                IL_0047:  leave.s    IL_005d
+              }
+              IL_0049:  ldarg.0
+              IL_004a:  ldc.i4.s   -2
+              IL_004c:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+              IL_0051:  ldarg.0
+              IL_0052:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+              IL_0057:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_005c:  nop
+              IL_005d:  ret
+            }
+            """);
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<>c.<<<Main>$>b__0_0>d.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size       87 (0x57)
+              .maxstack  2
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Exception V_2)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  newobj     "System.Threading.Lock..ctor()"
+                IL_000c:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0011:  stloc.1
+                .try
+                {
+                  IL_0012:  ldstr      "L"
+                  IL_0017:  call       "void System.Console.Write(string)"
+                  IL_001c:  leave.s    IL_002a
+                }
+                finally
+                {
+                  IL_001e:  ldloc.0
+                  IL_001f:  ldc.i4.0
+                  IL_0020:  bge.s      IL_0029
+                  IL_0022:  ldloca.s   V_1
+                  IL_0024:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_0029:  endfinally
+                }
+                IL_002a:  leave.s    IL_0043
+              }
+              catch System.Exception
+              {
+                IL_002c:  stloc.2
+                IL_002d:  ldarg.0
+                IL_002e:  ldc.i4.s   -2
+                IL_0030:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_0035:  ldarg.0
+                IL_0036:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+                IL_003b:  ldloc.2
+                IL_003c:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_0041:  leave.s    IL_0056
+              }
+              IL_0043:  ldarg.0
+              IL_0044:  ldc.i4.s   -2
+              IL_0046:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+              IL_004b:  ldarg.0
+              IL_004c:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+              IL_0051:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_0056:  ret
+            }
+            """);
     }
 
     [Fact]
@@ -1981,13 +3128,301 @@ public void AsyncLambda_WithAwait()
             var lam = async () =>
             {
                 await Task.Yield();
-                lock (new Lock()) { }
+                lock (new Lock()) { System.Console.Write("L"); }
+                await Task.Yield();
             };
+
+            await lam();
             """;
-        CreateCompilation([source, LockTypeDefinition]).VerifyDiagnostics(
-            // (7,11): error CS9217: A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.
-            //     lock (new Lock()) { }
-            Diagnostic(ErrorCode.ERR_BadSpecialByRefLock, "new Lock()").WithLocation(7, 11));
+        var expectedOutput = "ELD";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<>c.<<<Main>$>b__0_0>d.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      311 (0x137)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_1,
+                            System.Runtime.CompilerServices.YieldAwaitable V_2,
+                            Program.<>c.<<<Main>$>b__0_0>d V_3,
+                            System.Threading.Lock.Scope V_4,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_5,
+                            System.Exception V_6)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  brfalse.s  IL_0012
+                IL_000a:  br.s       IL_000c
+                IL_000c:  ldloc.0
+                IL_000d:  ldc.i4.1
+                IL_000e:  beq.s      IL_0014
+                IL_0010:  br.s       IL_0019
+                IL_0012:  br.s       IL_0058
+                IL_0014:  br         IL_00e1
+                IL_0019:  nop
+                IL_001a:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_001f:  stloc.2
+                IL_0020:  ldloca.s   V_2
+                IL_0022:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_0027:  stloc.1
+                IL_0028:  ldloca.s   V_1
+                IL_002a:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_002f:  brtrue.s   IL_0074
+                IL_0031:  ldarg.0
+                IL_0032:  ldc.i4.0
+                IL_0033:  dup
+                IL_0034:  stloc.0
+                IL_0035:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_003a:  ldarg.0
+                IL_003b:  ldloc.1
+                IL_003c:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_0041:  ldarg.0
+                IL_0042:  stloc.3
+                IL_0043:  ldarg.0
+                IL_0044:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+                IL_0049:  ldloca.s   V_1
+                IL_004b:  ldloca.s   V_3
+                IL_004d:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<>c.<<<Main>$>b__0_0>d>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<>c.<<<Main>$>b__0_0>d)"
+                IL_0052:  nop
+                IL_0053:  leave      IL_0136
+                IL_0058:  ldarg.0
+                IL_0059:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_005e:  stloc.1
+                IL_005f:  ldarg.0
+                IL_0060:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_0065:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_006b:  ldarg.0
+                IL_006c:  ldc.i4.m1
+                IL_006d:  dup
+                IL_006e:  stloc.0
+                IL_006f:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_0074:  ldloca.s   V_1
+                IL_0076:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_007b:  nop
+                IL_007c:  newobj     "System.Threading.Lock..ctor()"
+                IL_0081:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0086:  stloc.s    V_4
+                .try
+                {
+                  IL_0088:  nop
+                  IL_0089:  ldstr      "L"
+                  IL_008e:  call       "void System.Console.Write(string)"
+                  IL_0093:  nop
+                  IL_0094:  nop
+                  IL_0095:  leave.s    IL_00a4
+                }
+                finally
+                {
+                  IL_0097:  ldloc.0
+                  IL_0098:  ldc.i4.0
+                  IL_0099:  bge.s      IL_00a3
+                  IL_009b:  ldloca.s   V_4
+                  IL_009d:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_00a2:  nop
+                  IL_00a3:  endfinally
+                }
+                IL_00a4:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_00a9:  stloc.2
+                IL_00aa:  ldloca.s   V_2
+                IL_00ac:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_00b1:  stloc.s    V_5
+                IL_00b3:  ldloca.s   V_5
+                IL_00b5:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_00ba:  brtrue.s   IL_00fe
+                IL_00bc:  ldarg.0
+                IL_00bd:  ldc.i4.1
+                IL_00be:  dup
+                IL_00bf:  stloc.0
+                IL_00c0:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_00c5:  ldarg.0
+                IL_00c6:  ldloc.s    V_5
+                IL_00c8:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_00cd:  ldarg.0
+                IL_00ce:  stloc.3
+                IL_00cf:  ldarg.0
+                IL_00d0:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+                IL_00d5:  ldloca.s   V_5
+                IL_00d7:  ldloca.s   V_3
+                IL_00d9:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<>c.<<<Main>$>b__0_0>d>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<>c.<<<Main>$>b__0_0>d)"
+                IL_00de:  nop
+                IL_00df:  leave.s    IL_0136
+                IL_00e1:  ldarg.0
+                IL_00e2:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_00e7:  stloc.s    V_5
+                IL_00e9:  ldarg.0
+                IL_00ea:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_00ef:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_00f5:  ldarg.0
+                IL_00f6:  ldc.i4.m1
+                IL_00f7:  dup
+                IL_00f8:  stloc.0
+                IL_00f9:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_00fe:  ldloca.s   V_5
+                IL_0100:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_0105:  nop
+                IL_0106:  leave.s    IL_0122
+              }
+              catch System.Exception
+              {
+                IL_0108:  stloc.s    V_6
+                IL_010a:  ldarg.0
+                IL_010b:  ldc.i4.s   -2
+                IL_010d:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_0112:  ldarg.0
+                IL_0113:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+                IL_0118:  ldloc.s    V_6
+                IL_011a:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_011f:  nop
+                IL_0120:  leave.s    IL_0136
+              }
+              IL_0122:  ldarg.0
+              IL_0123:  ldc.i4.s   -2
+              IL_0125:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+              IL_012a:  ldarg.0
+              IL_012b:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+              IL_0130:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_0135:  nop
+              IL_0136:  ret
+            }
+            """);
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<>c.<<<Main>$>b__0_0>d.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      282 (0x11a)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_1,
+                            System.Runtime.CompilerServices.YieldAwaitable V_2,
+                            System.Threading.Lock.Scope V_3,
+                            System.Exception V_4)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  brfalse.s  IL_004b
+                IL_000a:  ldloc.0
+                IL_000b:  ldc.i4.1
+                IL_000c:  beq        IL_00c8
+                IL_0011:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_0016:  stloc.2
+                IL_0017:  ldloca.s   V_2
+                IL_0019:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_001e:  stloc.1
+                IL_001f:  ldloca.s   V_1
+                IL_0021:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_0026:  brtrue.s   IL_0067
+                IL_0028:  ldarg.0
+                IL_0029:  ldc.i4.0
+                IL_002a:  dup
+                IL_002b:  stloc.0
+                IL_002c:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_0031:  ldarg.0
+                IL_0032:  ldloc.1
+                IL_0033:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_0038:  ldarg.0
+                IL_0039:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+                IL_003e:  ldloca.s   V_1
+                IL_0040:  ldarg.0
+                IL_0041:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<>c.<<<Main>$>b__0_0>d>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<>c.<<<Main>$>b__0_0>d)"
+                IL_0046:  leave      IL_0119
+                IL_004b:  ldarg.0
+                IL_004c:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_0051:  stloc.1
+                IL_0052:  ldarg.0
+                IL_0053:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_0058:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_005e:  ldarg.0
+                IL_005f:  ldc.i4.m1
+                IL_0060:  dup
+                IL_0061:  stloc.0
+                IL_0062:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_0067:  ldloca.s   V_1
+                IL_0069:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_006e:  newobj     "System.Threading.Lock..ctor()"
+                IL_0073:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0078:  stloc.3
+                .try
+                {
+                  IL_0079:  ldstr      "L"
+                  IL_007e:  call       "void System.Console.Write(string)"
+                  IL_0083:  leave.s    IL_0091
+                }
+                finally
+                {
+                  IL_0085:  ldloc.0
+                  IL_0086:  ldc.i4.0
+                  IL_0087:  bge.s      IL_0090
+                  IL_0089:  ldloca.s   V_3
+                  IL_008b:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_0090:  endfinally
+                }
+                IL_0091:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_0096:  stloc.2
+                IL_0097:  ldloca.s   V_2
+                IL_0099:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_009e:  stloc.1
+                IL_009f:  ldloca.s   V_1
+                IL_00a1:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_00a6:  brtrue.s   IL_00e4
+                IL_00a8:  ldarg.0
+                IL_00a9:  ldc.i4.1
+                IL_00aa:  dup
+                IL_00ab:  stloc.0
+                IL_00ac:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_00b1:  ldarg.0
+                IL_00b2:  ldloc.1
+                IL_00b3:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_00b8:  ldarg.0
+                IL_00b9:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+                IL_00be:  ldloca.s   V_1
+                IL_00c0:  ldarg.0
+                IL_00c1:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<>c.<<<Main>$>b__0_0>d>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<>c.<<<Main>$>b__0_0>d)"
+                IL_00c6:  leave.s    IL_0119
+                IL_00c8:  ldarg.0
+                IL_00c9:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_00ce:  stloc.1
+                IL_00cf:  ldarg.0
+                IL_00d0:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<>c.<<<Main>$>b__0_0>d.<>u__1"
+                IL_00d5:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_00db:  ldarg.0
+                IL_00dc:  ldc.i4.m1
+                IL_00dd:  dup
+                IL_00de:  stloc.0
+                IL_00df:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_00e4:  ldloca.s   V_1
+                IL_00e6:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_00eb:  leave.s    IL_0106
+              }
+              catch System.Exception
+              {
+                IL_00ed:  stloc.s    V_4
+                IL_00ef:  ldarg.0
+                IL_00f0:  ldc.i4.s   -2
+                IL_00f2:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+                IL_00f7:  ldarg.0
+                IL_00f8:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+                IL_00fd:  ldloc.s    V_4
+                IL_00ff:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)"
+                IL_0104:  leave.s    IL_0119
+              }
+              IL_0106:  ldarg.0
+              IL_0107:  ldc.i4.s   -2
+              IL_0109:  stfld      "int Program.<>c.<<<Main>$>b__0_0>d.<>1__state"
+              IL_010e:  ldarg.0
+              IL_010f:  ldflda     "System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<>c.<<<Main>$>b__0_0>d.<>t__builder"
+              IL_0114:  call       "void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()"
+              IL_0119:  ret
+            }
+            """);
     }
 
     [Fact]
@@ -2011,9 +3446,93 @@ IEnumerable<int> M()
             }
             """;
         CreateCompilation([source, LockTypeDefinition]).VerifyEmitDiagnostics(
-            // (9,15): error CS4013: Instance of type 'Lock.Scope' cannot be used inside a nested function, query expression, iterator block or async method
+            // (9,15): error CS4007: Instance of type 'System.Threading.Lock.Scope' cannot be preserved across 'await' or 'yield' boundary.
             //         lock (new Lock())
-            Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "new Lock()").WithArguments("System.Threading.Lock.Scope").WithLocation(9, 15));
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "new Lock()").WithArguments("System.Threading.Lock.Scope").WithLocation(9, 15),
+            // (11,13): warning CS9237: 'yield return' should not be used in the body of a lock statement
+            //             yield return 2;
+            Diagnostic(ErrorCode.WRN_BadYieldInLock, "yield").WithLocation(11, 13));
+    }
+
+    [Fact]
+    public void Yield_Break()
+    {
+        var source = """
+            #pragma warning disable CS0162 // Unreachable code detected
+            using System;
+            using System.Collections.Generic;
+            using System.Threading;
+
+            static class Program
+            {
+                static void Main()
+                {
+                    foreach (var x in M())
+                    {
+                        Console.Write(x);
+                    }
+                }
+
+                static IEnumerable<int> M()
+                {
+                    yield return 1;
+                    lock (new Lock())
+                    {
+                        Console.Write("L");
+                        yield break;
+                        Console.Write("B");
+                    }
+                    yield return 2;
+                }
+            }
+            """;
+        var expectedOutput = "1ELD";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            verify: Verification.FailsILVerify, expectedOutput: expectedOutput);
+        verifier.VerifyDiagnostics();
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            verify: Verification.FailsILVerify, expectedOutput: expectedOutput);
+        verifier.VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void Yield_AroundOnly()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading;
+
+            static class Program
+            {
+                static void Main()
+                {
+                    foreach (var x in M())
+                    {
+                        Console.Write(x);
+                    }
+                }
+
+                static IEnumerable<int> M()
+                {
+                    yield return 1;
+                    lock (new Lock())
+                    {
+                        Console.Write("L");
+                    }
+                    yield return 2;
+                }
+            }
+            """;
+        var expectedOutput = "1ELD2";
+        var verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.ReleaseExe,
+            verify: Verification.FailsILVerify, expectedOutput: expectedOutput);
+        verifier.VerifyDiagnostics();
+
+        verifier = CompileAndVerify([source, LockTypeDefinition], options: TestOptions.DebugExe,
+            verify: Verification.FailsILVerify, expectedOutput: expectedOutput);
+        verifier.VerifyDiagnostics();
     }
 
     [Fact]
@@ -2038,10 +3557,455 @@ async IAsyncEnumerable<int> M()
                 }
             }
             """;
-        CreateCompilationWithTasksExtensions([source, LockTypeDefinition, AsyncStreamsTypes]).VerifyDiagnostics(
-            // (10,15): error CS9217: A lock statement on a value of type 'System.Threading.Lock' cannot be used in async methods or async lambda expressions.
+        CreateCompilationWithTasksExtensions([source, LockTypeDefinition, AsyncStreamsTypes]).VerifyEmitDiagnostics(
+            // (10,15): error CS4007: Instance of type 'System.Threading.Lock.Scope' cannot be preserved across 'await' or 'yield' boundary.
             //         lock (new Lock())
-            Diagnostic(ErrorCode.ERR_BadSpecialByRefLock, "new Lock()").WithLocation(10, 15));
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "new Lock()").WithArguments("System.Threading.Lock.Scope").WithLocation(10, 15),
+            // (12,13): warning CS9237: 'yield return' should not be used in the body of a lock statement
+            //             yield return 2;
+            Diagnostic(ErrorCode.WRN_BadYieldInLock, "yield").WithLocation(12, 13));
+    }
+
+    [Fact]
+    public void Yield_Async_Break()
+    {
+        var source = """
+            #pragma warning disable CS0162 // Unreachable code detected
+            using System;
+            using System.Collections.Generic;
+            using System.Threading;
+            using System.Threading.Tasks;
+
+            static class Program
+            {
+                static async Task Main()
+                {
+                    await foreach (var x in M())
+                    {
+                        Console.Write(x);
+                    }
+                }
+
+                async static IAsyncEnumerable<int> M()
+                {
+                    yield return 1;
+                    await Task.Yield();
+                    lock (new Lock())
+                    {
+                        Console.Write("L");
+                        yield break;
+                        Console.Write("B");
+                    }
+                    await Task.Yield();
+                    yield return 2;
+                }
+            }
+            """;
+        var expectedOutput = "1ELD";
+        var comp = CreateCompilationWithTasksExtensions([source, LockTypeDefinition, AsyncStreamsTypes], options: TestOptions.ReleaseExe);
+        var verifier = CompileAndVerify(comp, verify: Verification.FailsILVerify, expectedOutput: expectedOutput);
+        verifier.VerifyDiagnostics();
+
+        comp = CreateCompilationWithTasksExtensions([source, LockTypeDefinition, AsyncStreamsTypes], options: TestOptions.DebugExe);
+        verifier = CompileAndVerify(comp, verify: Verification.FailsILVerify, expectedOutput: expectedOutput);
+        verifier.VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void Yield_Async_AroundOnly()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading;
+            using System.Threading.Tasks;
+
+            static class Program
+            {
+                static async Task Main()
+                {
+                    await foreach (var x in M())
+                    {
+                        Console.Write(x);
+                    }
+                }
+
+                static async IAsyncEnumerable<int> M()
+                {
+                    yield return 1;
+                    lock (new Lock())
+                    {
+                        Console.Write("L");
+                    }
+                    await Task.Yield();
+                    yield return 2;
+                }
+            }
+            """;
+        var expectedOutput = "1ELD2";
+        var comp = CreateCompilationWithTasksExtensions([source, LockTypeDefinition, AsyncStreamsTypes], options: TestOptions.DebugExe);
+        var verifier = CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<M>d__1.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      414 (0x19e)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_2,
+                            System.Runtime.CompilerServices.YieldAwaitable V_3,
+                            Program.<M>d__1 V_4,
+                            System.Exception V_5)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<M>d__1.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  ldc.i4.s   -5
+                IL_000a:  sub
+                IL_000b:  switch    (
+                    IL_002a,
+                    IL_002f,
+                    IL_0031,
+                    IL_0038,
+                    IL_0038,
+                    IL_0033)
+                IL_0028:  br.s       IL_0038
+                IL_002a:  br         IL_0125
+                IL_002f:  br.s       IL_0065
+                IL_0031:  br.s       IL_0038
+                IL_0033:  br         IL_00ee
+                IL_0038:  ldarg.0
+                IL_0039:  ldfld      "bool Program.<M>d__1.<>w__disposeMode"
+                IL_003e:  brfalse.s  IL_0045
+                IL_0040:  leave      IL_0167
+                IL_0045:  ldarg.0
+                IL_0046:  ldc.i4.m1
+                IL_0047:  dup
+                IL_0048:  stloc.0
+                IL_0049:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_004e:  nop
+                IL_004f:  ldarg.0
+                IL_0050:  ldc.i4.1
+                IL_0051:  stfld      "int Program.<M>d__1.<>2__current"
+                IL_0056:  ldarg.0
+                IL_0057:  ldc.i4.s   -4
+                IL_0059:  dup
+                IL_005a:  stloc.0
+                IL_005b:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_0060:  leave      IL_0190
+                IL_0065:  ldarg.0
+                IL_0066:  ldc.i4.m1
+                IL_0067:  dup
+                IL_0068:  stloc.0
+                IL_0069:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_006e:  ldarg.0
+                IL_006f:  ldfld      "bool Program.<M>d__1.<>w__disposeMode"
+                IL_0074:  brfalse.s  IL_007b
+                IL_0076:  leave      IL_0167
+                IL_007b:  newobj     "System.Threading.Lock..ctor()"
+                IL_0080:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0085:  stloc.1
+                .try
+                {
+                  IL_0086:  nop
+                  IL_0087:  ldstr      "L"
+                  IL_008c:  call       "void System.Console.Write(string)"
+                  IL_0091:  nop
+                  IL_0092:  nop
+                  IL_0093:  leave.s    IL_00a2
+                }
+                finally
+                {
+                  IL_0095:  ldloc.0
+                  IL_0096:  ldc.i4.m1
+                  IL_0097:  bne.un.s   IL_00a1
+                  IL_0099:  ldloca.s   V_1
+                  IL_009b:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_00a0:  nop
+                  IL_00a1:  endfinally
+                }
+                IL_00a2:  ldarg.0
+                IL_00a3:  ldfld      "bool Program.<M>d__1.<>w__disposeMode"
+                IL_00a8:  brfalse.s  IL_00af
+                IL_00aa:  leave      IL_0167
+                IL_00af:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_00b4:  stloc.3
+                IL_00b5:  ldloca.s   V_3
+                IL_00b7:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_00bc:  stloc.2
+                IL_00bd:  ldloca.s   V_2
+                IL_00bf:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_00c4:  brtrue.s   IL_010a
+                IL_00c6:  ldarg.0
+                IL_00c7:  ldc.i4.0
+                IL_00c8:  dup
+                IL_00c9:  stloc.0
+                IL_00ca:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_00cf:  ldarg.0
+                IL_00d0:  ldloc.2
+                IL_00d1:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<M>d__1.<>u__1"
+                IL_00d6:  ldarg.0
+                IL_00d7:  stloc.s    V_4
+                IL_00d9:  ldarg.0
+                IL_00da:  ldflda     "System.Runtime.CompilerServices.AsyncIteratorMethodBuilder Program.<M>d__1.<>t__builder"
+                IL_00df:  ldloca.s   V_2
+                IL_00e1:  ldloca.s   V_4
+                IL_00e3:  call       "void System.Runtime.CompilerServices.AsyncIteratorMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<M>d__1>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<M>d__1)"
+                IL_00e8:  nop
+                IL_00e9:  leave      IL_019d
+                IL_00ee:  ldarg.0
+                IL_00ef:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<M>d__1.<>u__1"
+                IL_00f4:  stloc.2
+                IL_00f5:  ldarg.0
+                IL_00f6:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<M>d__1.<>u__1"
+                IL_00fb:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_0101:  ldarg.0
+                IL_0102:  ldc.i4.m1
+                IL_0103:  dup
+                IL_0104:  stloc.0
+                IL_0105:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_010a:  ldloca.s   V_2
+                IL_010c:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_0111:  nop
+                IL_0112:  ldarg.0
+                IL_0113:  ldc.i4.2
+                IL_0114:  stfld      "int Program.<M>d__1.<>2__current"
+                IL_0119:  ldarg.0
+                IL_011a:  ldc.i4.s   -5
+                IL_011c:  dup
+                IL_011d:  stloc.0
+                IL_011e:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_0123:  leave.s    IL_0190
+                IL_0125:  ldarg.0
+                IL_0126:  ldc.i4.m1
+                IL_0127:  dup
+                IL_0128:  stloc.0
+                IL_0129:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_012e:  ldarg.0
+                IL_012f:  ldfld      "bool Program.<M>d__1.<>w__disposeMode"
+                IL_0134:  brfalse.s  IL_0138
+                IL_0136:  leave.s    IL_0167
+                IL_0138:  leave.s    IL_0167
+              }
+              catch System.Exception
+              {
+                IL_013a:  stloc.s    V_5
+                IL_013c:  ldarg.0
+                IL_013d:  ldc.i4.s   -2
+                IL_013f:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_0144:  ldarg.0
+                IL_0145:  ldc.i4.0
+                IL_0146:  stfld      "int Program.<M>d__1.<>2__current"
+                IL_014b:  ldarg.0
+                IL_014c:  ldflda     "System.Runtime.CompilerServices.AsyncIteratorMethodBuilder Program.<M>d__1.<>t__builder"
+                IL_0151:  call       "void System.Runtime.CompilerServices.AsyncIteratorMethodBuilder.Complete()"
+                IL_0156:  nop
+                IL_0157:  ldarg.0
+                IL_0158:  ldflda     "System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool> Program.<M>d__1.<>v__promiseOfValueOrEnd"
+                IL_015d:  ldloc.s    V_5
+                IL_015f:  call       "void System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool>.SetException(System.Exception)"
+                IL_0164:  nop
+                IL_0165:  leave.s    IL_019d
+              }
+              IL_0167:  ldarg.0
+              IL_0168:  ldc.i4.s   -2
+              IL_016a:  stfld      "int Program.<M>d__1.<>1__state"
+              IL_016f:  ldarg.0
+              IL_0170:  ldc.i4.0
+              IL_0171:  stfld      "int Program.<M>d__1.<>2__current"
+              IL_0176:  ldarg.0
+              IL_0177:  ldflda     "System.Runtime.CompilerServices.AsyncIteratorMethodBuilder Program.<M>d__1.<>t__builder"
+              IL_017c:  call       "void System.Runtime.CompilerServices.AsyncIteratorMethodBuilder.Complete()"
+              IL_0181:  nop
+              IL_0182:  ldarg.0
+              IL_0183:  ldflda     "System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool> Program.<M>d__1.<>v__promiseOfValueOrEnd"
+              IL_0188:  ldc.i4.0
+              IL_0189:  call       "void System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool>.SetResult(bool)"
+              IL_018e:  nop
+              IL_018f:  ret
+              IL_0190:  ldarg.0
+              IL_0191:  ldflda     "System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool> Program.<M>d__1.<>v__promiseOfValueOrEnd"
+              IL_0196:  ldc.i4.1
+              IL_0197:  call       "void System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool>.SetResult(bool)"
+              IL_019c:  nop
+              IL_019d:  ret
+            }
+            """);
+
+        comp = CreateCompilationWithTasksExtensions([source, LockTypeDefinition, AsyncStreamsTypes], options: TestOptions.ReleaseExe);
+        verifier = CompileAndVerify(comp, expectedOutput: expectedOutput, verify: Verification.FailsILVerify);
+        verifier.VerifyDiagnostics();
+        verifier.VerifyIL("Program.<M>d__1.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext", """
+            {
+              // Code size      383 (0x17f)
+              .maxstack  3
+              .locals init (int V_0,
+                            System.Threading.Lock.Scope V_1,
+                            System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter V_2,
+                            System.Runtime.CompilerServices.YieldAwaitable V_3,
+                            Program.<M>d__1 V_4,
+                            System.Exception V_5)
+              IL_0000:  ldarg.0
+              IL_0001:  ldfld      "int Program.<M>d__1.<>1__state"
+              IL_0006:  stloc.0
+              .try
+              {
+                IL_0007:  ldloc.0
+                IL_0008:  ldc.i4.s   -5
+                IL_000a:  sub
+                IL_000b:  switch    (
+                    IL_010e,
+                    IL_0054,
+                    IL_0028,
+                    IL_0028,
+                    IL_0028,
+                    IL_00d8)
+                IL_0028:  ldarg.0
+                IL_0029:  ldfld      "bool Program.<M>d__1.<>w__disposeMode"
+                IL_002e:  brfalse.s  IL_0035
+                IL_0030:  leave      IL_014b
+                IL_0035:  ldarg.0
+                IL_0036:  ldc.i4.m1
+                IL_0037:  dup
+                IL_0038:  stloc.0
+                IL_0039:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_003e:  ldarg.0
+                IL_003f:  ldc.i4.1
+                IL_0040:  stfld      "int Program.<M>d__1.<>2__current"
+                IL_0045:  ldarg.0
+                IL_0046:  ldc.i4.s   -4
+                IL_0048:  dup
+                IL_0049:  stloc.0
+                IL_004a:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_004f:  leave      IL_0172
+                IL_0054:  ldarg.0
+                IL_0055:  ldc.i4.m1
+                IL_0056:  dup
+                IL_0057:  stloc.0
+                IL_0058:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_005d:  ldarg.0
+                IL_005e:  ldfld      "bool Program.<M>d__1.<>w__disposeMode"
+                IL_0063:  brfalse.s  IL_006a
+                IL_0065:  leave      IL_014b
+                IL_006a:  newobj     "System.Threading.Lock..ctor()"
+                IL_006f:  call       "System.Threading.Lock.Scope System.Threading.Lock.EnterScope()"
+                IL_0074:  stloc.1
+                .try
+                {
+                  IL_0075:  ldstr      "L"
+                  IL_007a:  call       "void System.Console.Write(string)"
+                  IL_007f:  leave.s    IL_008d
+                }
+                finally
+                {
+                  IL_0081:  ldloc.0
+                  IL_0082:  ldc.i4.m1
+                  IL_0083:  bne.un.s   IL_008c
+                  IL_0085:  ldloca.s   V_1
+                  IL_0087:  call       "void System.Threading.Lock.Scope.Dispose()"
+                  IL_008c:  endfinally
+                }
+                IL_008d:  ldarg.0
+                IL_008e:  ldfld      "bool Program.<M>d__1.<>w__disposeMode"
+                IL_0093:  brfalse.s  IL_009a
+                IL_0095:  leave      IL_014b
+                IL_009a:  call       "System.Runtime.CompilerServices.YieldAwaitable System.Threading.Tasks.Task.Yield()"
+                IL_009f:  stloc.3
+                IL_00a0:  ldloca.s   V_3
+                IL_00a2:  call       "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter System.Runtime.CompilerServices.YieldAwaitable.GetAwaiter()"
+                IL_00a7:  stloc.2
+                IL_00a8:  ldloca.s   V_2
+                IL_00aa:  call       "bool System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.IsCompleted.get"
+                IL_00af:  brtrue.s   IL_00f4
+                IL_00b1:  ldarg.0
+                IL_00b2:  ldc.i4.0
+                IL_00b3:  dup
+                IL_00b4:  stloc.0
+                IL_00b5:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_00ba:  ldarg.0
+                IL_00bb:  ldloc.2
+                IL_00bc:  stfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<M>d__1.<>u__1"
+                IL_00c1:  ldarg.0
+                IL_00c2:  stloc.s    V_4
+                IL_00c4:  ldarg.0
+                IL_00c5:  ldflda     "System.Runtime.CompilerServices.AsyncIteratorMethodBuilder Program.<M>d__1.<>t__builder"
+                IL_00ca:  ldloca.s   V_2
+                IL_00cc:  ldloca.s   V_4
+                IL_00ce:  call       "void System.Runtime.CompilerServices.AsyncIteratorMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, Program.<M>d__1>(ref System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter, ref Program.<M>d__1)"
+                IL_00d3:  leave      IL_017e
+                IL_00d8:  ldarg.0
+                IL_00d9:  ldfld      "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<M>d__1.<>u__1"
+                IL_00de:  stloc.2
+                IL_00df:  ldarg.0
+                IL_00e0:  ldflda     "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter Program.<M>d__1.<>u__1"
+                IL_00e5:  initobj    "System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter"
+                IL_00eb:  ldarg.0
+                IL_00ec:  ldc.i4.m1
+                IL_00ed:  dup
+                IL_00ee:  stloc.0
+                IL_00ef:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_00f4:  ldloca.s   V_2
+                IL_00f6:  call       "void System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter.GetResult()"
+                IL_00fb:  ldarg.0
+                IL_00fc:  ldc.i4.2
+                IL_00fd:  stfld      "int Program.<M>d__1.<>2__current"
+                IL_0102:  ldarg.0
+                IL_0103:  ldc.i4.s   -5
+                IL_0105:  dup
+                IL_0106:  stloc.0
+                IL_0107:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_010c:  leave.s    IL_0172
+                IL_010e:  ldarg.0
+                IL_010f:  ldc.i4.m1
+                IL_0110:  dup
+                IL_0111:  stloc.0
+                IL_0112:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_0117:  ldarg.0
+                IL_0118:  ldfld      "bool Program.<M>d__1.<>w__disposeMode"
+                IL_011d:  pop
+                IL_011e:  leave.s    IL_014b
+              }
+              catch System.Exception
+              {
+                IL_0120:  stloc.s    V_5
+                IL_0122:  ldarg.0
+                IL_0123:  ldc.i4.s   -2
+                IL_0125:  stfld      "int Program.<M>d__1.<>1__state"
+                IL_012a:  ldarg.0
+                IL_012b:  ldc.i4.0
+                IL_012c:  stfld      "int Program.<M>d__1.<>2__current"
+                IL_0131:  ldarg.0
+                IL_0132:  ldflda     "System.Runtime.CompilerServices.AsyncIteratorMethodBuilder Program.<M>d__1.<>t__builder"
+                IL_0137:  call       "void System.Runtime.CompilerServices.AsyncIteratorMethodBuilder.Complete()"
+                IL_013c:  ldarg.0
+                IL_013d:  ldflda     "System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool> Program.<M>d__1.<>v__promiseOfValueOrEnd"
+                IL_0142:  ldloc.s    V_5
+                IL_0144:  call       "void System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool>.SetException(System.Exception)"
+                IL_0149:  leave.s    IL_017e
+              }
+              IL_014b:  ldarg.0
+              IL_014c:  ldc.i4.s   -2
+              IL_014e:  stfld      "int Program.<M>d__1.<>1__state"
+              IL_0153:  ldarg.0
+              IL_0154:  ldc.i4.0
+              IL_0155:  stfld      "int Program.<M>d__1.<>2__current"
+              IL_015a:  ldarg.0
+              IL_015b:  ldflda     "System.Runtime.CompilerServices.AsyncIteratorMethodBuilder Program.<M>d__1.<>t__builder"
+              IL_0160:  call       "void System.Runtime.CompilerServices.AsyncIteratorMethodBuilder.Complete()"
+              IL_0165:  ldarg.0
+              IL_0166:  ldflda     "System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool> Program.<M>d__1.<>v__promiseOfValueOrEnd"
+              IL_016b:  ldc.i4.0
+              IL_016c:  call       "void System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool>.SetResult(bool)"
+              IL_0171:  ret
+              IL_0172:  ldarg.0
+              IL_0173:  ldflda     "System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool> Program.<M>d__1.<>v__promiseOfValueOrEnd"
+              IL_0178:  ldc.i4.1
+              IL_0179:  call       "void System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore<bool>.SetResult(bool)"
+              IL_017e:  ret
+            }
+            """);
     }
 
     [Theory, CombinatorialData]
diff --git a/src/Compilers/CSharp/Test/Emit2/Semantics/OutVarTests.cs b/src/Compilers/CSharp/Test/Emit2/Semantics/OutVarTests.cs
index 1c2c8c2f59050..213a94c3598ea 100644
--- a/src/Compilers/CSharp/Test/Emit2/Semantics/OutVarTests.cs
+++ b/src/Compilers/CSharp/Test/Emit2/Semantics/OutVarTests.cs
@@ -19581,9 +19581,9 @@ static void Test2(object x, System.ArgIterator y)
                 // (11,25): error CS1601: Cannot make reference to variable of type 'ArgIterator'
                 //     static object Test1(out System.ArgIterator x)
                 Diagnostic(ErrorCode.ERR_MethodArgCantBeRefAny, "out System.ArgIterator x").WithArguments("System.ArgIterator").WithLocation(11, 25),
-                // (8,25): error CS4012: Parameters or locals of type 'ArgIterator' cannot be declared in async methods or async lambda expressions.
+                // (8,25): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         Test2(Test1(out var x1), x1);
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("System.ArgIterator").WithLocation(8, 25),
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 25),
                 // (6,16): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
                 //     async void Test()
                 Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "Test").WithLocation(6, 16)
@@ -19630,12 +19630,12 @@ static void Test2(object x, System.ArgIterator y)
                 // (12,25): error CS1601: Cannot make reference to variable of type 'ArgIterator'
                 //     static object Test1(out System.ArgIterator x)
                 Diagnostic(ErrorCode.ERR_MethodArgCantBeRefAny, "out System.ArgIterator x").WithArguments("System.ArgIterator").WithLocation(12, 25),
-                // (8,25): error CS4012: Parameters or locals of type 'ArgIterator' cannot be declared in async methods or async lambda expressions.
+                // (8,25): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         Test2(Test1(out System.ArgIterator x1), x1);
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "System.ArgIterator").WithArguments("System.ArgIterator").WithLocation(8, 25),
-                // (9,9): error CS4012: Parameters or locals of type 'ArgIterator' cannot be declared in async methods or async lambda expressions.
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "System.ArgIterator").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 25),
+                // (9,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         var x = default(System.ArgIterator);
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("System.ArgIterator").WithLocation(9, 9),
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(9, 9),
                 // (9,13): warning CS0219: The variable 'x' is assigned but its value is never used
                 //         var x = default(System.ArgIterator);
                 Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "x").WithArguments("x").WithLocation(9, 13),
diff --git a/src/Compilers/CSharp/Test/Emit3/RefStructInterfacesTests.cs b/src/Compilers/CSharp/Test/Emit3/RefStructInterfacesTests.cs
index 5b8b68abbd7b0..a582a8a14aa22 100644
--- a/src/Compilers/CSharp/Test/Emit3/RefStructInterfacesTests.cs
+++ b/src/Compilers/CSharp/Test/Emit3/RefStructInterfacesTests.cs
@@ -11305,9 +11305,10 @@ static async Task Main()
             var comp = CreateCompilation(src, targetFramework: s_targetFrameworkSupportingByRefLikeGenerics, options: TestOptions.ReleaseExe);
 
             comp.VerifyDiagnostics(
+                // PROTOTYPE(RefStructInterfaces): follow up on recent changes due to https://github.com/dotnet/csharplang/blob/main/proposals/ref-unsafe-in-iterators-async.md
                 // (18,22): error CS9104: A using statement resource of type 'S2' cannot be used in async methods or async lambda expressions.
                 //         await using (new S2())
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new S2()").WithArguments("S2").WithLocation(18, 22)
+                //Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new S2()").WithArguments("S2").WithLocation(18, 22)
                 );
         }
 
@@ -11341,9 +11342,10 @@ static async Task Main()
             var comp = CreateCompilation(src, targetFramework: s_targetFrameworkSupportingByRefLikeGenerics, options: TestOptions.ReleaseExe);
 
             comp.VerifyDiagnostics(
+                // PROTOTYPE(RefStructInterfaces): follow up on recent changes due to https://github.com/dotnet/csharplang/blob/main/proposals/ref-unsafe-in-iterators-async.md
                 // (18,22): error CS9104: A using statement resource of type 'S2' cannot be used in async methods or async lambda expressions.
                 //         await using (new S2())
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new S2()").WithArguments("S2").WithLocation(18, 22)
+                //Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new S2()").WithArguments("S2").WithLocation(18, 22)
                 );
         }
 
@@ -11382,9 +11384,10 @@ static async Task Main()
             var comp = CreateCompilation(src, targetFramework: s_targetFrameworkSupportingByRefLikeGenerics, options: TestOptions.ReleaseExe);
 
             comp.VerifyDiagnostics(
+                // PROTOTYPE(RefStructInterfaces): follow up on recent changes due to https://github.com/dotnet/csharplang/blob/main/proposals/ref-unsafe-in-iterators-async.md
                 // (23,22): error CS9104: A using statement resource of type 'T' cannot be used in async methods or async lambda expressions.
                 //         await using (new T())
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new T()").WithArguments("T").WithLocation(23, 22)
+                //Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new T()").WithArguments("T").WithLocation(23, 22)
                 );
         }
 
@@ -11427,9 +11430,10 @@ static async Task Main()
             var comp = CreateCompilation(src, targetFramework: s_targetFrameworkSupportingByRefLikeGenerics, options: TestOptions.ReleaseExe);
 
             comp.VerifyDiagnostics(
+                // PROTOTYPE(RefStructInterfaces): follow up on recent changes due to https://github.com/dotnet/csharplang/blob/main/proposals/ref-unsafe-in-iterators-async.md
                 // (27,22): error CS9104: A using statement resource of type 'T' cannot be used in async methods or async lambda expressions.
                 //         await using (new T())
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new T()").WithArguments("T").WithLocation(27, 22)
+                //Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new T()").WithArguments("T").WithLocation(27, 22)
                 );
         }
 
@@ -11482,10 +11486,12 @@ static async Task Main()
                 Diagnostic(ErrorCode.ERR_AmbigCall, "new T()").WithArguments("IMyAsyncDisposable1.DisposeAsync()", "IMyAsyncDisposable2.DisposeAsync()").WithLocation(32, 22),
                 // (32,22): error CS8410: 'T': type used in an asynchronous using statement must be implicitly convertible to 'System.IAsyncDisposable' or implement a suitable 'DisposeAsync' method.
                 //         await using (new T())
-                Diagnostic(ErrorCode.ERR_NoConvToIAsyncDisp, "new T()").WithArguments("T").WithLocation(32, 22),
+                Diagnostic(ErrorCode.ERR_NoConvToIAsyncDisp, "new T()").WithArguments("T").WithLocation(32, 22)
+                // PROTOTYPE(RefStructInterfaces): follow up on recent changes due to https://github.com/dotnet/csharplang/blob/main/proposals/ref-unsafe-in-iterators-async.md
+                //,
                 // (32,22): error CS9104: A using statement resource of type 'T' cannot be used in async methods or async lambda expressions.
                 //         await using (new T())
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new T()").WithArguments("T").WithLocation(32, 22)
+                //Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new T()").WithArguments("T").WithLocation(32, 22)
                 );
         }
 
@@ -11541,10 +11547,12 @@ static async Task Main()
             comp.VerifyDiagnostics(
                 // (36,22): error CS0121: The call is ambiguous between the following methods or properties: 'IMyAsyncDisposable1.DisposeAsync()' and 'IMyAsyncDisposable2.DisposeAsync()'
                 //         await using (new T())
-                Diagnostic(ErrorCode.ERR_AmbigCall, "new T()").WithArguments("IMyAsyncDisposable1.DisposeAsync()", "IMyAsyncDisposable2.DisposeAsync()").WithLocation(36, 22),
+                Diagnostic(ErrorCode.ERR_AmbigCall, "new T()").WithArguments("IMyAsyncDisposable1.DisposeAsync()", "IMyAsyncDisposable2.DisposeAsync()").WithLocation(36, 22)
+                // PROTOTYPE(RefStructInterfaces): follow up on recent changes due to https://github.com/dotnet/csharplang/blob/main/proposals/ref-unsafe-in-iterators-async.md
+                //,
                 // (36,22): error CS9104: A using statement resource of type 'T' cannot be used in async methods or async lambda expressions.
                 //         await using (new T())
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new T()").WithArguments("T").WithLocation(36, 22)
+                //Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "new T()").WithArguments("T").WithLocation(36, 22)
                 );
         }
 
@@ -18513,12 +18521,12 @@ ref struct S
 
             var comp = CreateCompilation(src, targetFramework: s_targetFrameworkSupportingByRefLikeGenerics);
             comp.VerifyDiagnostics(
-                // (6,34): error CS4012: Parameters or locals of type 'T' cannot be declared in async methods or async lambda expressions.
+                // (6,34): error CS4012: Parameters of type 'T' cannot be declared in async methods or async lambda expressions.
                 //     static async void Test1<T>(T x)
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "x").WithArguments("T").WithLocation(6, 34),
-                // (11,31): error CS4012: Parameters or locals of type 'S' cannot be declared in async methods or async lambda expressions.
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefParameter, "x").WithArguments("T").WithLocation(6, 34),
+                // (11,31): error CS4012: Parameters of type 'S' cannot be declared in async methods or async lambda expressions.
                 //     static async void Test2(S y)
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "y").WithArguments("S").WithLocation(11, 31)
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefParameter, "y").WithArguments("S").WithLocation(11, 31)
                 );
         }
 
diff --git a/src/Compilers/CSharp/Test/Emit3/RefUnsafeInIteratorAndAsyncTests.cs b/src/Compilers/CSharp/Test/Emit3/RefUnsafeInIteratorAndAsyncTests.cs
new file mode 100644
index 0000000000000..d3b9880ee2a9d
--- /dev/null
+++ b/src/Compilers/CSharp/Test/Emit3/RefUnsafeInIteratorAndAsyncTests.cs
@@ -0,0 +1,1099 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Roslyn.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests;
+
+public class RefUnsafeInIteratorAndAsyncTests : CSharpTestBase
+{
+    private static string? IfSpans(string expectedOutput)
+        => ExecutionConditionUtil.IsDesktop ? null : expectedOutput;
+
+    [Fact]
+    public void LangVersion_RefLocalInAsync()
+    {
+        var source = """
+            using System.Threading.Tasks;
+            class C
+            {
+                async Task M(int x)
+                {
+                    ref int y = ref x;
+                    ref readonly int z = ref y;
+                    await Task.Yield();
+                }
+            }
+            """;
+
+        CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+            // (6,17): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         ref int y = ref x;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 17),
+            // (7,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         ref readonly int z = ref y;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "z").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 26));
+
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+        CreateCompilation(source).VerifyEmitDiagnostics();
+    }
+
+    [Fact]
+    public void LangVersion_RefLocalInIterator()
+    {
+        var source = """
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> M(int x)
+                {
+                    ref int y = ref x;
+                    ref readonly int z = ref y;
+                    yield return x;
+                }
+            }
+            """;
+
+        CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+            // (6,17): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         ref int y = ref x;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 17),
+            // (7,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         ref readonly int z = ref y;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "z").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 26));
+
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+        CreateCompilation(source).VerifyEmitDiagnostics();
+    }
+
+    [Fact]
+    public void LangVersion_RefLocalInIterator_IEnumerator()
+    {
+        var source = """
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerator<int> M(int x)
+                {
+                    ref int y = ref x;
+                    ref readonly int z = ref y;
+                    yield return x;
+                }
+            }
+            """;
+
+        CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+            // (6,17): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         ref int y = ref x;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 17),
+            // (7,26): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         ref readonly int z = ref y;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "z").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 26));
+
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+        CreateCompilation(source).VerifyEmitDiagnostics();
+    }
+
+    [Fact]
+    public void LangVersion_RefStructInAsync()
+    {
+        var source = """
+            #pragma warning disable CS0219 // variable unused
+            using System.Threading.Tasks;
+            class C
+            {
+                async Task M()
+                {
+                    R y = default;
+                    scoped R z = default;
+                    await Task.Yield();
+                }
+            }
+            ref struct R { }
+            """;
+
+        CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+            // (7,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         R y = default;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "R").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 9),
+            // (8,16): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         scoped R z = default;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "R").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 16));
+
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+        CreateCompilation(source).VerifyEmitDiagnostics();
+    }
+
+    [Fact]
+    public void LangVersion_RefStructInIterator()
+    {
+        var source = """
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> M(R r)
+                {
+                    M(r);
+                    yield return -1;        
+                }
+            }
+            ref struct R { }
+            """;
+
+        var expectedDiagnostics = new[]
+        {
+            // (6,11): error CS4007: Instance of type 'R' cannot be preserved across 'await' or 'yield' boundary.
+            //         M(r);
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "r").WithArguments("R").WithLocation(6, 11)
+        };
+
+        CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics(expectedDiagnostics);
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+        CreateCompilation(source).VerifyEmitDiagnostics(expectedDiagnostics);
+    }
+
+    [Fact]
+    public void LangVersion_RestrictedInAsync()
+    {
+        var source = """
+            #pragma warning disable CS0219 // variable unused
+            using System.Threading.Tasks;
+            class C
+            {
+                async Task M()
+                {
+                    System.TypedReference t = default;
+                    await Task.Yield();
+                }
+            }
+            """;
+
+        CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+            // (7,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+            //         System.TypedReference t = default;
+            Diagnostic(ErrorCode.ERR_FeatureInPreview, "System.TypedReference").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 9));
+
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+        CreateCompilation(source).VerifyEmitDiagnostics();
+    }
+
+    [Fact]
+    public void LangVersion_RestrictedInIterator()
+    {
+        var source = """
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> M(System.TypedReference t)
+                {
+                    t.GetHashCode();
+                    yield return -1;
+                }
+            }
+            """;
+
+        var expectedDiagnostics = new[]
+        {
+            // (6,9): error CS4007: Instance of type 'System.TypedReference' cannot be preserved across 'await' or 'yield' boundary.
+            //         t.GetHashCode();
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "t").WithArguments("System.TypedReference").WithLocation(6, 9)
+        };
+
+        CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics(expectedDiagnostics);
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+        CreateCompilation(source).VerifyEmitDiagnostics(expectedDiagnostics);
+    }
+
+    [Fact]
+    public void Await_RefLocal_Across()
+    {
+        var source = """
+            using System.Threading.Tasks;
+            class C
+            {
+                async Task M(int x)
+                {
+                    ref int y = ref x;
+                    await Task.Yield();
+                    System.Console.Write(y);
+                }
+            }
+            """;
+        CreateCompilation(source).VerifyEmitDiagnostics(
+            // (8,30): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+            //         System.Console.Write(y);
+            Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(8, 30));
+    }
+
+    [Fact]
+    public void Await_RefLocal_Across_Reassign()
+    {
+        var source = """
+            using System.Threading.Tasks;
+            class C
+            {
+                static Task Main() => M(123, 456);
+                static async Task M(int x, int z)
+                {
+                    ref int y = ref x;
+                    await Task.Yield();
+                    y = ref z;
+                    System.Console.Write(y);
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "456").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void Await_RefLocal_Between()
+    {
+        var source = """
+            using System.Threading.Tasks;
+            class C
+            {
+                static Task Main() => M(123);
+                static async Task M(int x)
+                {
+                    ref int y = ref x;
+                    System.Console.Write(y);
+                    await Task.Yield();
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "123").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void Await_RefStruct_Across()
+    {
+        var source = """
+            using System;
+            using System.Threading.Tasks;
+            class C
+            {
+                async Task M(int x)
+                {
+                    Span<int> y = new(ref x);
+                    await Task.Yield();
+                    Console.Write(y.ToString());
+                }
+            }
+            """;
+        CreateCompilation(source, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+            // (9,23): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+            //         Console.Write(y.ToString());
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.Span<int>").WithLocation(9, 23));
+    }
+
+    [Fact]
+    public void Await_RefStruct_Across_Reassign()
+    {
+        var source = """
+            using System;
+            using System.Threading.Tasks;
+            class C
+            {
+                static Task Main() => M(123, 456);
+                static async Task M(int x, int z)
+                {
+                    Span<int> y = new(ref x);
+                    await Task.Yield();
+                    y = new(ref z);
+                    Console.Write(y[0]);
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: IfSpans("456"), verify: Verification.FailsPEVerify, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void Await_RefStruct_Between()
+    {
+        var source = """
+            using System;
+            using System.Threading.Tasks;
+            class C
+            {
+                static Task Main() => M(123);
+                static async Task M(int x)
+                {
+                    Span<int> y = new(ref x);
+                    Console.Write(y[0]);
+                    await Task.Yield();
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: IfSpans("123"), verify: Verification.FailsPEVerify, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void Await_Restricted_Across()
+    {
+        var source = """
+            using System;
+            using System.Threading.Tasks;
+            class C
+            {
+                async Task M()
+                {
+                    TypedReference y = default;
+                    await Task.Yield();
+                    Console.Write(y.GetHashCode());
+                }
+            }
+            """;
+        CreateCompilation(source, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+            // (9,23): error CS4007: Instance of type 'System.TypedReference' cannot be preserved across 'await' or 'yield' boundary.
+            //         Console.Write(y.GetHashCode());
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.TypedReference").WithLocation(9, 23));
+    }
+
+    [Fact]
+    public void Await_Restricted_Across_Reassign()
+    {
+        var source = """
+            using System;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    TypedReference y = default;
+                    await Task.Yield();
+                    y = default;
+                    Console.Write(y.GetHashCode());
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "0").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void Await_Restricted_Between()
+    {
+        var source = """
+            using System;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    TypedReference y = default;
+                    Console.Write(y.GetHashCode());
+                    await Task.Yield();
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "0").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Across()
+    {
+        var source = """
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> M(int x)
+                {
+                    ref int y = ref x;
+                    yield return 1;
+                    System.Console.Write(y);
+                }
+            }
+            """;
+        CreateCompilation(source).VerifyEmitDiagnostics(
+            // (8,30): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+            //         System.Console.Write(y);
+            Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(8, 30));
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Across_Indexer()
+    {
+        var source = """
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> this[int x]
+                {
+                    get
+                    {
+                        ref int y = ref x;
+                        yield return 1;
+                        System.Console.Write(y);
+                    }
+                }
+            }
+            """;
+        CreateCompilation(source).VerifyEmitDiagnostics(
+            // (10,34): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+            //             System.Console.Write(y);
+            Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(10, 34));
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Across_NestedBlock()
+    {
+        var source = """
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> M(int x)
+                {
+                    ref int y = ref x;
+                    if (x != 0) { yield return 1; }
+                    System.Console.Write(y);
+                }
+            }
+            """;
+        CreateCompilation(source).VerifyEmitDiagnostics(
+            // (8,30): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+            //         System.Console.Write(y);
+            Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(8, 30));
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Across_Async()
+    {
+        var source = """
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                async IAsyncEnumerable<int> M(int x)
+                {
+                    ref int y = ref x;
+                    yield return 1; await Task.Yield();
+                    System.Console.Write(y);
+                }
+            }
+            """ + AsyncStreamsTypes;
+        CreateCompilationWithTasksExtensions(source).VerifyEmitDiagnostics(
+            // (9,30): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+            //         System.Console.Write(y);
+            Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(9, 30));
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Across_Reassign()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var i in M(123, 456))
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static IEnumerable<int> M(int x, int z)
+                {
+                    ref int y = ref x;
+                    yield return -1;
+                    y = ref z;
+                    Console.Write(y);
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "-1 456").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Across_Reassign_Indexer()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var i in new C()[123, 456])
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                IEnumerable<int> this[int x, int z]
+                {
+                    get
+                    {
+                        ref int y = ref x;
+                        yield return -1;
+                        y = ref z;
+                        Console.Write(y);
+                    }
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "-1 456").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Across_Reassign_Async()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    await foreach (var i in M(123, 456))
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static async IAsyncEnumerable<int> M(int x, int z)
+                {
+                    ref int y = ref x;
+                    yield return -1; await Task.Yield();
+                    y = ref z;
+                    Console.Write(y);
+                }
+            }
+            """ + AsyncStreamsTypes;
+        var comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+        CompileAndVerify(comp, expectedOutput: "-1 456").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Between()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var i in M(123))
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static IEnumerable<int> M(int x)
+                {
+                    ref int y = ref x;
+                    Console.Write(y);
+                    yield return -1;
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "123-1").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefLocal_Between_Async()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    await foreach (var i in M(123))
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static async IAsyncEnumerable<int> M(int x)
+                {
+                    ref int y = ref x;
+                    Console.Write(y);
+                    yield return -1; await Task.Yield();
+                }
+            }
+            """ + AsyncStreamsTypes;
+        var comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+        CompileAndVerify(comp, expectedOutput: "123-1").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefStruct_Across()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> M(int x)
+                {
+                    Span<int> y = new(ref x);
+                    yield return -1;
+                    Console.Write(y.ToString());
+                }
+            }
+            """;
+        CreateCompilation(source, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+            // (9,23): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+            //         Console.Write(y.ToString());
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.Span<int>").WithLocation(9, 23));
+    }
+
+    [Fact]
+    public void YieldReturn_RefStruct_Across_Indexer()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> this[int x]
+                {
+                    get
+                    {
+                        Span<int> y = new(ref x);
+                        yield return -1;
+                        Console.Write(y.ToString());
+                    }
+                }
+            }
+            """;
+        CreateCompilation(source, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+            // (11,27): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+            //             Console.Write(y.ToString());
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.Span<int>").WithLocation(11, 27));
+    }
+
+    [Fact]
+    public void YieldReturn_RefStruct_Across_NestedBlock()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> M(int x)
+                {
+                    Span<int> y = new(ref x);
+                    if (x != 0) { yield return -1; }
+                    Console.Write(y.ToString());
+                }
+            }
+            """;
+        CreateCompilation(source, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+            // (9,23): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+            //         Console.Write(y.ToString());
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.Span<int>").WithLocation(9, 23));
+    }
+
+    [Fact]
+    public void YieldReturn_RefStruct_Across_Async()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                async IAsyncEnumerable<int> M(int x)
+                {
+                    Span<int> y = new(ref x);
+                    yield return -1; await Task.Yield();
+                    Console.Write(y.ToString());
+                }
+            }
+            """;
+        CreateCompilation(source, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+            // (10,23): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+            //         Console.Write(y.ToString());
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.Span<int>").WithLocation(10, 23));
+    }
+
+    [Fact]
+    public void YieldReturn_RefStruct_Across_Reassign()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var i in M(123, 456))
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static IEnumerable<int> M(int x, int z)
+                {
+                    Span<int> y = new(ref x);
+                    yield return -1;
+                    y = new(ref z);
+                    Console.Write(y[0]);
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: IfSpans("-1 456"), verify: Verification.FailsPEVerify, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefStruct_Across_Reassign_Indexer()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var i in new C()[123, 456])
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                IEnumerable<int> this[int x, int z]
+                {
+                    get
+                    {
+                        Span<int> y = new(ref x);
+                        yield return -1;
+                        y = new(ref z);
+                        Console.Write(y[0]);
+                    }
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: IfSpans("-1 456"), verify: Verification.FailsPEVerify, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefStruct_Across_Reassign_Async()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    await foreach (var i in M(123, 456))
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static async IAsyncEnumerable<int> M(int x, int z)
+                {
+                    Span<int> y = new(ref x);
+                    yield return -1; await Task.Yield();
+                    y = new(ref z);
+                    Console.Write(y[0]);
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: IfSpans("-1 456"), verify: Verification.FailsPEVerify, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_RefStruct_Between()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var i in M(123))
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static IEnumerable<int> M(int x)
+                {
+                    Span<int> y = new(ref x);
+                    Console.Write(y[0]);
+                    yield return -1;
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: IfSpans("123-1"), verify: Verification.FailsPEVerify, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_Restricted_Across()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                IEnumerable<int> M()
+                {
+                    TypedReference y = default;
+                    yield return -1;
+                    Console.Write(y.GetHashCode());
+                }
+            }
+            """;
+        CreateCompilation(source, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+            // (9,23): error CS4007: Instance of type 'System.TypedReference' cannot be preserved across 'await' or 'yield' boundary.
+            //         Console.Write(y.GetHashCode());
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.TypedReference").WithLocation(9, 23));
+    }
+
+    [Fact]
+    public void YieldReturn_Restricted_Across_Async()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                async IAsyncEnumerable<int> M()
+                {
+                    TypedReference y = default;
+                    yield return -1; await Task.Yield();
+                    Console.Write(y.GetHashCode());
+                }
+            }
+            """;
+        CreateCompilation(source, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+            // (10,23): error CS4007: Instance of type 'System.TypedReference' cannot be preserved across 'await' or 'yield' boundary.
+            //         Console.Write(y.GetHashCode());
+            Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.TypedReference").WithLocation(10, 23));
+    }
+
+    [Fact]
+    public void YieldReturn_Restricted_Across_Reassign()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var i in M())
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static IEnumerable<int> M()
+                {
+                    TypedReference y = default;
+                    yield return -1;
+                    y = default;
+                    Console.Write(y.GetHashCode());
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "-1 0").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_Restricted_Across_Reassign_Async()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    await foreach (var i in M())
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static async IAsyncEnumerable<int> M()
+                {
+                    TypedReference y = default;
+                    yield return -1; await Task.Yield();
+                    y = default;
+                    Console.Write(y.GetHashCode());
+                }
+            }
+            """ + AsyncStreamsTypes;
+        var comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+        CompileAndVerify(comp, expectedOutput: "-1 0").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldReturn_Restricted_Between()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var i in M())
+                    {
+                        Console.Write(i + " ");
+                    }
+                }
+                static IEnumerable<int> M()
+                {
+                    TypedReference y = default;
+                    Console.Write(y.GetHashCode());
+                    yield return -1;
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "0-1").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldBreak_RefLocal_Across()
+    {
+        var source = """
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var a in M(10)) { throw null; }
+                    foreach (var b in M(123)) { throw null; }
+                }
+                static IEnumerable<int> M(int x)
+                {
+                    ref int y = ref x;
+                    if (x < 100) yield break;
+                    System.Console.Write(y);
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "123").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldBreak_RefLocal_Across_Async()
+    {
+        var source = """
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    await foreach (var a in M(10)) { throw null; }
+                    await foreach (var b in M(123)) { throw null; }
+                }
+                static async IAsyncEnumerable<int> M(int x)
+                {
+                    ref int y = ref x;
+                    if (x < 100) { await Task.Yield(); yield break; }
+                    System.Console.Write(y);
+                }
+            }
+            """ + AsyncStreamsTypes;
+        var comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+        CompileAndVerify(comp, expectedOutput: "123").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldBreak_RefStruct_Across()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var a in M(10)) { throw null; }
+                    foreach (var b in M(123)) { throw null; }
+                }
+                static IEnumerable<int> M(int x)
+                {
+                    Span<int> y = new(ref x);
+                    if (x < 100) yield break;
+                    Console.Write(y[0]);
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: IfSpans("123"), verify: Verification.FailsPEVerify, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldBreak_RefStruct_Across_Async()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    await foreach (var a in M(10)) { throw null; }
+                    await foreach (var b in M(123)) { throw null; }
+                }
+                static async IAsyncEnumerable<int> M(int x)
+                {
+                    Span<int> y = new(ref x);
+                    if (x < 100) { await Task.Yield(); yield break; }
+                    Console.Write(y[0]);
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: IfSpans("123"), verify: Verification.FailsPEVerify, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldBreak_Restricted_Across()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            class C
+            {
+                static void Main()
+                {
+                    foreach (var a in M(10)) { throw null; }
+                    foreach (var b in M(123)) { throw null; }
+                }
+                static IEnumerable<int> M(int x)
+                {
+                    TypedReference t = default;
+                    if (x < 100) yield break;
+                    Console.Write(x + t.GetHashCode());
+                }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "123").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void YieldBreak_Restricted_Across_Async()
+    {
+        var source = """
+            using System;
+            using System.Collections.Generic;
+            using System.Threading.Tasks;
+            class C
+            {
+                static async Task Main()
+                {
+                    await foreach (var a in M(10)) { throw null; }
+                    await foreach (var b in M(123)) { throw null; }
+                }
+                static async IAsyncEnumerable<int> M(int x)
+                {
+                    TypedReference t = default;
+                    if (x < 100) { await Task.Yield(); yield break; }
+                    Console.Write(x + t.GetHashCode());
+                }
+            }
+            """ + AsyncStreamsTypes;
+        var comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseExe);
+        CompileAndVerify(comp, expectedOutput: "123").VerifyDiagnostics();
+    }
+}
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/AwaitExpressionTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/AwaitExpressionTests.cs
index 58b403f3ed35f..0cdb4221d7784 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/AwaitExpressionTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/AwaitExpressionTests.cs
@@ -241,9 +241,9 @@ static async Task Goo()
 }";
             var comp = CreateCompilationWithMscorlib45(text, options: TestOptions.ReleaseDll);
             comp.VerifyEmitDiagnostics(
-                // (8,27): error CS4007: 'await' cannot be used in an expression containing the type 'System.TypedReference'
+                // (8,27): error CS4007: Instance of type 'System.TypedReference' cannot be preserved across 'await' or 'yield' boundary.
                 //         Console.WriteLine(new TypedReference().Equals(await Task.FromResult(0)));
-                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "await Task.FromResult(0)").WithArguments("System.TypedReference").WithLocation(8, 55));
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "new TypedReference()").WithArguments("System.TypedReference").WithLocation(8, 27));
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/BindingAsyncTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/BindingAsyncTests.cs
index 740346725b17a..8e550a2a565e8 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/BindingAsyncTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/BindingAsyncTests.cs
@@ -3218,9 +3218,9 @@ async Task M1(TypedReference tr)
     }
 }";
             CreateCompilationWithMscorlib45(source).VerifyDiagnostics(
-                // (7,34): error CS4012: Parameters or locals of type 'System.TypedReference' cannot be declared in async methods or async lambda expressions
+                // (7,34): error CS4012: Parameters of type 'System.TypedReference' cannot be declared in async methods or async lambda expressions
                 //     async Task M1(TypedReference tr)
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "tr").WithArguments("System.TypedReference"));
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefParameter, "tr").WithArguments("System.TypedReference"));
         }
 
         [Fact]
@@ -3238,10 +3238,7 @@ async Task M1()
         await Task.Factory.StartNew(() => { });
     }
 }";
-            CreateCompilationWithMscorlib45(source).VerifyDiagnostics(
-                // (9,9): error CS4012: Parameters or locals of type 'System.TypedReference' cannot be declared in async methods or async lambda expressions
-                //         TypedReference tr;
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "TypedReference").WithArguments("System.TypedReference"),
+            CreateCompilationWithMscorlib45(source).VerifyEmitDiagnostics(
                 // (9,24): warning CS0168: The variable 'tr' is declared but never used
                 //         TypedReference tr;
                 Diagnostic(ErrorCode.WRN_UnreferencedVar, "tr").WithArguments("tr"));
@@ -3257,18 +3254,33 @@ public void BadSpecialByRefVarDeclLocal()
 class Test
 {
     async Task M1(bool truth)
+    {
+        var tr = new TypedReference(); // 1
+        await Task.Factory.StartNew(() => { });
+    }
+
+    async Task M2(bool truth)
     {
         var tr = new TypedReference();
         await Task.Factory.StartNew(() => { });
+        var tr2 = tr; // 2
+    }
+
+    async Task M3()
+    {
+        var tr = new TypedReference();
+        await Task.Factory.StartNew(() => { });
+        tr = default;
+        var tr2 = tr;
     }
 }";
-            CreateCompilationWithMscorlib45(source).VerifyDiagnostics(
-                // (9,9): error CS4012: Parameters or locals of type 'TypedReference' cannot be declared in async methods or async lambda expressions.
-                //         var tr = new TypedReference();
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("System.TypedReference").WithLocation(9, 9),
+            CreateCompilationWithMscorlib45(source).VerifyEmitDiagnostics(
                 // (9,13): warning CS0219: The variable 'tr' is assigned but its value is never used
-                //         var tr = new TypedReference();
-                Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "tr").WithArguments("tr").WithLocation(9, 13));
+                //         var tr = new TypedReference(); // 1
+                Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "tr").WithArguments("tr").WithLocation(9, 13),
+                // (17,19): error CS4007: Instance of type 'System.TypedReference' cannot be preserved across 'await' or 'yield' boundary.
+                //         var tr2 = tr; // 2
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "tr").WithArguments("System.TypedReference").WithLocation(17, 19));
         }
 
         [Fact]
@@ -3288,9 +3300,6 @@ unsafe async public static void F()
                 // (8,31): error CS0209: The type of a local declared in a fixed statement must be a pointer type
                 //         fixed (TypedReference tr) { }
                 Diagnostic(ErrorCode.ERR_BadFixedInitType, "tr"),
-                // (8,16): error CS4012: Parameters or locals of type 'System.TypedReference' cannot be declared in async methods or async lambda expressions.
-                //         fixed (TypedReference tr) { }
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "TypedReference").WithArguments("System.TypedReference"),
                 // (8,31): error CS0210: You must provide an initializer in a fixed or using statement declaration
                 //         fixed (TypedReference tr) { }
                 Diagnostic(ErrorCode.ERR_FixedMustInit, "tr"),
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/ForEachTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/ForEachTests.cs
index bb59d99fef196..fd120694a384c 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/ForEachTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/ForEachTests.cs
@@ -3531,10 +3531,7 @@ public async static Task Test()
             System.Console.Write(x);
         }
     }
-}").VerifyDiagnostics(
-                // (20,26): error CS8177: Async methods cannot have by-reference locals
-                //         foreach (ref int x in new E())
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "x").WithLocation(20, 26));
+}").VerifyEmitDiagnostics();
         }
 
         [Fact]
@@ -3565,10 +3562,7 @@ public async static Task Test()
             System.Console.Write(x);
         }
     }
-}").VerifyDiagnostics(
-                // (20,35): error CS8177: Async methods cannot have by-reference locals
-                //         foreach (ref readonly int x in new E())
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "x").WithLocation(20, 35));
+}").VerifyEmitDiagnostics();
         }
 
         [Fact]
@@ -3597,10 +3591,7 @@ public static IEnumerable<int> Test()
             yield return x;
         }
     }
-}").VerifyDiagnostics(
-                // (18,26): error CS8176: Iterators cannot have by-reference locals
-                //         foreach (ref int x in new E())
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "x").WithLocation(18, 26));
+}").VerifyEmitDiagnostics();
         }
 
         [Fact]
@@ -3629,10 +3620,7 @@ public static IEnumerable<int> Test()
             yield return x;
         }
     }
-}").VerifyDiagnostics(
-                // (18,35): error CS8176: Iterators cannot have by-reference locals
-                //         foreach (ref readonly int x in new E())
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "x").WithLocation(18, 35));
+}").VerifyEmitDiagnostics();
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/IteratorTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/IteratorTests.cs
index 34be3f12005c3..06055448cc8b6 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/IteratorTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/IteratorTests.cs
@@ -106,6 +106,168 @@ IEnumerable<int> I()
                 );
         }
 
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/72443")]
+        public void YieldInLock_Async()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+
+                public class C
+                {
+                    public async Task ProcessValueAsync()
+                    {
+                        await foreach (int item in GetValuesAsync())
+                        {
+                            await Task.Yield();
+                            Console.Write(item);
+                        }
+                    }
+
+                    private async IAsyncEnumerable<int> GetValuesAsync()
+                    {
+                        await Task.Yield();
+                        lock (this)
+                        {
+                            for (int i = 0; i < 10; i++)
+                            {
+                                yield return i;
+
+                                if (i == 3)
+                                {
+                                    yield break;
+                                }
+                            }
+                        }
+                    }
+                }
+                """ + AsyncStreamsTypes;
+
+            var comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseDll.WithWarningLevel(8));
+            CompileAndVerify(comp).VerifyDiagnostics();
+
+            var expectedDiagnostics = new[]
+            {
+                // (23,17): warning CS9237: 'yield return' should not be used in the body of a lock statement
+                //                 yield return i;
+                Diagnostic(ErrorCode.WRN_BadYieldInLock, "yield").WithLocation(23, 17)
+            };
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseDll.WithWarningLevel(9));
+            CompileAndVerify(comp).VerifyDiagnostics(expectedDiagnostics);
+
+            comp = CreateCompilationWithTasksExtensions(source, options: TestOptions.ReleaseDll);
+            CompileAndVerify(comp).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/72443")]
+        public void YieldInLock_Sync()
+        {
+            var source = """
+                using System;
+                using System.Collections.Generic;
+                using System.Threading;
+
+                object o = new object();
+                Console.WriteLine($"Before: {Monitor.IsEntered(o)}");
+                using (IEnumerator<int> e = GetValues(o).GetEnumerator())
+                {
+                    Console.WriteLine($"Inside: {Monitor.IsEntered(o)}");
+                    while (e.MoveNext())
+                    {
+                        Console.WriteLine($"{e.Current}: {Monitor.IsEntered(o)}");
+                    }
+                    Console.WriteLine($"Done: {Monitor.IsEntered(o)}");
+                }
+                Console.WriteLine($"After: {Monitor.IsEntered(o)}");
+
+                static IEnumerable<int> GetValues(object obj)
+                {
+                    lock (obj)
+                    {
+                        for (int i = 0; i < 3; i++)
+                        {
+                            yield return i;
+
+                            if (i == 1)
+                            {
+                                yield break;
+                            }
+                        }
+                    }
+                }
+                """;
+
+            var expectedOutput = """
+                Before: False
+                Inside: False
+                0: True
+                1: True
+                Done: False
+                After: False
+                """;
+
+            CompileAndVerify(source, options: TestOptions.ReleaseExe.WithWarningLevel(8),
+                expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            var expectedDiagnostics = new[]
+            {
+                // (24,13): warning CS9237: 'yield return' should not be used in the body of a lock statement
+                //             yield return i;
+                Diagnostic(ErrorCode.WRN_BadYieldInLock, "yield").WithLocation(24, 13)
+            };
+
+            CompileAndVerify(source, options: TestOptions.ReleaseExe.WithWarningLevel(9),
+                expectedOutput: expectedOutput).VerifyDiagnostics(expectedDiagnostics);
+
+            CompileAndVerify(source, expectedOutput: expectedOutput).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/72443")]
+        public void YieldInLock_Nested()
+        {
+            var source = """
+                using System.Collections.Generic;
+
+                class C
+                {
+                    IEnumerable<int> M()
+                    {
+                        yield return 1;
+                        lock (this)
+                        {
+                            yield return 2;
+
+                            local();
+
+                            IEnumerable<int> local()
+                            {
+                                yield return 3;
+
+                                lock (this)
+                                {
+                                    yield return 4;
+
+                                    yield break;
+                                }
+                            }
+
+                            yield break;
+                        }
+                    }
+                }
+                """;
+
+            CreateCompilation(source).VerifyDiagnostics(
+                // (10,13): warning CS9237: 'yield return' should not be used in the body of a lock statement
+                //             yield return 2;
+                Diagnostic(ErrorCode.WRN_BadYieldInLock, "yield").WithLocation(10, 13),
+                // (20,21): warning CS9237: 'yield return' should not be used in the body of a lock statement
+                //                     yield return 4;
+                Diagnostic(ErrorCode.WRN_BadYieldInLock, "yield").WithLocation(20, 21));
+        }
+
         [WorkItem(546081, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/546081")]
         [Fact]
         public void IteratorBlockWithUnreachableCode()
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/LambdaTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/LambdaTests.cs
index 9b2f16c772425..2b32616b85553 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/LambdaTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/LambdaTests.cs
@@ -5981,15 +5981,15 @@ static void Main()
 }";
             var comp = CreateCompilation(source, parseOptions: TestOptions.RegularPreview);
             comp.VerifyDiagnostics(
-                // (10,39): error CS4012: Parameters or locals of type 'TypedReference' cannot be declared in async methods or async lambda expressions.
+                // (10,39): error CS4012: Parameters of type 'TypedReference' cannot be declared in async methods or async lambda expressions.
                 //         D1 d1 = async (TypedReference r) => { await Task.Yield(); };
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "r").WithArguments("System.TypedReference").WithLocation(10, 39),
-                // (11,46): error CS4012: Parameters or locals of type 'RuntimeArgumentHandle' cannot be declared in async methods or async lambda expressions.
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefParameter, "r").WithArguments("System.TypedReference").WithLocation(10, 39),
+                // (11,46): error CS4012: Parameters of type 'RuntimeArgumentHandle' cannot be declared in async methods or async lambda expressions.
                 //         D2 d2 = async (RuntimeArgumentHandle h) => { await Task.Yield(); };
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "h").WithArguments("System.RuntimeArgumentHandle").WithLocation(11, 46),
-                // (12,36): error CS4012: Parameters or locals of type 'ArgIterator' cannot be declared in async methods or async lambda expressions.
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefParameter, "h").WithArguments("System.RuntimeArgumentHandle").WithLocation(11, 46),
+                // (12,36): error CS4012: Parameters of type 'ArgIterator' cannot be declared in async methods or async lambda expressions.
                 //         D3 d3 = async (ArgIterator i) => { await Task.Yield(); };
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "i").WithArguments("System.ArgIterator").WithLocation(12, 36));
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefParameter, "i").WithArguments("System.ArgIterator").WithLocation(12, 36));
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/LocalFunctionTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/LocalFunctionTests.cs
index 74f8efa9b607c..7e4c694e57ba1 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/LocalFunctionTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/LocalFunctionTests.cs
@@ -2394,21 +2394,52 @@ public unsafe IEnumerable<int> M4(int* a)
                 // (33,44): error CS1637: Iterators cannot have pointer type parameters
                 //     public unsafe IEnumerable<int> M4(int* a)
                 Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "a").WithLocation(33, 44),
-                // (33,36): error CS1629: Unsafe code may not appear in iterators
+                // (33,36): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //     public unsafe IEnumerable<int> M4(int* a)
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "M4").WithLocation(33, 36),
-                // (37,40): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "M4").WithArguments("ref and unsafe in async and iterator methods").WithLocation(33, 36),
+                // (37,40): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //                 IEnumerable<int> Local(int* b) { yield break; }
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "int*").WithLocation(37, 40),
-                // (39,23): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "int*").WithArguments("ref and unsafe in async and iterator methods").WithLocation(37, 40),
+                // (39,23): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //                 Local(&x);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "&x").WithLocation(39, 23),
-                // (39,17): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "&x").WithArguments("ref and unsafe in async and iterator methods").WithLocation(39, 23),
+                // (39,17): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //                 Local(&x);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "Local(&x)").WithLocation(39, 17),
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "Local(&x)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(39, 17),
                 // (37,45): error CS1637: Iterators cannot have pointer type parameters
                 //                 IEnumerable<int> Local(int* b) { yield break; }
                 Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "b").WithLocation(37, 45));
+
+            var expectedDiagnostics = new[]
+            {
+                // (8,37): error CS1637: Iterators cannot have pointer type parameters
+                //         IEnumerable<int> Local(int* a) { yield break; }
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "a").WithLocation(8, 37),
+                // (17,41): error CS1637: Iterators cannot have pointer type parameters
+                //             IEnumerable<int> Local(int* x) { yield break; }
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "x").WithLocation(17, 41),
+                // (27,37): error CS1637: Iterators cannot have pointer type parameters
+                //         IEnumerable<int> Local(int* a) { yield break; }
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "a").WithLocation(27, 37),
+                // (33,44): error CS1637: Iterators cannot have pointer type parameters
+                //     public unsafe IEnumerable<int> M4(int* a)
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "a").WithLocation(33, 44),
+                // (37,40): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //                 IEnumerable<int> Local(int* b) { yield break; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(37, 40),
+                // (37,45): error CS1637: Iterators cannot have pointer type parameters
+                //                 IEnumerable<int> Local(int* b) { yield break; }
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "b").WithLocation(37, 45),
+                // (39,17): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //                 Local(&x);
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "Local(&x)").WithLocation(39, 17),
+                // (39,23): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //                 Local(&x);
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "&x").WithLocation(39, 23)
+            };
+
+            CreateCompilation(src, options: TestOptions.UnsafeDebugDll, parseOptions: TestOptions.RegularNext.WithFeature("run-nullable-analysis", "never")).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(src, options: TestOptions.UnsafeDebugDll, parseOptions: TestOptions.RegularPreview.WithFeature("run-nullable-analysis", "never")).VerifyDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
@@ -3516,7 +3547,7 @@ static void Main()
                 // (10,31): error CS0190: The __arglist construct is valid only within a variable argument method
                 //             Console.WriteLine(__arglist);
                 Diagnostic(ErrorCode.ERR_ArgsInvalid, "__arglist").WithLocation(10, 31),
-                // (18,31): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
+                // (18,31): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside a nested function, query expression, iterator block or async method
                 //             Console.WriteLine(__arglist);
                 Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "__arglist").WithArguments("System.RuntimeArgumentHandle").WithLocation(18, 31),
                 // (24,20): error CS1669: __arglist is not valid in this context
@@ -3528,7 +3559,7 @@ static void Main()
                 // (32,20): error CS1669: __arglist is not valid in this context
                 //         void Local(__arglist)
                 Diagnostic(ErrorCode.ERR_IllegalVarArgs, "__arglist").WithLocation(32, 20),
-                // (34,31): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
+                // (34,31): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside a nested function, query expression, iterator block or async method
                 //             Console.WriteLine(__arglist);
                 Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "__arglist").WithArguments("System.RuntimeArgumentHandle").WithLocation(34, 31)
     );
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/NativeIntegerTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/NativeIntegerTests.cs
index 320f13951a7f9..07bd155f06f33 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/NativeIntegerTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/NativeIntegerTests.cs
@@ -4369,14 +4369,31 @@ static IEnumerable<int> F()
         yield return sizeof(nuint);
     }
 }";
-            var comp = CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular9);
-            comp.VerifyDiagnostics(
-                // (6,22): error CS1629: Unsafe code may not appear in iterators
+            var expectedDiagnostics = new[]
+            {
+                // (6,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "sizeof(nint)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 22),
+                // (7,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         yield return sizeof(nuint);
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "sizeof(nuint)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 22)
+            };
+
+            CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular9).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(expectedDiagnostics);
+
+            expectedDiagnostics = new[]
+            {
+                // (6,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
                 //         yield return sizeof(nint);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "sizeof(nint)").WithLocation(6, 22),
-                // (7,22): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(6, 22),
+                // (7,22): error CS0233: 'nuint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
                 //         yield return sizeof(nuint);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "sizeof(nuint)").WithLocation(7, 22));
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nuint)").WithArguments("nuint").WithLocation(7, 22)
+            };
+
+            CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(source, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/RefEscapingTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/RefEscapingTests.cs
index 893f9732de04e..f97c3f0a283ce 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/RefEscapingTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/RefEscapingTests.cs
@@ -4969,9 +4969,10 @@ public static unsafe void Test(TestStruct[] ar)
         [Theory]
         [InlineData(LanguageVersion.CSharp10)]
         [InlineData(LanguageVersion.CSharp11)]
+        [InlineData(LanguageVersionFacts.CSharpNext)]
         public void AwaitRefStruct(LanguageVersion languageVersion)
         {
-            CreateCompilation(@"
+            var comp = CreateCompilation(@"
 using System.Threading.Tasks;
 
 ref struct S { }
@@ -4991,20 +4992,67 @@ async Task M(Task<S> t)
     void M(S t, ref S t1)
     {
     }
-}", parseOptions: TestOptions.Regular.WithLanguageVersion(languageVersion), options: TestOptions.ReleaseDll).VerifyDiagnostics(
-                // (8,26): error CS9504: The type 'S' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'TResult' in the generic type or method 'Task<TResult>'
-                //     async Task M(Task<S> t)
-                Diagnostic(ErrorCode.ERR_NotRefStructConstraintNotSatisfied, "t").WithArguments("System.Threading.Tasks.Task<TResult>", "TResult", "S").WithLocation(8, 26),
-                // (12,9): error CS4012: Parameters or locals of type 'S' cannot be declared in async methods or async lambda expressions.
-                //         var a = await t;
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("S").WithLocation(12, 9),
-                // (14,9): error CS4012: Parameters or locals of type 'S' cannot be declared in async methods or async lambda expressions.
-                //         var r = t.Result;
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("S").WithLocation(14, 9),
-                // (15,9): error CS8350: This combination of arguments to 'C.M(S, ref S)' is disallowed because it may expose variables referenced by parameter 't' outside of their declaration scope
-                //         M(await t, ref r);
-                Diagnostic(ErrorCode.ERR_CallArgMixing, "M(await t, ref r)").WithArguments("C.M(S, ref S)", "t").WithLocation(15, 9)
-                );
+}", parseOptions: TestOptions.Regular.WithLanguageVersion(languageVersion), options: TestOptions.ReleaseDll);
+            if (languageVersion < LanguageVersionFacts.CSharpNext)
+            {
+                comp.VerifyDiagnostics(
+                    // (8,26): error CS9504: The type 'S' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'TResult' in the generic type or method 'Task<TResult>'
+                    //     async Task M(Task<S> t)
+                    Diagnostic(ErrorCode.ERR_NotRefStructConstraintNotSatisfied, "t").WithArguments("System.Threading.Tasks.Task<TResult>", "TResult", "S").WithLocation(8, 26),
+                    // (12,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                    //         var a = await t;
+                    Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(12, 9),
+                    // (14,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                    //         var r = t.Result;
+                    Diagnostic(ErrorCode.ERR_FeatureInPreview, "var").WithArguments("ref and unsafe in async and iterator methods").WithLocation(14, 9),
+                    // (15,9): error CS8350: This combination of arguments to 'C.M(S, ref S)' is disallowed because it may expose variables referenced by parameter 't' outside of their declaration scope
+                    //         M(await t, ref r);
+                    Diagnostic(ErrorCode.ERR_CallArgMixing, "M(await t, ref r)").WithArguments("C.M(S, ref S)", "t").WithLocation(15, 9)
+                    );
+            }
+            else
+            {
+                comp.VerifyDiagnostics(
+                    // (8,26): error CS9504: The type 'S' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'TResult' in the generic type or method 'Task<TResult>'
+                    //     async Task M(Task<S> t)
+                    Diagnostic(ErrorCode.ERR_NotRefStructConstraintNotSatisfied, "t").WithArguments("System.Threading.Tasks.Task<TResult>", "TResult", "S").WithLocation(8, 26),
+                    // (15,9): error CS8350: This combination of arguments to 'C.M(S, ref S)' is disallowed because it may expose variables referenced by parameter 't' outside of their declaration scope
+                    //         M(await t, ref r);
+                    Diagnostic(ErrorCode.ERR_CallArgMixing, "M(await t, ref r)").WithArguments("C.M(S, ref S)", "t").WithLocation(15, 9)
+                    );
+            }
+        }
+
+        [Fact]
+        public void AsyncLocals_Reassignment()
+        {
+            var code = """
+                using System.Threading.Tasks;
+                class C
+                {
+                    async Task M1()
+                    {
+                        int x = 42;
+                        ref int y = ref x;
+                        y.ToString();
+                        await Task.Yield();
+                        y.ToString(); // 1
+                    }
+                    async Task M2()
+                    {
+                        int x = 42;
+                        ref int y = ref x;
+                        y.ToString();
+                        await Task.Yield();
+                        y = ref x;
+                        y.ToString();
+                    }
+                }
+                """;
+            CreateCompilation(code).VerifyEmitDiagnostics(
+                // (10,9): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         y.ToString(); // 1
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(10, 9));
         }
 
         [WorkItem(25398, "https://github.com/dotnet/roslyn/issues/25398")]
@@ -5345,9 +5393,9 @@ public static IEnumerable<int> Iterator() {
 }
 """);
             compilation.VerifyEmitDiagnostics(
-                // (20,19): error CS4013: Instance of type 'PooledArrayHandle<int>' cannot be used inside a nested function, query expression, iterator block or async method
+                // (20,19): error CS4007: Instance of type 'PooledArrayHandle<int>' cannot be preserved across 'await' or 'yield' boundary.
                 //         using var handle = RentArray<int>(200, out var array);
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "handle = RentArray<int>(200, out var array)").WithArguments("PooledArrayHandle<int>").WithLocation(20, 19));
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "handle = RentArray<int>(200, out var array)").WithArguments("PooledArrayHandle<int>").WithLocation(20, 19));
         }
 
         [Theory(Skip = "https://github.com/dotnet/roslyn/issues/40583")]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/RefLocalsAndReturnsTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/RefLocalsAndReturnsTests.cs
index 825e55fd10807..7299939536559 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/RefLocalsAndReturnsTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/RefLocalsAndReturnsTests.cs
@@ -1209,10 +1209,7 @@ async Task M()
         await Task.FromResult(false);
     }
 }");
-            comp.VerifyDiagnostics(
-                // (7,26): error CS8177: Async methods cannot have by-reference locals
-                //         ref readonly int x = ref (new int[1])[0];
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "x").WithLocation(7, 26));
+            comp.VerifyEmitDiagnostics();
         }
 
         [Fact]
@@ -1229,10 +1226,87 @@ IEnumerable<int> M()
         yield return i;
     }
 }");
-            comp.VerifyDiagnostics(
-                // (7,26): error CS8176: Iterators cannot have by-reference locals
-                //         ref readonly int x = ref (new int[1])[0];
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "x").WithLocation(7, 26));
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void RefReadonlyInIterator_ForEach()
+        {
+            var source = """
+                using System.Collections.Generic;
+                class C
+                {
+                    int[] arr = new int[2];
+                    IEnumerable<int> M()
+                    {
+                        ref readonly int[] x = ref arr;
+
+                        foreach (var i in x)
+                        {
+                            System.Console.Write(i);
+                            yield return 1;
+                        }
+                
+                        foreach (var j in x)
+                        {
+                            System.Console.Write(j);
+                            yield return 2;
+                        }
+                    }
+                }
+                """;
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (7,28): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref readonly int[] x = ref arr;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "x").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 28));
+
+            var expectedDiagnostics = new[]
+            {
+                // (15,27): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //         foreach (var j in x)
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "x").WithLocation(15, 27)
+            };
+
+            CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+
+            CreateCompilation(source).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void RefReadonlyInIterator_ForEach_02()
+        {
+            var source = """
+                using System.Collections.Generic;
+
+                foreach (var i in new C().M()) System.Console.Write(i);
+
+                class C
+                {
+                    int[] arr = new[] { 4, 5 };
+                    public IEnumerable<int> M()
+                    {
+                        ref readonly int[] x = ref arr;
+
+                        foreach (var i in x)
+                        {
+                            System.Console.Write(i);
+                            yield return 1;
+                        }
+                    }
+                }
+                """;
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (10,28): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         ref readonly int[] x = ref arr;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "x").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 28));
+
+            var expectedOutput = "4151";
+
+            CompileAndVerify(source, parseOptions: TestOptions.RegularNext, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            CompileAndVerify(source, expectedOutput: expectedOutput).VerifyDiagnostics();
         }
 
         [Fact]
@@ -1247,7 +1321,7 @@ IEnumerable<int> M()
         switch (this)
         {
             default:
-                ref readonly int x = ref (new int[1])[0]; // 1
+                ref readonly int x = ref (new int[1])[0];
                 yield return 1;
                 yield return x;
 
@@ -1260,10 +1334,10 @@ void local()
         }
     }
 }");
-            comp.VerifyDiagnostics(
-                // (10,34): error CS8176: Iterators cannot have by-reference locals
-                //                 ref readonly int x = ref (new int[1])[0]; // 1
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "x").WithLocation(10, 34));
+            comp.VerifyEmitDiagnostics(
+                // (12,30): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //                 yield return x;
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "x").WithLocation(12, 30));
         }
 
         [Fact]
@@ -1278,22 +1352,19 @@ IEnumerable<int> M()
         switch (this)
         {
             default:
-                ref readonly int x; // 1, 2
+                ref readonly int x; // 1
                 yield return 1;
-                yield return x; // 3
+                yield return x; // 2
                 break;
         }
     }
 }");
             comp.VerifyDiagnostics(
-                // (10,34): error CS8176: Iterators cannot have by-reference locals
-                //                 ref readonly int x; // 1, 2
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "x").WithLocation(10, 34),
                 // (10,34): error CS8174: A declaration of a by-reference variable must have an initializer
-                //                 ref readonly int x; // 1, 2
+                //                 ref readonly int x; // 1
                 Diagnostic(ErrorCode.ERR_ByReferenceVariableMustBeInitialized, "x").WithLocation(10, 34),
                 // (12,30): error CS0165: Use of unassigned local variable 'x'
-                //                 yield return x; // 3
+                //                 yield return x; // 2
                 Diagnostic(ErrorCode.ERR_UseDefViolation, "x").WithArguments("x").WithLocation(12, 30));
         }
 
@@ -1316,9 +1387,9 @@ IEnumerable<int> M()
     }
 }");
             comp.VerifyDiagnostics(
-                // (10,43): error CS8176: Iterators cannot have by-reference locals
+                // (10,49): error CS1510: A ref or out value must be an assignable variable
                 //                 foreach (ref readonly int x in (new int[1]))
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "x").WithLocation(10, 43));
+                Diagnostic(ErrorCode.ERR_RefLvalueExpected, "new int[1]").WithLocation(10, 49));
         }
 
         [Fact]
@@ -1331,18 +1402,15 @@ class C
     IEnumerable<int> M()
     {
         if (true)
-            ref int x = ref (new int[1])[0]; // 1, 2
+            ref int x = ref (new int[1])[0]; // 1
         
         yield return 1;
     }
 }");
             comp.VerifyDiagnostics(
                 // (8,13): error CS1023: Embedded statement cannot be a declaration or labeled statement
-                //             ref int x = ref (new int[1])[0]; // 1, 2
-                Diagnostic(ErrorCode.ERR_BadEmbeddedStmt, "ref int x = ref (new int[1])[0];").WithLocation(8, 13),
-                // (8,21): error CS8176: Iterators cannot have by-reference locals
-                //             ref int x = ref (new int[1])[0]; // 1, 2
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "x").WithLocation(8, 21));
+                //             ref int x = ref (new int[1])[0]; // 1
+                Diagnostic(ErrorCode.ERR_BadEmbeddedStmt, "ref int x = ref (new int[1])[0];").WithLocation(8, 13));
         }
 
         [Fact]
@@ -1355,18 +1423,15 @@ class C
     async Task M()
     {
         if (true)
-            ref int x = ref (new int[1])[0]; // 1, 2
+            ref int x = ref (new int[1])[0]; // 1
         
         await Task.Yield();
     }
 }");
             comp.VerifyDiagnostics(
                 // (8,13): error CS1023: Embedded statement cannot be a declaration or labeled statement
-                //             ref int x = ref (new int[1])[0]; // 1, 2
-                Diagnostic(ErrorCode.ERR_BadEmbeddedStmt, "ref int x = ref (new int[1])[0];").WithLocation(8, 13),
-                // (8,21): error CS8177: Async methods cannot have by-reference locals
-                //             ref int x = ref (new int[1])[0]; // 1, 2
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "x").WithLocation(8, 21));
+                //             ref int x = ref (new int[1])[0]; // 1
+                Diagnostic(ErrorCode.ERR_BadEmbeddedStmt, "ref int x = ref (new int[1])[0];").WithLocation(8, 13));
         }
 
         [Fact]
@@ -3087,14 +3152,147 @@ IEnumerable<int> localFunction()
         }
     }
 }";
-
-            CreateCompilation(code).VerifyDiagnostics(
-                // (13,21): error CS8176: Iterators cannot have by-reference locals
-                //             ref int z = ref x;
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "z").WithLocation(13, 21),
-                // (8,17): error CS8176: Iterators cannot have by-reference locals
+            CreateCompilation(code, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (8,17): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         ref int y = ref x;
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "y").WithLocation(8, 17));
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 17),
+                // (13,21): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             ref int z = ref x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "z").WithArguments("ref and unsafe in async and iterator methods").WithLocation(13, 21));
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(code).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void RefLocal_Iterator_LocalFunction_01()
+        {
+            var code = """
+                using System;
+                using System.Collections.Generic;
+
+                int x = 5;
+
+                foreach (var z in func())
+                {
+                    Console.Write(z);
+                    x++;
+                }
+
+                IEnumerable<int> func()
+                {
+                    ref int y = ref x;
+                    yield return y;
+                    y = ref x;
+                    yield return y;
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (14,13): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     ref int y = ref x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(14, 13));
+
+            var expectedOutput = "56";
+
+            CompileAndVerify(code, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext).VerifyDiagnostics();
+            CompileAndVerify(code, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefLocal_Iterator_LocalFunction_02()
+        {
+            var code = """
+                using System.Collections.Generic;
+
+                int x = 5;
+
+                IEnumerable<int> func()
+                {
+                    ref int y = ref x;
+                    yield return y;
+                    yield return y;
+                }
+
+                func();
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (7,13): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     ref int y = ref x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 13));
+
+            var expectedDiagnostics = new[]
+            {
+                // (9,18): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //     yield return y;
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(9, 18)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(code).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void RefStruct_Iterator_LocalFunction_01()
+        {
+            var code = """
+                using System;
+                using System.Collections.Generic;
+
+                int x = 5;
+
+                foreach (var z in func())
+                {
+                    Console.Write(z);
+                    x++;
+                }
+
+                IEnumerable<int> func()
+                {
+                    Span<int> y = new(ref x);
+                    yield return y[0];
+                    y = new(ref x);
+                    yield return y[0];
+                }
+                """;
+
+            var expectedOutput = ExecutionConditionUtil.IsDesktop ? null : "56";
+
+            CompileAndVerify(code, expectedOutput: expectedOutput, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net70, verify: Verification.FailsPEVerify).VerifyDiagnostics();
+            CompileAndVerify(code, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net70, verify: Verification.FailsPEVerify).VerifyDiagnostics();
+            CompileAndVerify(code, expectedOutput: expectedOutput, targetFramework: TargetFramework.Net70, verify: Verification.FailsPEVerify).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefStruct_Iterator_LocalFunction_02()
+        {
+            var code = """
+                using System;
+                using System.Collections.Generic;
+
+                int x = 5;
+
+                IEnumerable<int> func()
+                {
+                    Span<int> y = new(ref x);
+                    yield return y[0];
+                    yield return y[0];
+                }
+
+                func();
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (10,18): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //     yield return y[0];
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.Span<int>").WithLocation(10, 18)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [Fact, WorkItem(13073, "https://github.com/dotnet/roslyn/issues/13073")]
@@ -3115,13 +3313,152 @@ await Task.Run(async () =>
         });
     }
 }";
-            CreateCompilationWithMscorlib45(code).VerifyDiagnostics(
-                // (8,17): error CS8177: Async methods cannot have by-reference locals
+            CreateCompilation(code, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (8,17): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //         ref int y = ref x;
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "y").WithLocation(8, 17),
-                // (11,21): error CS8177: Async methods cannot have by-reference locals
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 17),
+                // (11,21): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //             ref int z = ref x;
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "z").WithLocation(11, 21));
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "z").WithArguments("ref and unsafe in async and iterator methods").WithLocation(11, 21));
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(code).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void RefLocal_Async_LocalFunction_01()
+        {
+            var code = """
+                using System;
+                using System.Threading.Tasks;
+
+                int x = 5;
+
+                await func();
+
+                async Task func()
+                {
+                    ref int y = ref x;
+                    Console.Write(y);
+                    await Task.Yield();
+                    y = ref x;
+                    Console.Write(y);
+                    await Task.Yield();
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (10,13): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     ref int y = ref x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 13));
+
+            var expectedOutput = "55";
+
+            CompileAndVerify(code, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext).VerifyDiagnostics();
+            CompileAndVerify(code, expectedOutput: expectedOutput).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefLocal_Async_LocalFunction_02()
+        {
+            var code = """
+                using System;
+                using System.Threading.Tasks;
+
+                int x = 5;
+
+                async Task func()
+                {
+                    ref int y = ref x;
+                    await Task.Yield();
+                    Console.Write(y);
+                }
+
+                await func();
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (8,13): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     ref int y = ref x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "y").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 13));
+
+            var expectedDiagnostics = new[]
+            {
+                // (10,19): error CS9217: A 'ref' local cannot be preserved across 'await' or 'yield' boundary.
+                //     Console.Write(y);
+                Diagnostic(ErrorCode.ERR_RefLocalAcrossAwait, "y").WithLocation(10, 19)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(code).VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void RefStruct_Async_LocalFunction_01()
+        {
+            var code = """
+                using System;
+                using System.Threading.Tasks;
+                
+                int x = 5;
+                
+                await func();
+                
+                async Task func()
+                {
+                    Span<int> y = new(ref x);
+                    Console.Write(y[0]);
+                    await Task.Yield();
+                    y = new(ref x);
+                    Console.Write(y[0]);
+                    await Task.Yield();
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net70).VerifyDiagnostics(
+                // (10,5): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     Span<int> y = new(ref x);
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "Span<int>").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 5));
+
+            var expectedOutput = ExecutionConditionUtil.IsDesktop ? null : "55";
+
+            CompileAndVerify(code, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net70, verify: Verification.FailsPEVerify).VerifyDiagnostics();
+            CompileAndVerify(code, expectedOutput: expectedOutput, targetFramework: TargetFramework.Net70, verify: Verification.FailsPEVerify).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void RefStruct_Async_LocalFunction_02()
+        {
+            var code = """
+                using System;
+                using System.Threading.Tasks;
+
+                int x = 5;
+
+                async Task func()
+                {
+                    Span<int> y = new(ref x);
+                    await Task.Yield();
+                    Console.Write(y[0]);
+                }
+
+                await func();
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, targetFramework: TargetFramework.Net70).VerifyDiagnostics(
+                // (8,5): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     Span<int> y = new(ref x);
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "Span<int>").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 5));
+
+            var expectedDiagnostics = new[]
+            {
+                // (10,19): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //     Console.Write(y[0]);
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.Span<int>").WithLocation(10, 19)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [Fact, WorkItem(13073, "https://github.com/dotnet/roslyn/issues/13073")]
@@ -3434,15 +3771,40 @@ static async void Goo()
 }
 ";
 
-            CreateCompilationWithMscorlib46(text).VerifyDiagnostics(
-                // (8,17): error CS8177: Async methods cannot have by-reference locals
-                //         ref int i = ref field;
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "i").WithLocation(8, 17),
+            CreateCompilationWithMscorlib46(text).VerifyEmitDiagnostics(
                 // (6,23): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
                 //     static async void Goo()
                 Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "Goo").WithLocation(6, 23));
         }
 
+        [Fact]
+        public void RefReturnAcrossAwaitExpression()
+        {
+            var code = """
+                using System.Threading.Tasks;
+
+                class Program
+                {
+                    static int field = 1;
+
+                    static async Task Main()
+                    {
+                        M1(ref GiveMeRef(), await M2());
+                    }
+
+                    static void M1(ref int i, int j) { }
+
+                    static ref int GiveMeRef() => ref field;
+
+                    static Task<int> M2() => Task.FromResult(field++);
+                }
+                """;
+            CreateCompilation(code).VerifyEmitDiagnostics(
+                // (9,16): error CS8178: A reference returned by a call to 'Program.GiveMeRef()' cannot be preserved across 'await' or 'yield' boundary.
+                //         M1(ref GiveMeRef(), await M2());
+                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait, "GiveMeRef()").WithArguments("Program.GiveMeRef()").WithLocation(9, 16));
+        }
+
         [Fact]
         public void BadRefLocalInIteratorMethod()
         {
@@ -3461,10 +3823,7 @@ static IEnumerable ObjEnumerable()
 }
 ";
 
-            CreateCompilationWithMscorlib46(text).VerifyDiagnostics(
-                // (10,17): error CS8931: Iterators cannot have by-reference locals
-                //         ref int i = ref field;
-                Diagnostic(ErrorCode.ERR_BadIteratorLocalType, "i").WithLocation(10, 17));
+            CreateCompilationWithMscorlib46(text).VerifyEmitDiagnostics();
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/SemanticErrorTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/SemanticErrorTests.cs
index fc678a39411b1..d361b612ede78 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/SemanticErrorTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/SemanticErrorTests.cs
@@ -7269,15 +7269,15 @@ static void Main()
 }";
             var comp = CreateCompilationWithMscorlib40AndSystemCore(source);
             comp.VerifyDiagnostics(
-                // (10,14): error CS4013: Instance of type 'System.RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
+                // (10,14): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside a nested function, query expression, iterator block or async method
                 //     f = x=>f(__arglist);
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "__arglist").WithArguments("System.RuntimeArgumentHandle"),
-                // (11,29): error CS4013: Instance of type 'System.RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
+                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "__arglist").WithArguments("System.RuntimeArgumentHandle").WithLocation(10, 14),
+                // (11,29): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside a nested function, query expression, iterator block or async method
                 //     f = delegate { return f(__arglist); };
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "__arglist").WithArguments("System.RuntimeArgumentHandle"),
-                // (12,44): error CS4013: Instance of type 'System.RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
+                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "__arglist").WithArguments("System.RuntimeArgumentHandle").WithLocation(11, 29),
+                // (12,44): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside a nested function, query expression, iterator block or async method
                 //     var q = from x in new int[10] select f(__arglist);
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "__arglist").WithArguments("System.RuntimeArgumentHandle")
+                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "__arglist").WithArguments("System.RuntimeArgumentHandle").WithLocation(12, 44)
                 );
         }
 
@@ -7304,12 +7304,12 @@ static void Main()
   }
 }";
             CreateCompilationWithMscorlib45(source).VerifyEmitDiagnostics(
-                // (10,34): error CS4013: Instance of type 'System.RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
+                // (10,34): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside a nested function, query expression, iterator block or async method
                 //       RuntimeArgumentHandle h2 = h; // Bad use of h
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "h").WithArguments("System.RuntimeArgumentHandle"),
-                // (11,43): error CS4013: Instance of type 'System.RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
+                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "h").WithArguments("System.RuntimeArgumentHandle").WithLocation(10, 34),
+                // (11,43): error CS4013: Instance of type 'RuntimeArgumentHandle' cannot be used inside a nested function, query expression, iterator block or async method
                 //       ArgIterator args1 = new ArgIterator(h); // Bad use of h
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "h").WithArguments("System.RuntimeArgumentHandle"));
+                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "h").WithArguments("System.RuntimeArgumentHandle").WithLocation(11, 43));
         }
 
         [Fact]
@@ -7321,9 +7321,9 @@ public void CS4013ERR_SpecialByRefInLambda03()
 public class C
 {
   static void N(RuntimeArgumentHandle x) {}
-  static IEnumerable<int> M(RuntimeArgumentHandle h1) // Error: hoisted to field
+  static IEnumerable<int> M(RuntimeArgumentHandle h1)
   {
-    N(h1);
+    N(h1); // Error: hoisted to field
     yield return 1;
     RuntimeArgumentHandle h2 = default(RuntimeArgumentHandle);
     yield return 2;
@@ -7339,12 +7339,12 @@ static void Main()
 
             CreateCompilation(source).Emit(new System.IO.MemoryStream()).Diagnostics
                 .Verify(
-                // (7,51): error CS4013: Instance of type 'System.RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
-                //   static IEnumerable<int> M(RuntimeArgumentHandle h1) // Error: hoisted to field
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "h1").WithArguments("System.RuntimeArgumentHandle"),
-                // (13,7): error CS4013: Instance of type 'System.RuntimeArgumentHandle' cannot be used inside an anonymous function, query expression, iterator block or async method
+                // (9,7): error CS4007: Instance of type 'System.RuntimeArgumentHandle' cannot be preserved across 'await' or 'yield' boundary.
+                //     N(h1); // Error: hoisted to field
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "h1").WithArguments("System.RuntimeArgumentHandle").WithLocation(9, 7),
+                // (13,7): error CS4007: Instance of type 'System.RuntimeArgumentHandle' cannot be preserved across 'await' or 'yield' boundary.
                 //     N(h2); // Error: hoisted to field
-                Diagnostic(ErrorCode.ERR_SpecialByRefInLambda, "h2").WithArguments("System.RuntimeArgumentHandle")
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "h2").WithArguments("System.RuntimeArgumentHandle").WithLocation(13, 7)
                 );
         }
 
@@ -15318,35 +15318,48 @@ class C
 {
    IEnumerator<int> IteratorMeth() {
       int i;
-      unsafe  // CS1629
+      unsafe
       {
          int *p = &i;
          yield return *p;
       }
    }
 
-    unsafe IEnumerator<int> IteratorMeth2() {   // CS1629
+    unsafe IEnumerator<int> IteratorMeth2() {
         yield break;
     }
 }
 ";
-            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (7,7): error CS1629: Unsafe code may not appear in iterators
-                //       unsafe  // CS1629
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "unsafe"),
-                // (9,10): error CS1629: Unsafe code may not appear in iterators
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (7,7): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //       unsafe
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 7),
+                // (9,10): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //          int *p = &i;
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "int *"),
-                // (9,19): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "int *").WithArguments("ref and unsafe in async and iterator methods").WithLocation(9, 10),
+                // (9,19): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //          int *p = &i;
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "&i"),
-                // (10,24): error CS1629: Unsafe code may not appear in iterators
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "&i").WithArguments("ref and unsafe in async and iterator methods").WithLocation(9, 19),
+                // (10,24): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //          yield return *p;
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "p"),
-                // (14,29): error CS1629: Unsafe code may not appear in iterators
-                //     unsafe IEnumerator<int> IteratorMeth2() {   // CS1629
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "IteratorMeth2")
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "p").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 24),
+                // (14,29): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     unsafe IEnumerator<int> IteratorMeth2() {
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "IteratorMeth2").WithArguments("ref and unsafe in async and iterator methods").WithLocation(14, 29)
                 );
+
+            var expectedDiagnostics = new[]
+            {
+                // (9,20): error CS9239: The '&' operator cannot be used on parameters or local variables in iterator methods.
+                //          int *p = &i;
+                Diagnostic(ErrorCode.ERR_AddressOfInIterator, "i").WithLocation(9, 20),
+                // (10,10): error CS9238: Cannot use 'yield return' in an 'unsafe' block
+                //          yield return *p;
+                Diagnostic(ErrorCode.ERR_BadYieldInUnsafe, "yield").WithLocation(10, 10)
+            };
+
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/SpanStackSafetyTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/SpanStackSafetyTests.cs
index a66cc2a843bb2..fac0d3fe54188 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/SpanStackSafetyTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/SpanStackSafetyTests.cs
@@ -1158,9 +1158,9 @@ public static async Task<int> M1(Span<int> arg)
             CSharpCompilation comp = CreateCompilationWithMscorlibAndSpan(text);
 
             comp.VerifyDiagnostics(
-                // (11,48): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
+                // (11,48): error CS4012: Parameters of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
                 //     public static async Task<int> M1(Span<int> arg)
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "arg").WithArguments("System.Span<int>").WithLocation(11, 48)
+                Diagnostic(ErrorCode.ERR_BadSpecialByRefParameter, "arg").WithArguments("System.Span<int>").WithLocation(11, 48)
             );
         }
 
@@ -1179,48 +1179,77 @@ static void Main()
 
     public static async Task<int> M1()
     {
-        Span<int> local1 = default(Span<int>); // 1
-        var local2 = default(Span<int>); // 2
+        Span<int> local1 = default(Span<int>);
+        var local2 = default(Span<int>);
 
         await Task.Yield();
         return 42;
     }
+
+    public static async Task<int> M2()
+    {
+        Span<int> local1 = default(Span<int>);
+        var local2 = default(Span<int>);
+
+        await Task.Yield();
+        return local1.Length; // 1
+    }
+
+    public static async Task<int> M3()
+    {
+        Span<int> local1 = default(Span<int>);
+        var local2 = default(Span<int>);
+
+        await Task.Yield();
+        return local2.Length; // 2
+    }
+
+    public static async Task<int> M4()
+    {
+        Span<int> local1 = default(Span<int>);
+        var local2 = default(Span<int>);
+
+        await Task.Yield();
+        return local1.Length + local2.Length; // 3, 4
+    }
 }
 ";
 
-            CSharpCompilation comp = CreateCompilationWithMscorlibAndSpan(text);
-
-            comp.VerifyDiagnostics(
-                // (13,9): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         Span<int> local1 = default(Span<int>); // 1
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "Span<int>").WithArguments("System.Span<int>").WithLocation(13, 9),
+            var expectedDiagnostics = new[]
+            {
                 // (13,19): warning CS0219: The variable 'local1' is assigned but its value is never used
-                //         Span<int> local1 = default(Span<int>); // 1
+                //         Span<int> local1 = default(Span<int>);
                 Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "local1").WithArguments("local1").WithLocation(13, 19),
-                // (14,9): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         var local2 = default(Span<int>); // 2
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("System.Span<int>").WithLocation(14, 9),
                 // (14,13): warning CS0219: The variable 'local2' is assigned but its value is never used
-                //         var local2 = default(Span<int>); // 2
-                Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "local2").WithArguments("local2").WithLocation(14, 13)
-            );
+                //         var local2 = default(Span<int>);
+                Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "local2").WithArguments("local2").WithLocation(14, 13),
+                // (23,13): warning CS0219: The variable 'local2' is assigned but its value is never used
+                //         var local2 = default(Span<int>);
+                Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "local2").WithArguments("local2").WithLocation(23, 13),
+                // (26,16): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         return local1.Length; // 1
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "local1").WithArguments("System.Span<int>").WithLocation(26, 16),
+                // (31,19): warning CS0219: The variable 'local1' is assigned but its value is never used
+                //         Span<int> local1 = default(Span<int>);
+                Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "local1").WithArguments("local1").WithLocation(31, 19),
+                // (35,16): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         return local2.Length; // 2
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "local2").WithArguments("System.Span<int>").WithLocation(35, 16),
+                // (44,16): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         return local1.Length + local2.Length; // 3, 4
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "local1").WithArguments("System.Span<int>").WithLocation(44, 16),
+                // (44,32): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         return local1.Length + local2.Length; // 3, 4
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "local2").WithArguments("System.Span<int>").WithLocation(44, 32)
+            };
+
+            CSharpCompilation comp = CreateCompilationWithMscorlibAndSpan(text);
+
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
 
             comp = CreateCompilationWithMscorlibAndSpan(text, TestOptions.DebugExe);
 
-            comp.VerifyDiagnostics(
-                // (13,9): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         Span<int> local1 = default(Span<int>); // 1
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "Span<int>").WithArguments("System.Span<int>").WithLocation(13, 9),
-                // (13,19): warning CS0219: The variable 'local1' is assigned but its value is never used
-                //         Span<int> local1 = default(Span<int>); // 1
-                Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "local1").WithArguments("local1").WithLocation(13, 19),
-                // (14,9): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         var local2 = default(Span<int>); // 2
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("System.Span<int>").WithLocation(14, 9),
-                // (14,13): warning CS0219: The variable 'local2' is assigned but its value is never used
-                //         var local2 = default(Span<int>); // 2
-                Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "local2").WithArguments("local2").WithLocation(14, 13)
-            );
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
@@ -1234,27 +1263,44 @@ public class Program
 {
     public static async Task<int> M1()
     {
-        M2(out var local1); // 1
-        M2(out Span<int> local2); // 2
+        M2(out var local1);
+        M2(out Span<int> local2);
 
         await Task.Yield();
         return 42;
     }
 
+    public static async Task<int> M3()
+    {
+        M2(out var local1);
+        M2(out Span<int> local2);
+
+        await Task.Yield();
+        return local1.Length; // 1
+    }
+
+    public static async Task<int> M4()
+    {
+        M2(out var local1);
+        M2(out Span<int> local2);
+
+        await Task.Yield();
+        return local2.Length; // 2
+    }
+
     static void M2(out Span<int> s) => throw null;
 }
 ";
 
             CSharpCompilation comp = CreateCompilationWithMscorlibAndSpan(text);
 
-            comp.VerifyDiagnostics(
-                // (9,16): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         M2(out var local1); // 1
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("System.Span<int>").WithLocation(9, 16),
-                // (10,16): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         M2(out Span<int> local2); // 2
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "Span<int>").WithArguments("System.Span<int>").WithLocation(10, 16)
-                );
+            comp.VerifyEmitDiagnostics(
+                // (22,16): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         return local1.Length; // 1
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "local1").WithArguments("System.Span<int>").WithLocation(22, 16),
+                // (31,16): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         return local2.Length; // 2
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "local2").WithArguments("System.Span<int>").WithLocation(31, 16));
         }
 
         [Fact, WorkItem(62747, "https://github.com/dotnet/roslyn/issues/62747")]
@@ -1269,30 +1315,49 @@ public class Program
 {
     public async Task M(IReadOnlyList<string> o)
     {
-        foreach (ReadOnlySpan<char> c1 in o) { } // 1
+        foreach (ReadOnlySpan<char> c1 in o) { }
 
         var enumerator = ((IEnumerable<string>)o).GetEnumerator();
         while (enumerator.MoveNext())
         {
-            ReadOnlySpan<char> c2 = (ReadOnlySpan<char>)(string)enumerator.Current; // 2
+            ReadOnlySpan<char> c2 = (ReadOnlySpan<char>)(string)enumerator.Current;
             _ = c2.Length;
         }
 
         await Task.Yield();
         return;
     }
+
+    public async Task M2(IReadOnlyList<string> o)
+    {
+        foreach (ReadOnlySpan<char> c1 in o)
+        {
+            await Task.Yield();
+            _ = c1.Length; // 1
+        }
+
+        var enumerator = ((IEnumerable<string>)o).GetEnumerator();
+        while (enumerator.MoveNext())
+        {
+            ReadOnlySpan<char> c2 = (ReadOnlySpan<char>)(string)enumerator.Current;
+            await Task.Yield();
+            _ = c2.Length; // 2
+        }
+
+        await Task.Yield();
+        return;
+    }
 }
 ";
 
             var comp = CreateCompilation(src, targetFramework: TargetFramework.Net70);
-            comp.VerifyDiagnostics(
-                // (10,18): error CS4012: Parameters or locals of type 'ReadOnlySpan<char>' cannot be declared in async methods or async lambda expressions.
-                //         foreach (ReadOnlySpan<char> c1 in o) { } // 1
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "ReadOnlySpan<char>").WithArguments("System.ReadOnlySpan<char>").WithLocation(10, 18),
-                // (15,13): error CS4012: Parameters or locals of type 'ReadOnlySpan<char>' cannot be declared in async methods or async lambda expressions.
-                //             ReadOnlySpan<char> c2 = (ReadOnlySpan<char>)(string)enumerator.Current; // 2
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "ReadOnlySpan<char>").WithArguments("System.ReadOnlySpan<char>").WithLocation(15, 13)
-                );
+            comp.VerifyEmitDiagnostics(
+                // (28,17): error CS4007: Instance of type 'System.ReadOnlySpan<char>' cannot be preserved across 'await' or 'yield' boundary.
+                //             _ = c1.Length; // 1
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "c1").WithArguments("System.ReadOnlySpan<char>").WithLocation(28, 17),
+                // (36,17): error CS4007: Instance of type 'System.ReadOnlySpan<char>' cannot be preserved across 'await' or 'yield' boundary.
+                //             _ = c2.Length; // 2
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "c2").WithArguments("System.ReadOnlySpan<char>").WithLocation(36, 17));
         }
 
         [Fact, WorkItem(62747, "https://github.com/dotnet/roslyn/issues/62747")]
@@ -1306,11 +1371,23 @@ public class Program
 {
     public async Task M()
     {
-        (Span<int> s1, Span<int> s2) = new Program(); // 1, 2
-        var (s3, s4) = new Program(); // 3, 4
-        (var s5, var s6) = new Program(); // 5, 6
+        (Span<int> s1, Span<int> s2) = new Program();
+        var (s3, s4) = new Program();
+        (var s5, var s6) = new Program();
+
+        await Task.Yield();
+        return;
+    }
+
+    public async Task M2()
+    {
+        (Span<int> s1, Span<int> s2) = new Program();
+        var (s3, s4) = new Program();
+        (var s5, var s6) = new Program();
 
         await Task.Yield();
+
+        _ = s1.Length + s4.Length + s5.Length; // 1, 2, 3
         return;
     }
 
@@ -1318,26 +1395,16 @@ public async Task M()
 }
 ";
             var comp = CreateCompilation(src, targetFramework: TargetFramework.Net70);
-            comp.VerifyDiagnostics(
-                // (9,10): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         (Span<int> s1, Span<int> s2) = new Program(); // 1, 2
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "Span<int>").WithArguments("System.Span<int>").WithLocation(9, 10),
-                // (9,24): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         (Span<int> s1, Span<int> s2) = new Program(); // 1, 2
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "Span<int>").WithArguments("System.Span<int>").WithLocation(9, 24),
-                // (10,14): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         var (s3, s4) = new Program(); // 3, 4
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "s3").WithArguments("System.Span<int>").WithLocation(10, 14),
-                // (10,18): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         var (s3, s4) = new Program(); // 3, 4
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "s4").WithArguments("System.Span<int>").WithLocation(10, 18),
-                // (11,10): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         (var s5, var s6) = new Program(); // 5, 6
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("System.Span<int>").WithLocation(11, 10),
-                // (11,18): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         (var s5, var s6) = new Program(); // 5, 6
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("System.Span<int>").WithLocation(11, 18)
-                );
+            comp.VerifyEmitDiagnostics(
+                // (25,13): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         _ = s1.Length + s4.Length + s5.Length; // 1, 2, 3
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s1").WithArguments("System.Span<int>").WithLocation(25, 13),
+                // (25,25): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         _ = s1.Length + s4.Length + s5.Length; // 1, 2, 3
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s4").WithArguments("System.Span<int>").WithLocation(25, 25),
+                // (25,37): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         _ = s1.Length + s4.Length + s5.Length; // 1, 2, 3
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s5").WithArguments("System.Span<int>").WithLocation(25, 37));
         }
 
         [Fact, WorkItem(62747, "https://github.com/dotnet/roslyn/issues/62747")]
@@ -1350,11 +1417,26 @@ public class Program
 {
     public async Task M()
     {
-        using (default(RS)) { } // 1
-        using (var s1 = default(RS)) { } // 2
-        using (RS s2 = default(RS)) { } // 3
-        using RS s3 = default(RS); // 4
-        using var s4 = default(RS); // 5
+        using (default(RS)) { }
+        using (var s1 = default(RS)) { }
+        using (RS s2 = default(RS)) { }
+        using RS s3 = default(RS); // 1
+        using var s4 = default(RS); // 2
+
+        await Task.Yield();
+        return;
+    }
+
+    public async Task M2()
+    {
+        using (default(RS)) { await Task.Yield(); } // 3
+        using (var s1 = default(RS)) { await Task.Yield(); } // 4
+        using (RS s2 = default(RS)) { await Task.Yield(); } // 5
+        {
+            using RS s3 = default(RS); // 6
+            await Task.Yield();
+            using var s4 = default(RS);
+        }
 
         await Task.Yield();
         return;
@@ -1367,23 +1449,25 @@ public void Dispose() { }
 }
 ";
             var comp = CreateCompilation(src, targetFramework: TargetFramework.Net70);
-            comp.VerifyDiagnostics(
-                // (8,16): error CS9104: A using statement resource of type 'RS' cannot be used in async methods or async lambda expressions.
-                //         using (default(RS)) { } // 1
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefUsing, "default(RS)").WithArguments("RS").WithLocation(8, 16),
-                // (9,16): error CS4012: Parameters or locals of type 'RS' cannot be declared in async methods or async lambda expressions.
-                //         using (var s1 = default(RS)) { } // 2
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("RS").WithLocation(9, 16),
-                // (10,16): error CS4012: Parameters or locals of type 'RS' cannot be declared in async methods or async lambda expressions.
-                //         using (RS s2 = default(RS)) { } // 3
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "RS").WithArguments("RS").WithLocation(10, 16),
-                // (11,15): error CS4012: Parameters or locals of type 'RS' cannot be declared in async methods or async lambda expressions.
-                //         using RS s3 = default(RS); // 4
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "RS").WithArguments("RS").WithLocation(11, 15),
-                // (12,15): error CS4012: Parameters or locals of type 'RS' cannot be declared in async methods or async lambda expressions.
-                //         using var s4 = default(RS); // 5
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "var").WithArguments("RS").WithLocation(12, 15)
-                );
+            comp.VerifyEmitDiagnostics(
+                // (11,18): error CS4007: Instance of type 'RS' cannot be preserved across 'await' or 'yield' boundary.
+                //         using RS s3 = default(RS); // 1
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s3 = default(RS)").WithArguments("RS").WithLocation(11, 18),
+                // (12,19): error CS4007: Instance of type 'RS' cannot be preserved across 'await' or 'yield' boundary.
+                //         using var s4 = default(RS); // 2
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s4 = default(RS)").WithArguments("RS").WithLocation(12, 19),
+                // (20,16): error CS4007: Instance of type 'RS' cannot be preserved across 'await' or 'yield' boundary.
+                //         using (default(RS)) { await Task.Yield(); } // 3
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "default(RS)").WithArguments("RS").WithLocation(20, 16),
+                // (21,20): error CS4007: Instance of type 'RS' cannot be preserved across 'await' or 'yield' boundary.
+                //         using (var s1 = default(RS)) { await Task.Yield(); } // 4
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s1 = default(RS)").WithArguments("RS").WithLocation(21, 20),
+                // (22,19): error CS4007: Instance of type 'RS' cannot be preserved across 'await' or 'yield' boundary.
+                //         using (RS s2 = default(RS)) { await Task.Yield(); } // 5
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s2 = default(RS)").WithArguments("RS").WithLocation(22, 19),
+                // (24,22): error CS4007: Instance of type 'RS' cannot be preserved across 'await' or 'yield' boundary.
+                //             using RS s3 = default(RS); // 6
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s3 = default(RS)").WithArguments("RS").WithLocation(24, 22));
         }
 
         [Fact]
@@ -1397,24 +1481,76 @@ public class Program
 {
     public async Task M()
     {
-        if (M2() is var s1) { } // 1
-        if (M2() is Span<int> s2) { } // 2
+        if (M2() is var s1) { }
+        if (M2() is Span<int> s2) { }
+        if (M2() is var s3) { await Task.Yield(); }
+        if (M2() is Span<int> s4) { await Task.Yield(); }
+
+        await Task.Yield();
+        return;
+    }
+
+    public async Task M3()
+    {
+        if (M2() is var s1)
+        {
+            await Task.Yield();
+            _ = s1.Length; // 1
+        }
+        if (M2() is Span<int> s2)
+        {
+            await Task.Yield();
+            _ = s2.Length; // 2
+        }
 
         await Task.Yield();
         return;
     }
+
     static Span<int> M2() => throw null;
 }
 ";
             var comp = CreateCompilation(src, targetFramework: TargetFramework.Net70);
-            comp.VerifyDiagnostics(
-                // (9,25): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         if (M2() is var s1) { } // 1
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "s1").WithArguments("System.Span<int>").WithLocation(9, 25),
-                // (10,21): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         if (M2() is Span<int> s2) { } // 2
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "Span<int>").WithArguments("System.Span<int>").WithLocation(10, 21)
-                );
+            comp.VerifyEmitDiagnostics(
+                // (23,17): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //             _ = s1.Length;
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s1").WithArguments("System.Span<int>").WithLocation(23, 17),
+                // (28,17): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //             _ = s2.Length;
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "s2").WithArguments("System.Span<int>").WithLocation(28, 17));
+        }
+
+        [Fact]
+        public void AsyncLocals_Reassignment()
+        {
+            var code = """
+                using System;
+                using System.Threading.Tasks;
+                class C
+                {
+                    async Task M1()
+                    {
+                        int x = 42;
+                        Span<int> y = new(ref x);
+                        y.ToString();
+                        await Task.Yield();
+                        y.ToString(); // 1
+                    }
+                    async Task M2()
+                    {
+                        int x = 42;
+                        Span<int> y = new(ref x);
+                        y.ToString();
+                        await Task.Yield();
+                        y = new(ref x);
+                        y.ToString();
+                    }
+                }
+                """;
+            CreateCompilation(code, targetFramework: TargetFramework.Net70).VerifyEmitDiagnostics(
+                // (11,9): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
+                //         y.ToString(); // 1
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "y").WithArguments("System.Span<int>").WithLocation(11, 9));
         }
 
         [Fact]
@@ -1463,19 +1599,18 @@ public static async Task<int> I1()
 
             CSharpCompilation comp = CreateCompilationWithMscorlibAndSpan(text);
 
-            comp.VerifyEmitDiagnostics(
-                // (17,39): error CS4007: 'await' cannot be used in an expression containing the type 'System.Span<int>'
+            var expectedDiagnostics = new[]
+            {
+                // (17,19): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
                 //         TakesSpan(default(Span<int>), await I1());
-                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "await I1()").WithArguments("System.Span<int>")
-            );
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "default(Span<int>)").WithArguments("System.Span<int>").WithLocation(17, 19)
+            };
+
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
 
             comp = CreateCompilationWithMscorlibAndSpan(text, TestOptions.DebugExe);
 
-            comp.VerifyEmitDiagnostics(
-                // (17,39): error CS4007: 'await' cannot be used in an expression containing the type 'System.Span<int>'
-                //         TakesSpan(default(Span<int>), await I1());
-                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "await I1()").WithArguments("System.Span<int>")
-            );
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
@@ -1514,19 +1649,49 @@ public static async Task<int> I1()
 
             CSharpCompilation comp = CreateCompilationWithMscorlibAndSpan(text);
 
-            comp.VerifyEmitDiagnostics(
-                // (14,45): error CS4007: 'await' cannot be used in an expression containing the type 'Span<int>'
+            var expectedDiagnostics = new[]
+            {
+                // (14,22): error CS4007: Instance of type 'System.Span<int>' cannot be preserved across 'await' or 'yield' boundary.
                 //         TakesSpan(s: default(Span<int>), i: await I1());
-                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "await I1()").WithArguments("System.Span<int>").WithLocation(14, 45)
-            );
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "default(Span<int>)").WithArguments("System.Span<int>").WithLocation(14, 22)
+            };
+
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
 
             comp = CreateCompilationWithMscorlibAndSpan(text, TestOptions.DebugExe);
 
-            comp.VerifyEmitDiagnostics(
-                // (14,45): error CS4007: 'await' cannot be used in an expression containing the type 'Span<int>'
-                //         TakesSpan(s: default(Span<int>), i: await I1());
-                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "await I1()").WithArguments("System.Span<int>").WithLocation(14, 45)
-            );
+            comp.VerifyEmitDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void AwaitAssignSpan()
+        {
+            var source = """
+                using System;
+                using System.Threading.Tasks;
+
+                ReadOnlySpan<int> r = await M();
+                Console.Write(r[1]);
+
+                async Task<int[]> M()
+                {
+                    await Task.Yield();
+                    return new[] { 4, 5, 6 };
+                }
+                """;
+
+            CreateCompilationWithSpan(source, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (4,1): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                // ReadOnlySpan<int> r = await M();
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "ReadOnlySpan<int>").WithArguments("ref and unsafe in async and iterator methods").WithLocation(4, 1));
+
+            var expectedOutput = "5";
+
+            var comp = CreateCompilationWithSpan(source, parseOptions: TestOptions.RegularNext);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
+
+            comp = CreateCompilationWithSpan(source);
+            CompileAndVerify(comp, expectedOutput: expectedOutput).VerifyDiagnostics();
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/StackAllocInitializerTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/StackAllocInitializerTests.cs
index 2bc18e34ce28a..232f168d07c00 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/StackAllocInitializerTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/StackAllocInitializerTests.cs
@@ -757,13 +757,29 @@ async void M()
             comp.VerifyDiagnostics(
                 // (8,38): error CS0150: A constant value is expected
                 //         Span<int> p = stackalloc int[await Task.FromResult(1)] { await Task.FromResult(2) };
-                Diagnostic(ErrorCode.ERR_ConstantExpected, "await Task.FromResult(1)").WithLocation(8, 38),
-                // (8,9): error CS4012: Parameters or locals of type 'Span<int>' cannot be declared in async methods or async lambda expressions.
-                //         Span<int> p = stackalloc int[await Task.FromResult(1)] { await Task.FromResult(2) };
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "Span<int>").WithArguments("System.Span<int>").WithLocation(8, 9)
+                Diagnostic(ErrorCode.ERR_ConstantExpected, "await Task.FromResult(1)").WithLocation(8, 38)
                 );
         }
 
+        [Fact]
+        public void TestAwait_Span_02()
+        {
+            var comp = CreateCompilationWithMscorlibAndSpan("""
+                using System;
+                using System.Threading.Tasks;
+                class Test
+                {
+                    static async Task Main()
+                    {
+                        Span<int> p = stackalloc int[1] { await Task.FromResult(2) };
+                        Console.WriteLine(p[0]);
+                    }
+                }
+                """, TestOptions.UnsafeReleaseExe);
+
+            CompileAndVerify(comp, expectedOutput: "2", verify: Verification.Fails).VerifyDiagnostics();
+        }
+
         [Fact]
         public void TestSelfInSize()
         {
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/TopLevelStatementsTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/TopLevelStatementsTests.cs
index 6ab50856c1131..a8723ce4cb72b 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/TopLevelStatementsTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/TopLevelStatementsTests.cs
@@ -1113,13 +1113,24 @@ public void LocalDeclarationStatement_18()
 await System.Threading.Tasks.Task.Yield();
 ";
 
+            var expectedDiagnostics = new[]
+            {
+                // (3,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                // ref int d = ref c;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "d").WithArguments("ref and unsafe in async and iterator methods").WithLocation(3, 9)
+            };
+
             var comp = CreateCompilation(text, options: TestOptions.DebugExe, parseOptions: DefaultParseOptions);
+            comp.VerifyDiagnostics(expectedDiagnostics);
 
-            comp.VerifyDiagnostics(
-                // (3,9): error CS8177: Async methods cannot have by-reference locals
-                // ref int d = ref c;
-                Diagnostic(ErrorCode.ERR_BadAsyncLocalType, "d").WithLocation(3, 9)
-                );
+            comp = CreateCompilation(text, options: TestOptions.DebugExe, parseOptions: TestOptions.Regular12);
+            comp.VerifyDiagnostics(expectedDiagnostics);
+
+            comp = CreateCompilation(text, options: TestOptions.DebugExe, parseOptions: TestOptions.RegularNext);
+            comp.VerifyEmitDiagnostics();
+
+            comp = CreateCompilation(text, options: TestOptions.DebugExe);
+            comp.VerifyEmitDiagnostics();
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/UnsafeTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/UnsafeTests.cs
index 12753c86accd8..ccfc049d18034 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/UnsafeTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/UnsafeTests.cs
@@ -275,67 +275,2326 @@ unsafe System.Collections.Generic.IEnumerator<int> Goo()
 }
 ";
 
-            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (4,56): error CS1629: Unsafe code may not appear in iterators
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "Goo"));
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (4,56): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     unsafe System.Collections.Generic.IEnumerator<int> Goo()
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "Goo").WithArguments("ref and unsafe in async and iterator methods").WithLocation(4, 56));
+
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void IteratorUnsafe3()
+        {
+            var text = @"
+class C
+{
+    System.Collections.Generic.IEnumerator<int> Goo()
+    {
+        unsafe { }
+        yield return 1;
+    }
+}
+";
+
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (6,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         unsafe { }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 9));
+
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void IteratorUnsafe4()
+        {
+            var text = @"
+unsafe class C
+{
+    System.Collections.Generic.IEnumerator<int> Goo()
+    {
+        unsafe { }
+        yield return 1;
+    }
+}
+";
+
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (6,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         unsafe { }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 9));
+
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [WorkItem(546657, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/546657")]
+        [Fact]
+        public void IteratorUnsafe5()
+        {
+            var text = @"
+unsafe class C
+{
+    System.Collections.Generic.IEnumerator<int> Goo()
+    {
+        System.Action a = () => { unsafe { } };
+        yield return 1;
+    }
+}
+";
+
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (6,35): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         System.Action a = () => { unsafe { } };
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 35));
+
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/73280")]
+        public void Iterator_UnsafeBlock_LangVersion()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M1()
+                    {
+                        unsafe // langversion error in C# 12
+                        {
+                            int* p = null; // unnecessary langversion error in C# 12
+                        }
+                        yield break;
+                    }
+                    System.Collections.Generic.IEnumerable<int> M2()
+                    {
+                        int* p = null; // necessary langversion error in C# 12
+                        yield break;
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         unsafe // langversion error in C# 12
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(5, 9),
+                // (7,13): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             int* p = null; // unnecessary langversion error in C# 12
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "int*").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 13),
+                // (13,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         int* p = null; // necessary langversion error in C# 12
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "int*").WithArguments("ref and unsafe in async and iterator methods").WithLocation(13, 9));
+
+            var expectedDiagnostics = new[]
+            {
+                // (13,9): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         int* p = null; // necessary langversion error in C# 12
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(13, 9)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_Local()
+        {
+            var code = """
+                class C
+                {
+                    public System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        int x = 1;
+                        unsafe
+                        {
+                            int *p = &x;
+                            *p = *p + 1;
+                        }
+                        yield return x;
+                    }
+                }
+                """;
+
+            // https://github.com/dotnet/roslyn/issues/73280 - diagnostics inside the unsafe block are unnecessary
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (6,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         unsafe
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 9),
+                // (8,13): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "int *").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 13),
+                // (8,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "&x").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 22),
+                // (9,14): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             *p = *p + 1;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "p").WithArguments("ref and unsafe in async and iterator methods").WithLocation(9, 14),
+                // (9,19): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             *p = *p + 1;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "p").WithArguments("ref and unsafe in async and iterator methods").WithLocation(9, 19));
+
+            var expectedDiagnostics = new[]
+            {
+                // (8,23): error CS9239: The '&' operator cannot be used on parameters or local variables in iterator methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_AddressOfInIterator, "x").WithLocation(8, 23)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_Parameter()
+        {
+            var code = """
+                class C
+                {
+                    public System.Collections.Generic.IEnumerable<int> M(int x)
+                    {
+                        unsafe
+                        {
+                            int *p = &x;
+                            *p = *p + 1;
+                        }
+                        yield return x;
+                    }
+                }
+                """;
+
+            // https://github.com/dotnet/roslyn/issues/73280 - diagnostics inside the unsafe block are unnecessary
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         unsafe
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(5, 9),
+                // (7,13): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "int *").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 13),
+                // (7,22): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "&x").WithArguments("ref and unsafe in async and iterator methods").WithLocation(7, 22),
+                // (8,14): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             *p = *p + 1;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "p").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 14),
+                // (8,19): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             *p = *p + 1;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "p").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 19));
+
+            var expectedDiagnostics = new[]
+            {
+                // (7,23): error CS9239: The '&' operator cannot be used on parameters or local variables in iterator methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_AddressOfInIterator, "x").WithLocation(7, 23)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_Field_01()
+        {
+            var code = """
+                class Program
+                {
+                    static int _f;
+                    static System.Collections.Generic.IEnumerable<int> F()
+                    {
+                        unsafe
+                        {
+                            fixed (int* p = &_f) { }
+                        }
+                        yield break;
+                    }
+                }
+                """;
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_Field_02()
+        {
+            var code = """
+                #pragma warning disable CS0649 // field is never assigned to
+                unsafe class Program
+                {
+                    static int* _p;
+                    static System.Collections.Generic.IEnumerable<int> F()
+                    {
+                        unsafe
+                        {
+                            int* p = _p;
+                            p = &p[1];
+                        }
+                        yield break;
+                    }
+                }
+                """;
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_Field_03()
+        {
+            var code = """
+                struct S
+                {
+                    public int F;
+                }
+                class Program
+                {
+                    static System.Collections.Generic.IEnumerable<int> F()
+                    {
+                        unsafe
+                        {
+                            S s = default;
+                            int* p = &s.F;
+                        }
+                        yield break;
+                    }
+                }
+                """;
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (12,23): error CS9239: The '&' operator cannot be used on parameters or local variables in iterator methods.
+                //             int* p = &s.F;
+                Diagnostic(ErrorCode.ERR_AddressOfInIterator, "s.F").WithLocation(12, 23));
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_Field_04()
+        {
+            var code = """
+                class Program
+                {
+                    static int _f;
+                    static System.Collections.Generic.IEnumerable<int> F()
+                    {
+                        unsafe
+                        {
+                            int* p = &_f;
+                            (*p)++;
+                        }
+                        yield break;
+                    }
+                }
+                """;
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (8,22): error CS0212: You can only take the address of an unfixed expression inside of a fixed statement initializer
+                //             int* p = &_f;
+                Diagnostic(ErrorCode.ERR_FixedNeeded, "&_f").WithLocation(8, 22));
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_SizeOf()
+        {
+            var code = """
+                foreach (var x in new C().M()) System.Console.Write(x);
+
+                class C
+                {
+                    public System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        int x;
+                        unsafe
+                        {
+                            x = sizeof(nint);
+                        }
+                        yield return x;
+                    }
+                }
+                """;
+
+            // https://github.com/dotnet/roslyn/issues/73280 - diagnostics inside the unsafe block are unnecessary
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(
+                // (8,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         unsafe
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(8, 9),
+                // (10,17): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //             x = sizeof(nint);
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "sizeof(nint)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(10, 17));
+
+            var expectedOutput = IntPtr.Size.ToString();
+            CompileAndVerify(code, expectedOutput: expectedOutput, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CompileAndVerify(code, expectedOutput: expectedOutput, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_YieldBreak()
+        {
+            var code = """
+                class C
+                {
+                    public System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        unsafe
+                        {
+                            yield break;
+                        }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         unsafe
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(5, 9));
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_YieldReturn()
+        {
+            var code = """
+                class C
+                {
+                    public System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        unsafe
+                        {
+                            yield return 1;
+                        }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         unsafe
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "unsafe").WithArguments("ref and unsafe in async and iterator methods").WithLocation(5, 9));
+
+            var expectedDiagnostics = new[]
+            {
+                // (7,13): error CS9238: Cannot use 'yield return' in an 'unsafe' block
+                //             yield return 1;
+                Diagnostic(ErrorCode.ERR_BadYieldInUnsafe, "yield").WithLocation(7, 13)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Iterator_UnsafeBlock_Async_01()
+        {
+            var code = """
+                await foreach (var x in new C().M()) System.Console.Write(x);
+
+                class C
+                {
+                    public async System.Collections.Generic.IAsyncEnumerable<int> M()
+                    {
+                        int x = 1;
+                        await System.Threading.Tasks.Task.Yield();
+                        unsafe
+                        {
+                            int *p = &x;
+                            *p = *p + 1;
+                        }
+                        yield return x;
+                    }
+                }
+                """ + AsyncStreamsTypes;
+
+            var comp = CreateCompilationWithTasksExtensions(code, options: TestOptions.UnsafeReleaseExe);
+            CompileAndVerify(comp, expectedOutput: "1", verify: Verification.Fails).VerifyDiagnostics(
+                // (11,23): warning CS9123: The '&' operator should not be used on parameters or local variables in async methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.WRN_AddressOfInAsync, "x").WithLocation(11, 23));
+        }
+
+        // Should behave equivalently to AwaitBetweenUnsafeBlocks.
+        [Fact]
+        public void Iterator_UnsafeBlock_Async_02()
+        {
+            var code = """
+                class C
+                {
+                    public System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        int x = 1;
+                        unsafe
+                        {
+                            int *p = &x;
+                            *p = *p + 1;
+                        }
+                        yield return x;
+                        unsafe
+                        {
+                            int *p = &x;
+                            *p = *p + 1;
+                        }
+                        yield return x;
+                        unsafe
+                        {
+                            int *p = &x;
+                            if (*p == 3) yield break;
+                        }
+                        yield return x;
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (8,23): error CS9239: The '&' operator cannot be used on parameters or local variables in iterator methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_AddressOfInIterator, "x").WithLocation(8, 23),
+                // (14,23): error CS9239: The '&' operator cannot be used on parameters or local variables in iterator methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_AddressOfInIterator, "x").WithLocation(14, 23),
+                // (20,23): error CS9239: The '&' operator cannot be used on parameters or local variables in iterator methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.ERR_AddressOfInIterator, "x").WithLocation(20, 23)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        // Should behave equivalently to Iterator_UnsafeBlock_Async_02.
+        [Fact]
+        public void AwaitBetweenUnsafeBlocks()
+        {
+            var code = """
+                using System;
+                using System.Threading.Tasks;
+
+                await new C().M();
+
+                class C
+                {
+                	async Task Report(int x)
+                    {
+                        Console.Write(x);
+                        await Task.Yield();
+                    }
+                    public async Task M()
+                    {
+                        int x = 1;
+                        unsafe
+                        {
+                            int *p = &x;
+                            *p = *p + 1;
+                        }
+                        await Report(x);
+                        unsafe
+                        {
+                            int *p = &x;
+                            *p = *p + 1;
+                        }
+                        await Report(x);
+                        unsafe
+                        {
+                            int *p = &x;
+                            if (*p == 3) return;
+                        }
+                        await Report(x);
+                    }
+                }
+                """;
+            var expectedDiagnostics = new[]
+            {
+                // (18,23): warning CS9123: The '&' operator should not be used on parameters or local variables in async methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.WRN_AddressOfInAsync, "x").WithLocation(18, 23),
+                // (24,23): warning CS9123: The '&' operator should not be used on parameters or local variables in async methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.WRN_AddressOfInAsync, "x").WithLocation(24, 23),
+                // (30,23): warning CS9123: The '&' operator should not be used on parameters or local variables in async methods.
+                //             int *p = &x;
+                Diagnostic(ErrorCode.WRN_AddressOfInAsync, "x").WithLocation(30, 23)
+            };
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void Iterator_LocalFunction_Nested()
+        {
+            var code = """
+                class Program
+                {
+                    static void F()
+                    {
+                        unsafe
+                        {
+                            static System.Collections.Generic.IEnumerable<int> G(int x)
+                            {
+                                unsafe
+                                {
+                                    x = sizeof(nint);
+                                }
+                                yield return x;
+
+                                unsafe
+                                {
+                                    G2(new int*[0]);
+                                    static System.Collections.Generic.IEnumerable<int> G2(int*[] x)
+                                    {
+                                        int y;
+                                        unsafe
+                                        {
+                                            y = *x[0];
+                                        }
+                                        yield return y;
+                                    }
+                                }
+                            }
+                            G(0);
+                        }
+                    }
+                }
+                """;
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_LambdaInIterator_Async()
+        {
+            var code = """
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                unsafe class C
+                {
+                    IEnumerable<int> M()
+                    {
+                        yield return 1;
+                        var lam = async () => await Task.Yield();
+                    }
+                }
+                """;
+
+            // https://github.com/dotnet/roslyn/issues/73280 - these should ideally be langversion errors
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (8,31): error CS4004: Cannot await in an unsafe context
+                //         var lam = async () => await Task.Yield();
+                Diagnostic(ErrorCode.ERR_AwaitInUnsafeContext, "await Task.Yield()").WithLocation(8, 31));
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunctionInIterator_Async()
+        {
+            var code = """
+                using System.Collections.Generic;
+                using System.Threading.Tasks;
+                unsafe class C
+                {
+                    IEnumerable<int> M()
+                    {
+                        yield return 1;
+                        local();
+                        async void local() { await Task.Yield(); }
+                    }
+                }
+                """;
+
+            // https://github.com/dotnet/roslyn/issues/73280 - these should ideally be langversion errors
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (9,30): error CS4004: Cannot await in an unsafe context
+                //         async void local() { await Task.Yield(); }
+                Diagnostic(ErrorCode.ERR_AwaitInUnsafeContext, "await Task.Yield()").WithLocation(9, 30));
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_LambdaInIterator_Unsafe()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return 1;
+                        var lam = () => sizeof(nint);
+                    }
+                }
+                """;
+
+            // https://github.com/dotnet/roslyn/issues/73280 - should not be a langversion error since this remains an error in C# 13
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (6,25): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         var lam = () => sizeof(nint);
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "sizeof(nint)").WithArguments("ref and unsafe in async and iterator methods").WithLocation(6, 25));
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,25): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         var lam = () => sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(6, 25)
+            };
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunctionInIterator_BreakingChange()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return 1;
+                        local();
+                        void local() { int* p = null; }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics();
+
+            var expectedDiagnostics = new[]
+            {
+                // (7,24): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         void local() { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(7, 24)
+            };
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunctionInIterator_BreakingChangeWorkaround()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return 1;
+                        local();
+                        unsafe void local() { int* p = null; }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunctionInIterator_BreakingChange_Property()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> P
+                    {
+                        get
+                        {
+                            yield return 1;
+                            local();
+                            void local() { int* p = null; }
+                        }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics();
+
+            var expectedDiagnostics = new[]
+            {
+                // (9,28): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //             void local() { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(9, 28)
+            };
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunctionInIterator_BreakingChange_Operator()
+        {
+            var code = """
+                unsafe class C
+                {
+                    public static System.Collections.Generic.IEnumerable<int> operator+(C c1, C c2)
+                    {
+                        yield return 1;
+                        local();
+                        void local() { int* p = null; }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics();
+
+            var expectedDiagnostics = new[]
+            {
+                // (7,24): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         void local() { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(7, 24)
+            };
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunctionInIterator_BreakingChange_Nested_01()
+        {
+            var code = """
+                unsafe class C
+                {
+                    void M()
+                    {
+                        local1();
+                        System.Collections.Generic.IEnumerable<int> local1()
+                        {
+                            yield return 1;
+                            local2();
+                            void local2() { int* p = null; }
+                        }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics();
+
+            var expectedDiagnostics = new[]
+            {
+                // (10,29): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //             void local2() { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(10, 29)
+            };
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunctionInIterator_BreakingChange_Nested_02()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return 1;
+                        local1();
+                        void local1()
+                        {
+                            local2();
+                            void local2() { int* p = null; }
+                        }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyEmitDiagnostics();
+
+            var expectedDiagnostics = new[]
+            {
+                // (10,29): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //             void local2() { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(10, 29)
+            };
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/73280")]
+        public void UnsafeContext_LangVersion()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        int* p = null;
+                        yield break;
+                    }
+                }
+                """;
+
+            // https://github.com/dotnet/roslyn/issues/73280 - should not be a langversion error since this remains an error in C# 13
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (5,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         int* p = null;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "int*").WithArguments("ref and unsafe in async and iterator methods").WithLocation(5, 9));
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,9): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         int* p = null;
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(5, 9)
+            };
+
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Method_Signature_Unsafe(bool unsafeClass, bool unsafeMethod)
+        {
+            if (!unsafeClass && !unsafeMethod)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeMethod ? "unsafe" : "")}} void M(int* p) { }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Method_Signature_Safe()
+        {
+            var code = """
+                class C
+                {
+                    void M(int* p) { }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,12): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //     void M(int* p)
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(3, 12)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Method_Body_Unsafe(bool unsafeClass, bool unsafeMethod)
+        {
+            if (!unsafeClass && !unsafeMethod)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeMethod ? "unsafe" : "")}} int M()
+                    {
+                        return sizeof(nint);
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Method_Body_Safe()
+        {
+            var code = """
+                class C
+                {
+                    int M()
+                    {
+                        return sizeof(nint);
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,16): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 16)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Method_Iterator_Signature_UnsafeClass()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                    {
+                        yield break;
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Method_Iterator_Signature_UnsafeMethod(bool unsafeClass)
+        {
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    unsafe System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                    {
+                        yield break;
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (3,56): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     unsafe System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "M").WithArguments("ref and unsafe in async and iterator methods").WithLocation(3, 56));
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Method_Iterator_Signature_Safe()
+        {
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                    {
+                        yield break;
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,51): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //     System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(3, 51)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Method_Iterator_Body_CSharp12_Safe()
+        {
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return sizeof(nint);
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 22));
+        }
+
+        [Fact]
+        public void UnsafeContext_Method_Iterator_Body_CSharp12_Safe_NestedBlock()
+        {
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        {
+                            yield return sizeof(nint);
+                        }
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (6,26): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(6, 26));
+        }
+
+        [Fact]
+        public void UnsafeContext_Method_Iterator_Body_CSharp12_Unsafe()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return local();
+                        int local() => sizeof(nint);
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Method_Iterator_Body_CSharp12_Unsafe_NestedBlock()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        {
+                            yield return local();
+                        }
+                        int local() => sizeof(nint);
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Method_Iterator_Body_CSharp13(bool unsafeClass, bool unsafeMethod)
+        {
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeMethod ? "unsafe" : "")}} System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return sizeof(nint);
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 22)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Method_Iterator_Body_CSharp13_NestedBlock(bool unsafeClass, bool unsafeMethod)
+        {
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeMethod ? "unsafe" : "")}} System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        {
+                            yield return sizeof(nint);
+                        }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,26): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(6, 26)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Operator_Signature_Unsafe(bool unsafeClass, bool unsafeOperator)
+        {
+            if (!unsafeClass && !unsafeOperator)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeOperator ? "unsafe" : "")}} public static C operator+(C c, int* p)
+                    {
+                        return c;
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Operator_Signature_Safe()
+        {
+            var code = """
+                class C
+                {
+                    public static C operator+(C c, int* p)
+                    {
+                        return c;
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,36): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //     public static C operator+(C c, int* p)
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(3, 36)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Operator_Body_Unsafe(bool unsafeClass, bool unsafeOperator)
+        {
+            if (!unsafeClass && !unsafeOperator)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeOperator ? "unsafe" : "")}} public static int operator+(C c1, C c2)
+                    {
+                        return sizeof(nint);
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Operator_Body_Safe()
+        {
+            var code = """
+                class C
+                {
+                    public static int operator+(C c1, C c2)
+                    {
+                        return sizeof(nint);
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,16): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 16)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Operator_Iterator_Signature_UnsafeClass()
+        {
+            var code = """
+                unsafe class C
+                {
+                    public static System.Collections.Generic.IEnumerable<int> operator+(C c, int*[] p)
+                    {
+                        yield break;
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Operator_Iterator_Signature_UnsafeOperator(bool unsafeClass)
+        {
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    public static unsafe System.Collections.Generic.IEnumerable<int> operator+(C c, int*[] p)
+                    {
+                        yield break;
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (3,78): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public static unsafe System.Collections.Generic.IEnumerable<int> operator+(C c, int*[] p)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "+").WithArguments("ref and unsafe in async and iterator methods").WithLocation(3, 78));
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Operator_Iterator_Signature_Safe()
+        {
+            var code = """
+                class C
+                {
+                    public static System.Collections.Generic.IEnumerable<int> operator+(C c, int*[] p)
+                    {
+                        yield break;
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,78): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //     public static System.Collections.Generic.IEnumerable<int> operator+(C c, int*[] p)
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(3, 78)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Operator_Iterator_Body_CSharp12_Safe()
+        {
+            var code = """
+                class C
+                {
+                    public static System.Collections.Generic.IEnumerable<int> operator+(C c1, C c2)
+                    {
+                        yield return sizeof(nint);
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 22));
+        }
+
+        [Fact]
+        public void UnsafeContext_Operator_Iterator_Body_CSharp12_Unsafe()
+        {
+            var code = """
+                unsafe class C
+                {
+                    public static System.Collections.Generic.IEnumerable<int> operator+(C c1, C c2)
+                    {
+                        yield return local();
+                        int local() => sizeof(nint);
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Operator_Iterator_Body_CSharp13(bool unsafeClass, bool unsafeIndexer)
+        {
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeIndexer ? "unsafe" : "")}} public static System.Collections.Generic.IEnumerable<int> operator+(C c1, C c2)
+                    {
+                        yield return sizeof(nint);
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 22)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Indexer_Signature_Unsafe(bool unsafeClass, bool unsafeIndexer)
+        {
+            if (!unsafeClass && !unsafeIndexer)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeIndexer ? "unsafe" : "")}} int this[int* p]
+                    {
+                        get { return 0; }
+                        set { }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Indexer_Signature_Safe()
+        {
+            var code = """
+                class C
+                {
+                    int this[int* p]
+                    {
+                        get { return 0; }
+                        set { }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,14): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //     int this[int* p]
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(3, 14)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Indexer_Body_Unsafe(bool unsafeClass, bool unsafeIndexer)
+        {
+            if (!unsafeClass && !unsafeIndexer)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeIndexer ? "unsafe" : "")}} int this[int x]
+                    {
+                        get { return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Indexer_Body_Safe()
+        {
+            var code = """
+                class C
+                {
+                    int this[int x]
+                    {
+                        get { return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         get { return sizeof(nint); }
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 22),
+                // (6,15): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         set { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(6, 15)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Indexer_Iterator_Signature_UnsafeClass()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> this[int*[] p]
+                    {
+                        get { yield break; }
+                        set { }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Indexer_Iterator_Signature_UnsafeIndexer(bool unsafeClass)
+        {
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    unsafe System.Collections.Generic.IEnumerable<int> this[int*[] p]
+                    {
+                        get { yield break; }
+                        set { }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         get { yield break; }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "get").WithArguments("ref and unsafe in async and iterator methods").WithLocation(5, 9));
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Indexer_Iterator_Signature_Safe()
+        {
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int> this[int*[] p]
+                    {
+                        get { yield break; }
+                        set { }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,54): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //     System.Collections.Generic.IEnumerable<int> this[int*[] p]
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(3, 54)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Indexer_Iterator_Body_CSharp12_Safe()
+        {
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int> this[int x]
+                    {
+                        get { yield return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,28): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         get { yield return sizeof(nint); }
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 28),
+                // (6,15): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         set { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(6, 15));
+        }
+
+        [Fact]
+        public void UnsafeContext_Indexer_Iterator_Body_CSharp12_Unsafe()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> this[int x]
+                    {
+                        get
+                        {
+                            yield return local();
+                            int local() => sizeof(nint);
+                        }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Indexer_Iterator_Body_CSharp13_UnsafeSetter(bool unsafeClass, bool unsafeIndexer)
+        {
+            if (!unsafeClass && !unsafeIndexer)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeIndexer ? "unsafe" : "")}} System.Collections.Generic.IEnumerable<int> this[int x]
+                    {
+                        get { yield return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,28): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         get { yield return sizeof(nint); }
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 28)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Indexer_Iterator_Body_CSharp13_SafeSetter()
+        {
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int> this[int x]
+                    {
+                        get { yield return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,28): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         get { yield return sizeof(nint); }
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 28),
+                // (6,15): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         set { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(6, 15)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Property_Signature_Unsafe(bool unsafeClass, bool unsafeProperty)
+        {
+            if (!unsafeClass && !unsafeProperty)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeProperty ? "unsafe" : "")}} int* P
+                    {
+                        get { throw null; }
+                        set { }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Property_Signature_Safe()
+        {
+            var code = """
+                class C
+                {
+                    int* P
+                    {
+                        get { throw null; }
+                        set { }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,5): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //     int* P
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(3, 5)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Property_Body_Unsafe(bool unsafeClass, bool unsafeProperty)
+        {
+            if (!unsafeClass && !unsafeProperty)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeProperty ? "unsafe" : "")}} int P
+                    {
+                        get { return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Property_Body_Safe()
+        {
+            var code = """
+                class C
+                {
+                    int P
+                    {
+                        get { return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         get { return sizeof(nint); }
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 22),
+                // (6,15): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         set { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(6, 15)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Property_Iterator_Signature_UnsafeClass()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int*[]> P
+                    {
+                        get { yield break; }
+                        set { }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Property_Iterator_Signature_UnsafeProperty(bool unsafeClass)
+        {
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    unsafe System.Collections.Generic.IEnumerable<int*[]> P
+                    {
+                        get { yield break; }
+                        set { }
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,9): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         get { yield break; }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "get").WithArguments("ref and unsafe in async and iterator methods").WithLocation(5, 9));
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Property_Iterator_Signature_Safe()
+        {
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int*[]> P
+                    {
+                        get { yield break; }
+                        set { }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,44): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //     System.Collections.Generic.IEnumerable<int*[]> P
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(3, 44)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Property_Iterator_Body_CSharp12_Safe()
+        {
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int> P
+                    {
+                        get { yield return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (5,28): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         get { yield return sizeof(nint); }
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 28),
+                // (6,15): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         set { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(6, 15));
+        }
+
+        [Fact]
+        public void UnsafeContext_Property_Iterator_Body_CSharp12_Unsafe()
+        {
+            var code = """
+                unsafe class C
+                {
+                    System.Collections.Generic.IEnumerable<int> P
+                    {
+                        get
+                        {
+                            yield return local();
+                            int local() => sizeof(nint);
+                        }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_Property_Iterator_Body_CSharp13_UnsafeSetter(bool unsafeClass, bool unsafeProperty)
+        {
+            if (!unsafeClass && !unsafeProperty)
+            {
+                return;
+            }
+
+            var code = $$"""
+                {{(unsafeClass ? "unsafe" : "")}} class C
+                {
+                    {{(unsafeProperty ? "unsafe" : "")}} System.Collections.Generic.IEnumerable<int> P
+                    {
+                        get { yield return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,28): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         get { yield return sizeof(nint); }
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 28)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
-        public void IteratorUnsafe3()
+        public void UnsafeContext_Property_Iterator_Body_CSharp13_SafeSetter()
         {
-            var text = @"
-class C
-{
-    System.Collections.Generic.IEnumerator<int> Goo()
-    {
-        unsafe { }
-        yield return 1;
-    }
-}
-";
+            var code = """
+                class C
+                {
+                    System.Collections.Generic.IEnumerable<int> P
+                    {
+                        get { yield return sizeof(nint); }
+                        set { int* p = null; }
+                    }
+                }
+                """;
 
-            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (6,9): error CS1629: Unsafe code may not appear in iterators
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "unsafe"));
+            var expectedDiagnostics = new[]
+            {
+                // (5,28): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         get { yield return sizeof(nint); }
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(5, 28),
+                // (6,15): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                //         set { int* p = null; }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(6, 15)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_LocalFunction_Signature_Unsafe(bool unsafeBlock, bool unsafeFunction)
+        {
+            if (!unsafeBlock && !unsafeFunction)
+            {
+                return;
+            }
+
+            var code = $$"""
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                {{(unsafeBlock ? "unsafe" : "")}}
+                {
+                    {{(unsafeFunction ? "unsafe" : "")}} void M(int* p) { }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
         }
 
         [Fact]
-        public void IteratorUnsafe4()
+        public void UnsafeContext_LocalFunction_Signature_Safe()
         {
-            var text = @"
-unsafe class C
-{
-    System.Collections.Generic.IEnumerator<int> Goo()
-    {
-        unsafe { }
-        yield return 1;
-    }
-}
-";
+            var code = """
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                void M(int* p) { }
+                """;
 
-            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (6,9): error CS1629: Unsafe code may not appear in iterators
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "unsafe"));
+            var expectedDiagnostics = new[]
+            {
+                // (2,8): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                // void M(int* p) { }
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(2, 8)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_LocalFunction_Body_Unsafe(bool unsafeBlock, bool unsafeFunction)
+        {
+            if (!unsafeBlock && !unsafeFunction)
+            {
+                return;
+            }
+
+            var code = $$"""
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                {{(unsafeBlock ? "unsafe" : "")}}
+                {
+                    {{(unsafeFunction ? "unsafe" : "")}} int M()
+                    {
+                        return sizeof(nint);
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
         }
 
-        [WorkItem(546657, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/546657")]
         [Fact]
-        public void IteratorUnsafe5()
+        public void UnsafeContext_LocalFunction_Body_Safe()
         {
-            var text = @"
-unsafe class C
-{
-    System.Collections.Generic.IEnumerator<int> Goo()
-    {
-        System.Action a = () => { unsafe { } };
-        yield return 1;
-    }
-}
-";
+            var code = """
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                int M()
+                {
+                    return sizeof(nint);
+                }
+                """;
 
-            CreateCompilation(text, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (6,9): error CS1629: Unsafe code may not appear in iterators
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "unsafe"));
+            var expectedDiagnostics = new[]
+            {
+                // (4,12): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //     return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(4, 12)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunction_Iterator_Signature_UnsafeBlock()
+        {
+            var code = """
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                unsafe
+                {
+                    System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                    {
+                        yield break;
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_LocalFunction_Iterator_Signature_UnsafeFunction(bool unsafeBlock)
+        {
+            var code = $$"""
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                {{(unsafeBlock ? "unsafe" : "")}}
+                {
+                    unsafe System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                    {
+                        yield break;
+                    }
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(
+                // (4,56): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     unsafe System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "M").WithArguments("ref and unsafe in async and iterator methods").WithLocation(4, 56));
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunction_Iterator_Signature_Safe()
+        {
+            var code = """
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                {
+                    yield break;
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (2,47): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                // System.Collections.Generic.IEnumerable<int> M(int*[] p)
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(2, 47)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunction_Iterator_Body_CSharp12_Safe()
+        {
+            var code = """
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                System.Collections.Generic.IEnumerable<int> M()
+                {
+                    yield return sizeof(nint);
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(
+                // (4,18): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //     yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(4, 18));
+        }
+
+        [Fact]
+        public void UnsafeContext_LocalFunction_Iterator_Body_CSharp12_Unsafe()
+        {
+            var code = """
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                unsafe
+                {
+                    System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return local();
+                        int local() => sizeof(nint);
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+        }
+
+        [Theory, CombinatorialData]
+        public void UnsafeContext_LocalFunction_Iterator_Body_CSharp13(bool unsafeBlock, bool unsafeFunction)
+        {
+            var code = $$"""
+                #pragma warning disable CS8321 // The local function 'M' is declared but never used
+                {{(unsafeBlock ? "unsafe" : "")}}
+                {
+                    {{(unsafeFunction ? "unsafe" : "")}} System.Collections.Generic.IEnumerable<int> M()
+                    {
+                        yield return sizeof(nint);
+                    }
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,22): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(6, 22)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Lambda_Signature_Unsafe()
+        {
+            var code = """
+                unsafe
+                {
+                    var lam = (int* p) => { };
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Lambda_Signature_Safe()
+        {
+            var code = """
+                var lam = (int* p) => { };
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (1,12): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                // var lam = (int* p) => { };
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(1, 12),
+                // (1,17): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                // var lam = (int* p) => { };
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "p").WithLocation(1, 17)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Lambda_Body_Unsafe()
+        {
+            var code = """
+                unsafe
+                {
+                    var lam = () =>
+                    {
+                        return sizeof(nint);
+                    };
+                }
+                """;
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnsafeContext_Lambda_Body_Safe()
+        {
+            var code = """
+                var lam = () =>
+                {
+                    return sizeof(nint);
+                };
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,12): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //     return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(3, 12)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Lambda_Iterator_Signature_Unsafe()
+        {
+            var code = """
+                unsafe
+                {
+                    var lam = (int*[] p) =>
+                    {
+                        yield break;
+                    };
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,9): error CS1621: The yield statement cannot be used inside an anonymous method or lambda expression
+                //         yield break;
+                Diagnostic(ErrorCode.ERR_YieldInAnonMeth, "yield").WithLocation(5, 9)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Lambda_Iterator_Signature_Safe()
+        {
+            var code = """
+                var lam = (int*[] p) =>
+                {
+                    yield break;
+                };
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (1,12): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                // var lam = (int*[] p) =>
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "int*").WithLocation(1, 12),
+                // (1,19): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
+                // var lam = (int*[] p) =>
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "p").WithLocation(1, 19),
+                // (3,5): error CS1621: The yield statement cannot be used inside an anonymous method or lambda expression
+                //     yield break;
+                Diagnostic(ErrorCode.ERR_YieldInAnonMeth, "yield").WithLocation(3, 5)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Lambda_Iterator_Body_Unsafe()
+        {
+            var code = """
+                unsafe
+                {
+                    var lam = () =>
+                    {
+                        yield return sizeof(nint);
+                    };
+                }
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (5,9): error CS1621: The yield statement cannot be used inside an anonymous method or lambda expression
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_YieldInAnonMeth, "yield").WithLocation(5, 9)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+
+            expectedDiagnostics = [
+                // (5,9): error CS1621: The yield statement cannot be used inside an anonymous method or lambda expression
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_YieldInAnonMeth, "yield").WithLocation(5, 9),
+                // (5,9): error CS9238: Cannot use 'yield return' in an 'unsafe' block
+                //         yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_BadYieldInUnsafe, "yield").WithLocation(5, 9)
+            ];
+
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeContext_Lambda_Iterator_Body_Safe()
+        {
+            var code = """
+                var lam = () =>
+                {
+                    yield return sizeof(nint);
+                };
+                """;
+
+            var expectedDiagnostics = new[]
+            {
+                // (3,5): error CS1621: The yield statement cannot be used inside an anonymous method or lambda expression
+                //     yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_YieldInAnonMeth, "yield").WithLocation(3, 5),
+                // (3,18): error CS0233: 'nint' does not have a predefined size, therefore sizeof can only be used in an unsafe context
+                //     yield return sizeof(nint);
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(nint)").WithArguments("nint").WithLocation(3, 18)
+            };
+
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseExe).VerifyDiagnostics(expectedDiagnostics);
+        }
+
+        [Fact]
+        public void UnsafeBlock_InAsyncMethod()
+        {
+            var code = """
+                using System.Threading.Tasks;
+                class C
+                {
+                    async Task M(int x)
+                    {
+                        unsafe { x = sizeof(nint); }
+                        await Task.Yield();
+                    }
+                }
+                """;
+            CreateCompilation(code, parseOptions: TestOptions.Regular12, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+            CreateCompilation(code, parseOptions: TestOptions.RegularNext, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+            CreateCompilation(code, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
         }
 
         [Fact]
@@ -936,18 +3195,41 @@ public void UnsafeIteratorSignatures()
                 Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "p"));
 
             var withUnsafeOnMembers = string.Format(template, "", "unsafe");
-            CreateCompilation(withUnsafeOnMembers, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (4,64): error CS1637: Iterators cannot have pointer type parameters
-                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "p"),
-                // (4,56): error CS1629: Unsafe code may not appear in iterators
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "Iterator")); //this is for putting "unsafe" on an iterator, not for the parameter type
+            CreateCompilation(withUnsafeOnMembers, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (4,70): error CS1637: Iterators cannot have pointer type parameters
+                //     unsafe System.Collections.Generic.IEnumerable<int> Iterator(int* p)
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "p").WithLocation(4, 70),
+                // (4,56): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     unsafe System.Collections.Generic.IEnumerable<int> Iterator(int* p)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "Iterator").WithArguments("ref and unsafe in async and iterator methods").WithLocation(4, 56)); //this is for putting "unsafe" on an iterator, not for the parameter type
+
+            var expectedDiagnostics = new[]
+            {
+                // (4,70): error CS1637: Iterators cannot have pointer type parameters
+                //     unsafe System.Collections.Generic.IEnumerable<int> Iterator(int* p)
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "p").WithLocation(4, 70)
+            };
+
+            CreateCompilation(withUnsafeOnMembers, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(withUnsafeOnMembers, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
 
             var withUnsafeOnTypeAndMembers = string.Format(template, "unsafe", "unsafe");
-            CreateCompilation(withUnsafeOnTypeAndMembers, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (4,64): error CS1637: Iterators cannot have pointer type parameters
-                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "p"),
-                // (4,56): error CS1629: Unsafe code may not appear in iterators
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "Iterator")); //this is for putting "unsafe" on an iterator, not for the parameter type
+            CreateCompilation(withUnsafeOnTypeAndMembers, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (4,70): error CS1637: Iterators cannot have pointer type parameters
+                //     unsafe System.Collections.Generic.IEnumerable<int> Iterator(int* p)
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "p").WithLocation(4, 70),
+                // (4,56): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     unsafe System.Collections.Generic.IEnumerable<int> Iterator(int* p)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "Iterator").WithArguments("ref and unsafe in async and iterator methods").WithLocation(4, 56)); //this is for putting "unsafe" on an iterator, not for the parameter type
+
+            expectedDiagnostics = [
+                // (4,70): error CS1637: Iterators cannot have pointer type parameters
+                //     unsafe System.Collections.Generic.IEnumerable<int> Iterator(int* p)
+                Diagnostic(ErrorCode.ERR_UnsafeIteratorArgType, "p").WithLocation(4, 70)
+            ];
+
+            CreateCompilation(withUnsafeOnTypeAndMembers, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(withUnsafeOnTypeAndMembers, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
@@ -974,18 +3256,24 @@ public void UnsafeIteratorSignatures_PointerArray()
             CreateCompilation(withUnsafeOnType, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics();
 
             var withUnsafeOnMembers = string.Format(template, "", "unsafe");
-            CreateCompilation(withUnsafeOnMembers, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (4,56): error CS1629: Unsafe code may not appear in iterators
+            CreateCompilation(withUnsafeOnMembers, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (4,56): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //     unsafe System.Collections.Generic.IEnumerable<int> Iterator(int*[] p)
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "Iterator").WithLocation(4, 56)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "Iterator").WithArguments("ref and unsafe in async and iterator methods").WithLocation(4, 56)
                 );
 
+            CreateCompilation(withUnsafeOnMembers, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(withUnsafeOnMembers, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
+
             var withUnsafeOnTypeAndMembers = string.Format(template, "unsafe", "unsafe");
-            CreateCompilation(withUnsafeOnTypeAndMembers, options: TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
-                // (4,56): error CS1629: Unsafe code may not appear in iterators
+            CreateCompilation(withUnsafeOnTypeAndMembers, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.Regular12).VerifyDiagnostics(
+                // (4,56): error CS8652: The feature 'ref and unsafe in async and iterator methods' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
                 //     unsafe System.Collections.Generic.IEnumerable<int> Iterator(int*[] p)
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "Iterator").WithLocation(4, 56)
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "Iterator").WithArguments("ref and unsafe in async and iterator methods").WithLocation(4, 56)
                 );
+
+            CreateCompilation(withUnsafeOnTypeAndMembers, options: TestOptions.UnsafeReleaseDll, parseOptions: TestOptions.RegularNext).VerifyEmitDiagnostics();
+            CreateCompilation(withUnsafeOnTypeAndMembers, options: TestOptions.UnsafeReleaseDll).VerifyEmitDiagnostics();
         }
 
         [Fact]
@@ -8156,10 +10444,17 @@ System.Collections.Generic.IEnumerable<int> M()
     }
 }
 ";
-            CreateCompilation(text).VerifyDiagnostics(
-                // (6,22): error CS1629: Unsafe code may not appear in iterators
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,22): error CS0233: 'S' does not have a predefined size, therefore sizeof can only be used in an unsafe context
                 //         yield return sizeof(S);
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "sizeof(S)"));
+                Diagnostic(ErrorCode.ERR_SizeofUnsafe, "sizeof(S)").WithArguments("S").WithLocation(6, 22)
+            };
+
+            CreateCompilation(text, parseOptions: TestOptions.Regular12).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(text, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(text).VerifyDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
@@ -8522,10 +10817,17 @@ System.Collections.Generic.IEnumerable<int> M()
     }
 }
 ";
-            CreateCompilation(text).VerifyDiagnostics(
-                // (6,17): error CS1629: Unsafe code may not appear in iterators
+
+            var expectedDiagnostics = new[]
+            {
+                // (6,17): error CS0214: Pointers and fixed size buffers may only be used in an unsafe context
                 //         var p = stackalloc int[1];
-                Diagnostic(ErrorCode.ERR_IllegalInnerUnsafe, "stackalloc int[1]"));
+                Diagnostic(ErrorCode.ERR_UnsafeNeeded, "stackalloc int[1]").WithLocation(6, 17)
+            };
+
+            CreateCompilation(text, parseOptions: TestOptions.Regular12).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(text, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+            CreateCompilation(text).VerifyDiagnostics(expectedDiagnostics);
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/UsingStatementTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/UsingStatementTests.cs
index 3b9371a1c3769..610897b83d561 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/UsingStatementTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/UsingStatementTests.cs
@@ -1127,12 +1127,67 @@ static async Task Main()
         }
     }
 }";
-            var compilation = CreateCompilationWithTasksExtensions(new[] { source, IAsyncDisposableDefinition }).VerifyDiagnostics(
-                // (16,22): error CS4012: Parameters or locals of type 'S1' cannot be declared in async methods or async lambda expressions.
+            var compilation = CreateCompilationWithTasksExtensions(new[] { source, IAsyncDisposableDefinition }, options: TestOptions.ReleaseExe);
+            CompileAndVerify(compilation, expectedOutput: "Dispose async").VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UsingPatternAsyncTest_02()
+        {
+            var source = """
+                using System.Threading.Tasks;
+                ref struct S1
+                {
+                    public ValueTask DisposeAsync() 
+                    { 
+                        System.Console.WriteLine("Dispose async");
+                        return new ValueTask(Task.CompletedTask);
+                    }
+                }
+                class C2
+                {
+                    static async Task Main()
+                    {
+                        await using (S1 c = new S1())
+                        {
+                            await Task.Yield();
+                        }
+                    }
+                }
+                """;
+            var compilation = CreateCompilationWithTasksExtensions(new[] { source, IAsyncDisposableDefinition });
+            compilation.VerifyEmitDiagnostics(
+                // 0.cs(14,25): error CS4007: Instance of type 'S1' cannot be preserved across 'await' or 'yield' boundary.
                 //         await using (S1 c = new S1())
-                Diagnostic(ErrorCode.ERR_BadSpecialByRefLocal, "S1").WithArguments("S1").WithLocation(16, 22)
-                );
+                Diagnostic(ErrorCode.ERR_ByRefTypeAndAwait, "c = new S1()").WithArguments("S1").WithLocation(14, 25));
+        }
 
+        [Fact]
+        public void UsingPatternAsyncTest_03()
+        {
+            var source = """
+                using System.Threading.Tasks;
+                ref struct S1
+                {
+                    public S1(int x) { }
+                    public ValueTask DisposeAsync() 
+                    { 
+                        System.Console.WriteLine("Dispose async");
+                        return new ValueTask(Task.CompletedTask);
+                    }
+                }
+                class C2
+                {
+                    static async Task Main()
+                    {
+                        await using (S1 c = new S1(await Task.FromResult(1)))
+                        {
+                        }
+                    }
+                }
+                """;
+            var compilation = CreateCompilationWithTasksExtensions(new[] { source, IAsyncDisposableDefinition }, options: TestOptions.ReleaseExe);
+            CompileAndVerify(compilation, expectedOutput: "Dispose async").VerifyDiagnostics();
         }
 
         [Fact]
diff --git a/src/Compilers/CSharp/Test/Syntax/Diagnostics/DiagnosticTest.cs b/src/Compilers/CSharp/Test/Syntax/Diagnostics/DiagnosticTest.cs
index 587107287652c..9f5490d05adac 100644
--- a/src/Compilers/CSharp/Test/Syntax/Diagnostics/DiagnosticTest.cs
+++ b/src/Compilers/CSharp/Test/Syntax/Diagnostics/DiagnosticTest.cs
@@ -471,6 +471,10 @@ public void WarningLevel_2()
                             // These are the warnings introduced with the warning "wave" shipped with dotnet 8 and C# 12.
                             Assert.Equal(8, ErrorFacts.GetWarningLevel(errorCode));
                             break;
+                        case ErrorCode.WRN_BadYieldInLock:
+                            // These are the warnings introduced with the warning "wave" shipped with dotnet 9 and C# 13.
+                            Assert.Equal(9, ErrorFacts.GetWarningLevel(errorCode));
+                            break;
                         default:
                             // If a new warning is added, this test will fail
                             // and whoever is adding the new warning will have to update it with the expected error level.
@@ -2979,6 +2983,7 @@ public void TestIsBuildOnlyDiagnostic()
                     case ErrorCode.ERR_InterceptableMethodMustBeOrdinary:
                     case ErrorCode.ERR_PossibleAsyncIteratorWithoutYield:
                     case ErrorCode.ERR_PossibleAsyncIteratorWithoutYieldOrAwait:
+                    case ErrorCode.ERR_RefLocalAcrossAwait:
                         Assert.True(isBuildOnly, $"Check failed for ErrorCode.{errorCode}");
                         break;
 
diff --git a/src/Features/CSharp/Portable/Diagnostics/LanguageServer/CSharpLspBuildOnlyDiagnostics.cs b/src/Features/CSharp/Portable/Diagnostics/LanguageServer/CSharpLspBuildOnlyDiagnostics.cs
index 8a81b6d09d4b1..6bd117bf73410 100644
--- a/src/Features/CSharp/Portable/Diagnostics/LanguageServer/CSharpLspBuildOnlyDiagnostics.cs
+++ b/src/Features/CSharp/Portable/Diagnostics/LanguageServer/CSharpLspBuildOnlyDiagnostics.cs
@@ -59,7 +59,8 @@ namespace Microsoft.CodeAnalysis.CSharp.LanguageServer;
     "CS9178", // ErrorCode.ERR_InterceptorCannotBeGeneric
     "CS9207", // ErrorCode.ERR_InterceptableMethodMustBeOrdinary
     "CS8419", // ErrorCode.ERR_PossibleAsyncIteratorWithoutYield
-    "CS8420" // ErrorCode.ERR_PossibleAsyncIteratorWithoutYieldOrAwait
+    "CS8420", // ErrorCode.ERR_PossibleAsyncIteratorWithoutYieldOrAwait
+    "CS9217" // ErrorCode.ERR_RefLocalAcrossAwait
     )]
 [Shared]
 internal sealed class CSharpLspBuildOnlyDiagnostics : ILspBuildOnlyDiagnostics
diff --git a/src/Features/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/LanguageServer/RazorDynamicDocumentSyncRegistration.cs b/src/Features/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/LanguageServer/RazorDynamicDocumentSyncRegistration.cs
index d255899e89c82..c9e35dd4f0c46 100644
--- a/src/Features/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/LanguageServer/RazorDynamicDocumentSyncRegistration.cs
+++ b/src/Features/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/LanguageServer/RazorDynamicDocumentSyncRegistration.cs
@@ -46,7 +46,7 @@ public async Task OnInitializedAsync(ClientCapabilities clientCapabilities, Requ
             {
                 var languageServerManager = context.GetRequiredLspService<IClientLanguageServerManager>();
 
-                var documentFilters = new[] { new DocumentFilter() { Pattern = "**/*.razor" }, new DocumentFilter() { Pattern = "**/*.cshtml" } };
+                var documentFilters = new[] { new DocumentFilter() { Pattern = "**/*.{razor, cshtml}", Language = "aspnetcorerazor" } };
                 var registrationOptions = new TextDocumentRegistrationOptions()
                 {
                     DocumentSelector = documentFilters
@@ -66,7 +66,11 @@ await languageServerManager.SendRequestAsync(Methods.ClientRegisterCapabilityNam
                             {
                                 Id = Guid.NewGuid().ToString(), // No need to save this for unregistering
                                 Method = Methods.TextDocumentDidChangeName,
-                                RegisterOptions = registrationOptions
+                                RegisterOptions = new TextDocumentChangeRegistrationOptions()
+                                {
+                                    DocumentSelector = documentFilters,
+                                    SyncKind = TextDocumentSyncKind.Incremental
+                                }
                             },
                             new()
                             {
diff --git a/src/Features/LanguageServer/Protocol/Protocol/TextDocumentChangeRegistrationOptions.cs b/src/Features/LanguageServer/Protocol/Protocol/TextDocumentChangeRegistrationOptions.cs
new file mode 100644
index 0000000000000..39de99b3501a0
--- /dev/null
+++ b/src/Features/LanguageServer/Protocol/Protocol/TextDocumentChangeRegistrationOptions.cs
@@ -0,0 +1,34 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.Serialization;
+using System.Text;
+using System.Threading.Tasks;
+using Roslyn.LanguageServer.Protocol;
+
+namespace Roslyn.LanguageServer.Protocol
+{
+    /// <summary>
+    /// Class representing the registration options for didChange events.
+    ///
+    /// See the <see href="https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocumentChangeRegistrationOptions">Language Server Protocol specification</see> for additional information.
+    /// </summary>
+    [DataContract]
+    internal class TextDocumentChangeRegistrationOptions : TextDocumentRegistrationOptions
+    {
+        /// <summary>
+        /// How documents are synced to the server. See <see cref="TextDocumentSyncKind.Full"/>
+	    /// and <see cref="TextDocumentSyncKind.Incremental"/>.
+        /// </summary>
+        [DataMember(Name = "syncKind")]
+        public TextDocumentSyncKind SyncKind
+        {
+            get;
+            set;
+        }
+    }
+}
diff --git a/src/Workspaces/CSharp/Portable/Simplification/CSharpSimplificationService.cs b/src/Workspaces/CSharp/Portable/Simplification/CSharpSimplificationService.cs
index f3075ecfe3cea..7b46d9a52c515 100644
--- a/src/Workspaces/CSharp/Portable/Simplification/CSharpSimplificationService.cs
+++ b/src/Workspaces/CSharp/Portable/Simplification/CSharpSimplificationService.cs
@@ -10,18 +10,21 @@
 using System.Threading;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.CSharp.Utilities;
-using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Formatting;
 using Microsoft.CodeAnalysis.Host.Mef;
 using Microsoft.CodeAnalysis.Internal.Log;
 using Microsoft.CodeAnalysis.Options;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Simplification;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp.Simplification;
 
 [ExportLanguageService(typeof(ISimplificationService), LanguageNames.CSharp), Shared]
-internal partial class CSharpSimplificationService : AbstractSimplificationService<ExpressionSyntax, StatementSyntax, CrefSyntax>
+[method: ImportingConstructor]
+[method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
+internal partial class CSharpSimplificationService()
+    : AbstractSimplificationService<CompilationUnitSyntax, ExpressionSyntax, StatementSyntax, CrefSyntax>(s_reducers)
 {
     // 1. the cast simplifier should run earlier then everything else to minimize the type expressions
     // 2. Extension method reducer may insert parentheses.  So run it before the parentheses remover.
@@ -40,12 +43,6 @@ internal partial class CSharpSimplificationService : AbstractSimplificationServi
             new CSharpDefaultExpressionReducer(),
         ];
 
-    [ImportingConstructor]
-    [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
-    public CSharpSimplificationService() : base(s_reducers)
-    {
-    }
-
     public override SimplifierOptions DefaultOptions
         => CSharpSimplifierOptions.Default;
 
@@ -230,4 +227,26 @@ private static bool IsTupleInDeconstruction(SyntaxNode tuple)
         }
         while (true);
     }
+
+    protected override void AddImportDeclarations(CompilationUnitSyntax root, ArrayBuilder<SyntaxNode> importDeclarations)
+    {
+        importDeclarations.AddRange(root.Usings);
+
+        foreach (var member in root.Members)
+        {
+            if (member is BaseNamespaceDeclarationSyntax baseNamespace)
+                AddImportDeclarations(baseNamespace, importDeclarations);
+        }
+
+        static void AddImportDeclarations(BaseNamespaceDeclarationSyntax baseNamespace, ArrayBuilder<SyntaxNode> importDeclarations)
+        {
+            importDeclarations.AddRange(baseNamespace.Usings);
+
+            foreach (var member in baseNamespace.Members)
+            {
+                if (member is BaseNamespaceDeclarationSyntax childNamespace)
+                    AddImportDeclarations(childNamespace, importDeclarations);
+            }
+        }
+    }
 }
diff --git a/src/Workspaces/Core/Portable/Simplification/AbstractSimplificationService.cs b/src/Workspaces/Core/Portable/Simplification/AbstractSimplificationService.cs
index f852e4a0ef741..2ad302dcb1782 100644
--- a/src/Workspaces/Core/Portable/Simplification/AbstractSimplificationService.cs
+++ b/src/Workspaces/Core/Portable/Simplification/AbstractSimplificationService.cs
@@ -10,10 +10,10 @@
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
-using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Editing;
 using Microsoft.CodeAnalysis.Internal.Log;
 using Microsoft.CodeAnalysis.Options;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Shared.Collections;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Shared.Utilities;
@@ -22,7 +22,8 @@
 
 namespace Microsoft.CodeAnalysis.Simplification;
 
-internal abstract class AbstractSimplificationService<TExpressionSyntax, TStatementSyntax, TCrefSyntax> : ISimplificationService
+internal abstract class AbstractSimplificationService<TCompilationUnitSyntax, TExpressionSyntax, TStatementSyntax, TCrefSyntax> : ISimplificationService
+    where TCompilationUnitSyntax : SyntaxNode
     where TExpressionSyntax : SyntaxNode
     where TStatementSyntax : SyntaxNode
     where TCrefSyntax : SyntaxNode
@@ -38,6 +39,7 @@ protected AbstractSimplificationService(ImmutableArray<AbstractReducer> reducers
     protected abstract ImmutableArray<NodeOrTokenToReduce> GetNodesAndTokensToReduce(SyntaxNode root, Func<SyntaxNodeOrToken, bool> isNodeOrTokenOutsideSimplifySpans);
     protected abstract SemanticModel GetSpeculativeSemanticModel(ref SyntaxNode nodeToSpeculate, SemanticModel originalSemanticModel, SyntaxNode originalNode);
     protected abstract bool NodeRequiresNonSpeculativeSemanticModel(SyntaxNode node);
+    protected abstract void AddImportDeclarations(TCompilationUnitSyntax root, ArrayBuilder<SyntaxNode> importDeclarations);
 
     public abstract SimplifierOptions DefaultOptions { get; }
     public abstract SimplifierOptions GetSimplifierOptions(IOptionsReader options, SimplifierOptions? fallbackOptions);
@@ -103,27 +105,22 @@ private async Task<Document> ReduceCoreAsync(
         // Create a simple interval tree for simplification spans.
         var spansTree = new TextSpanIntervalTree(spans);
 
-        bool isNodeOrTokenOutsideSimplifySpans(SyntaxNodeOrToken nodeOrToken)
-            => !spansTree.HasIntervalThatOverlapsWith(nodeOrToken.FullSpan.Start, nodeOrToken.FullSpan.Length);
-
-        var semanticModel = await document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false);
-        var root = await semanticModel.SyntaxTree.GetRootAsync(cancellationToken).ConfigureAwait(false);
+        var root = (TCompilationUnitSyntax)await document.GetRequiredSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
 
         // prep namespace imports marked for simplification 
         var removeIfUnusedAnnotation = new SyntaxAnnotation();
         var originalRoot = root;
-        root = PrepareNamespaceImportsForRemovalIfUnused(document, root, removeIfUnusedAnnotation, isNodeOrTokenOutsideSimplifySpans);
+        root = PrepareNamespaceImportsForRemovalIfUnused(root, removeIfUnusedAnnotation, IsNodeOrTokenOutsideSimplifySpans);
         var hasImportsToSimplify = root != originalRoot;
 
         if (hasImportsToSimplify)
         {
             document = document.WithSyntaxRoot(root);
-            semanticModel = await document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false);
-            root = await semanticModel.SyntaxTree.GetRootAsync(cancellationToken).ConfigureAwait(false);
+            root = (TCompilationUnitSyntax)await document.GetRequiredSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
         }
 
         // Get the list of syntax nodes and tokens that need to be reduced.
-        var nodesAndTokensToReduce = this.GetNodesAndTokensToReduce(root, isNodeOrTokenOutsideSimplifySpans);
+        var nodesAndTokensToReduce = this.GetNodesAndTokensToReduce(root, IsNodeOrTokenOutsideSimplifySpans);
 
         if (nodesAndTokensToReduce.Any())
         {
@@ -143,7 +140,7 @@ bool isNodeOrTokenOutsideSimplifySpans(SyntaxNodeOrToken nodeOrToken)
             // Reduce all the nodesAndTokensToReduce using the given reducers/rewriters and
             // store the reduced nodes and/or tokens in the reduced nodes/tokens maps.
             // Note that this method doesn't update the original syntax tree.
-            await this.ReduceAsync(document, root, nodesAndTokensToReduce, reducers, options, semanticModel, reducedNodesMap, reducedTokensMap, cancellationToken).ConfigureAwait(false);
+            await this.ReduceAsync(document, root, nodesAndTokensToReduce, reducers, options, reducedNodesMap, reducedTokensMap, cancellationToken).ConfigureAwait(false);
 
             if (reducedNodesMap.Any() || reducedTokensMap.Any())
             {
@@ -167,6 +164,9 @@ bool isNodeOrTokenOutsideSimplifySpans(SyntaxNodeOrToken nodeOrToken)
         }
 
         return document;
+
+        bool IsNodeOrTokenOutsideSimplifySpans(SyntaxNodeOrToken nodeOrToken)
+            => !spansTree.HasIntervalThatOverlapsWith(nodeOrToken.FullSpan.Start, nodeOrToken.FullSpan.Length);
     }
 
     private async Task ReduceAsync(
@@ -175,7 +175,6 @@ private async Task ReduceAsync(
         ImmutableArray<NodeOrTokenToReduce> nodesAndTokensToReduce,
         ImmutableArray<AbstractReducer> reducers,
         SimplifierOptions options,
-        SemanticModel semanticModel,
         ConcurrentDictionary<SyntaxNode, SyntaxNode> reducedNodesMap,
         ConcurrentDictionary<SyntaxToken, SyntaxToken> reducedTokensMap,
         CancellationToken cancellationToken)
@@ -185,6 +184,9 @@ private async Task ReduceAsync(
 
         Contract.ThrowIfFalse(nodesAndTokensToReduce.Any());
 
+        var semanticModel = await document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+
+        // Reduce each node or token in the given list by running it through each reducer.
         if (executeSerially)
         {
             foreach (var nodeOrTokenToReduce in nodesAndTokensToReduce)
@@ -207,8 +209,8 @@ async ValueTask ReduceOneNodeOrTokenAsync(
 
             var nodeOrToken = nodeOrTokenToReduce.OriginalNodeOrToken;
             var simplifyAllDescendants = nodeOrTokenToReduce.SimplifyAllDescendants;
-            var semanticModelForReduce = semanticModel;
             var currentNodeOrToken = nodeOrTokenToReduce.NodeOrToken;
+            var semanticModelForReduce = semanticModel;
             var isNode = nodeOrToken.IsNode;
 
             foreach (var reducer in reducers)
@@ -230,10 +232,7 @@ async ValueTask ReduceOneNodeOrTokenAsync(
                         // However, the reduced nodes haven't been parented yet, so do the required parenting using the original node's parent.
                         if (currentNodeOrToken.Parent == null &&
                             nodeOrToken.Parent != null &&
-                            (currentNodeOrToken.IsToken ||
-                            currentNodeOrToken.AsNode() is TExpressionSyntax ||
-                            currentNodeOrToken.AsNode() is TStatementSyntax ||
-                            currentNodeOrToken.AsNode() is TCrefSyntax))
+                            (currentNodeOrToken.IsToken || currentNodeOrToken.AsNode() is TExpressionSyntax or TStatementSyntax or TCrefSyntax))
                         {
                             var annotation = new SyntaxAnnotation();
                             currentNodeOrToken = currentNodeOrToken.WithAdditionalAnnotations(annotation);
@@ -292,20 +291,18 @@ async ValueTask ReduceOneNodeOrTokenAsync(
 
     // find any namespace imports / using directives marked for simplification in the specified spans
     // and add removeIfUnused annotation
-    private static SyntaxNode PrepareNamespaceImportsForRemovalIfUnused(
-        Document document,
-        SyntaxNode root,
+    private TCompilationUnitSyntax PrepareNamespaceImportsForRemovalIfUnused(
+        TCompilationUnitSyntax root,
         SyntaxAnnotation removeIfUnusedAnnotation,
         Func<SyntaxNodeOrToken, bool> isNodeOrTokenOutsideSimplifySpan)
     {
-        var gen = SyntaxGenerator.GetGenerator(document);
+        using var _ = ArrayBuilder<SyntaxNode>.GetInstance(out var importDeclarations);
 
-        var importsToSimplify = root.DescendantNodes().Where(n =>
-            !isNodeOrTokenOutsideSimplifySpan(n)
-            && gen.GetDeclarationKind(n) == DeclarationKind.NamespaceImport
-            && n.HasAnnotation(Simplifier.Annotation));
+        this.AddImportDeclarations(root, importDeclarations);
 
-        return root.ReplaceNodes(importsToSimplify, (o, r) => r.WithAdditionalAnnotations(removeIfUnusedAnnotation));
+        return root.ReplaceNodes(
+            importDeclarations.Where(n => !isNodeOrTokenOutsideSimplifySpan(n) && n.HasAnnotation(Simplifier.Annotation)),
+            (o, r) => r.WithAdditionalAnnotations(removeIfUnusedAnnotation));
     }
 
     private async Task<Document> RemoveUnusedNamespaceImportsAsync(
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/CSharpFormatEngine.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/CSharpFormatEngine.cs
index 11c3eb47cef74..792e58bfdb761 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/CSharpFormatEngine.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/CSharpFormatEngine.cs
@@ -29,7 +29,7 @@ public CSharpFormatEngine(
     internal override IHeaderFacts HeaderFacts => CSharpHeaderFacts.Instance;
 
     protected override AbstractTriviaDataFactory CreateTriviaFactory()
-        => new TriviaDataFactory(this.TreeData, this.Options);
+        => new TriviaDataFactory(this.TreeData, this.Options.LineFormatting);
 
     protected override AbstractFormattingResult CreateFormattingResult(TokenStream tokenStream)
         => new FormattingResult(this.TreeData, tokenStream, this.SpanToFormat);
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/CSharpStructuredTriviaFormatEngine.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/CSharpStructuredTriviaFormatEngine.cs
index 182588d383675..cec283c043333 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/CSharpStructuredTriviaFormatEngine.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/CSharpStructuredTriviaFormatEngine.cs
@@ -43,7 +43,7 @@ private CSharpStructuredTriviaFormatEngine(
     internal override IHeaderFacts HeaderFacts => CSharpHeaderFacts.Instance;
 
     protected override AbstractTriviaDataFactory CreateTriviaFactory()
-        => new TriviaDataFactory(this.TreeData, this.Options);
+        => new TriviaDataFactory(this.TreeData, this.Options.LineFormatting);
 
     protected override FormattingContext CreateFormattingContext(TokenStream tokenStream, CancellationToken cancellationToken)
         => new(this, tokenStream);
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.ComplexTrivia.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.ComplexTrivia.cs
index 85172a3f82170..21c63a5e8f9a0 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.ComplexTrivia.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.ComplexTrivia.cs
@@ -19,9 +19,9 @@ internal partial class TriviaDataFactory
     /// represents a general trivia between two tokens. slightly more expensive than others since it
     /// needs to calculate stuff unlike other cases
     /// </summary>
-    private class ComplexTrivia : AbstractComplexTrivia
+    private sealed class ComplexTrivia : AbstractComplexTrivia
     {
-        public ComplexTrivia(SyntaxFormattingOptions options, TreeData treeInfo, SyntaxToken token1, SyntaxToken token2)
+        public ComplexTrivia(LineFormattingOptions options, TreeData treeInfo, SyntaxToken token1, SyntaxToken token2)
             : base(options, treeInfo, token1, token2)
         {
         }
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.FormattedComplexTrivia.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.FormattedComplexTrivia.cs
index 4a525360d5b33..d62c4c9a6d9a4 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.FormattedComplexTrivia.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.FormattedComplexTrivia.cs
@@ -13,7 +13,7 @@ namespace Microsoft.CodeAnalysis.CSharp.Formatting;
 
 internal partial class TriviaDataFactory
 {
-    private class FormattedComplexTrivia : TriviaDataWithList
+    private sealed class FormattedComplexTrivia : TriviaDataWithList
     {
         private readonly CSharpTriviaFormatter _formatter;
         private readonly IList<TextChange> _textChanges;
@@ -27,7 +27,7 @@ public FormattedComplexTrivia(
             int spaces,
             string originalString,
             CancellationToken cancellationToken)
-            : base(context.Options, LanguageNames.CSharp)
+            : base(context.Options.LineFormatting)
         {
             Contract.ThrowIfNull(context);
             Contract.ThrowIfNull(formattingRules);
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.ModifiedComplexTrivia.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.ModifiedComplexTrivia.cs
index 5bce6fbd0fd38..1db933c501f3b 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.ModifiedComplexTrivia.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.ModifiedComplexTrivia.cs
@@ -14,12 +14,12 @@ namespace Microsoft.CodeAnalysis.CSharp.Formatting;
 
 internal partial class TriviaDataFactory
 {
-    private class ModifiedComplexTrivia : TriviaDataWithList
+    private sealed class ModifiedComplexTrivia : TriviaDataWithList
     {
         private readonly ComplexTrivia _original;
 
-        public ModifiedComplexTrivia(SyntaxFormattingOptions options, ComplexTrivia original, int lineBreaks, int space)
-            : base(options, original.Token1.Language)
+        public ModifiedComplexTrivia(LineFormattingOptions options, ComplexTrivia original, int lineBreaks, int space)
+            : base(options)
         {
             Contract.ThrowIfNull(original);
 
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.cs
index a7313cb07f61c..1ec647df3cd22 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Formatting/Engine/Trivia/TriviaDataFactory.cs
@@ -18,7 +18,7 @@ namespace Microsoft.CodeAnalysis.CSharp.Formatting;
 /// </summary>
 internal partial class TriviaDataFactory : AbstractTriviaDataFactory
 {
-    public TriviaDataFactory(TreeData treeInfo, SyntaxFormattingOptions options)
+    public TriviaDataFactory(TreeData treeInfo, LineFormattingOptions options)
         : base(treeInfo, options)
     {
     }
@@ -118,7 +118,7 @@ private static bool ContainsOnlyWhitespace(Analyzer.AnalysisResult result)
         {
             // calculate actual space size from tab
             var spaces = CalculateSpaces(token1, token2);
-            return new ModifiedWhitespace(this.Options, result.LineBreaks, indentation: spaces, elastic: result.TreatAsElastic, language: LanguageNames.CSharp);
+            return new ModifiedWhitespace(this.Options, result.LineBreaks, indentation: spaces, elastic: result.TreatAsElastic);
         }
 
         // check whether we can cache trivia info for current indentation
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Extensions/SyntaxNodeExtensions.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Extensions/SyntaxNodeExtensions.cs
index 782139a28ab85..67fe1c29206da 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Extensions/SyntaxNodeExtensions.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Extensions/SyntaxNodeExtensions.cs
@@ -178,12 +178,18 @@ public static SyntaxNode GetCommonRoot(this SyntaxNode node1, SyntaxNode node2)
     {
         Contract.ThrowIfTrue(node1.RawKind == 0 || node2.RawKind == 0);
 
-        // find common starting node from two nodes.
-        // as long as two nodes belong to same tree, there must be at least one common root (Ex, compilation unit)
-        var ancestors = node1.GetAncestorsOrThis<SyntaxNode>();
-        var set = new HashSet<SyntaxNode>(node2.GetAncestorsOrThis<SyntaxNode>());
+        // find common starting node from two nodes. as long as two nodes belong to same tree, there must be at least
+        // one common root (Ex, compilation unit)
+        using var _ = PooledHashSet<SyntaxNode>.GetInstance(out var set);
+        set.AddRange(node2.GetAncestorsOrThis<SyntaxNode>());
 
-        return ancestors.First(set.Contains);
+        foreach (var ancestor in node1.AncestorsAndSelf())
+        {
+            if (set.Contains(ancestor))
+                return ancestor;
+        }
+
+        throw ExceptionUtilities.Unreachable();
     }
 
     public static int Width(this SyntaxNode node)
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.AbstractComplexTrivia.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.AbstractComplexTrivia.cs
index 8e14ebe0b9565..c081c5813c4b4 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.AbstractComplexTrivia.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.AbstractComplexTrivia.cs
@@ -3,7 +3,6 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Threading;
-using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Shared.Utilities;
 using Roslyn.Utilities;
 
@@ -21,8 +20,8 @@ protected abstract class AbstractComplexTrivia : TriviaDataWithList
 
         private readonly bool _treatAsElastic;
 
-        public AbstractComplexTrivia(SyntaxFormattingOptions options, TreeData treeInfo, SyntaxToken token1, SyntaxToken token2)
-            : base(options, token1.Language)
+        public AbstractComplexTrivia(LineFormattingOptions options, TreeData treeInfo, SyntaxToken token1, SyntaxToken token2)
+            : base(options)
         {
             Contract.ThrowIfNull(treeInfo);
 
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.FormattedWhitespace.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.FormattedWhitespace.cs
index 82ae7747c65b3..2e1c7f1a60ce4 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.FormattedWhitespace.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.FormattedWhitespace.cs
@@ -13,12 +13,12 @@ namespace Microsoft.CodeAnalysis.Formatting;
 
 internal abstract partial class AbstractTriviaDataFactory
 {
-    protected class FormattedWhitespace : TriviaData
+    protected sealed class FormattedWhitespace : TriviaData
     {
         private readonly string _newString;
 
-        public FormattedWhitespace(SyntaxFormattingOptions options, int lineBreaks, int indentation, string language)
-            : base(options, language)
+        public FormattedWhitespace(LineFormattingOptions options, int lineBreaks, int indentation)
+            : base(options)
         {
             this.LineBreaks = Math.Max(0, lineBreaks);
             this.Spaces = Math.Max(0, indentation);
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.ModifiedWhitespace.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.ModifiedWhitespace.cs
index bca9cdecd1006..474e09b6a354d 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.ModifiedWhitespace.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.ModifiedWhitespace.cs
@@ -11,18 +11,18 @@ namespace Microsoft.CodeAnalysis.Formatting;
 
 internal abstract partial class AbstractTriviaDataFactory
 {
-    protected class ModifiedWhitespace : Whitespace
+    protected sealed class ModifiedWhitespace : Whitespace
     {
         private readonly Whitespace? _original;
 
-        public ModifiedWhitespace(SyntaxFormattingOptions options, int lineBreaks, int indentation, bool elastic, string language)
-            : base(options, lineBreaks, indentation, elastic, language)
+        public ModifiedWhitespace(LineFormattingOptions options, int lineBreaks, int indentation, bool elastic)
+            : base(options, lineBreaks, indentation, elastic)
         {
             _original = null;
         }
 
-        public ModifiedWhitespace(SyntaxFormattingOptions options, Whitespace original, int lineBreaks, int indentation, bool elastic, string language)
-            : base(options, lineBreaks, indentation, elastic, language)
+        public ModifiedWhitespace(LineFormattingOptions options, Whitespace original, int lineBreaks, int indentation, bool elastic)
+            : base(options, lineBreaks, indentation, elastic)
         {
             Contract.ThrowIfNull(original);
             _original = original;
@@ -83,7 +83,7 @@ public override void Format(
             CancellationToken cancellationToken,
             int tokenPairIndex = TokenPairIndexNotNeeded)
         {
-            formattingResultApplier(tokenPairIndex, context.TokenStream, new FormattedWhitespace(this.Options, this.LineBreaks, this.Spaces, this.Language));
+            formattingResultApplier(tokenPairIndex, context.TokenStream, new FormattedWhitespace(this.Options, this.LineBreaks, this.Spaces));
         }
     }
 }
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.Whitespace.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.Whitespace.cs
index 116dda5456313..bda9b5b3785b4 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.Whitespace.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.Whitespace.cs
@@ -21,14 +21,14 @@ protected class Whitespace : TriviaData
     {
         private readonly bool _elastic;
 
-        public Whitespace(SyntaxFormattingOptions options, int space, bool elastic, string language)
-            : this(options, lineBreaks: 0, indentation: space, elastic: elastic, language: language)
+        public Whitespace(LineFormattingOptions options, int space, bool elastic)
+            : this(options, lineBreaks: 0, indentation: space, elastic: elastic)
         {
             Contract.ThrowIfFalse(space >= 0);
         }
 
-        public Whitespace(SyntaxFormattingOptions options, int lineBreaks, int indentation, bool elastic, string language)
-            : base(options, language)
+        public Whitespace(LineFormattingOptions options, int lineBreaks, int indentation, bool elastic)
+            : base(options)
         {
             _elastic = elastic;
 
@@ -47,11 +47,9 @@ public Whitespace(SyntaxFormattingOptions options, int lineBreaks, int indentati
         public override TriviaData WithSpace(int space, FormattingContext context, ChainedFormattingRules formattingRules)
         {
             if (this.LineBreaks == 0 && this.Spaces == space)
-            {
                 return this;
-            }
 
-            return new ModifiedWhitespace(this.Options, this, /*lineBreak*/0, space, elastic: false, language: this.Language);
+            return new ModifiedWhitespace(this.Options, this, /*lineBreak*/0, space, elastic: false);
         }
 
         public override TriviaData WithLine(int line, int indentation, FormattingContext context, ChainedFormattingRules formattingRules, CancellationToken cancellationToken)
@@ -63,7 +61,7 @@ public override TriviaData WithLine(int line, int indentation, FormattingContext
                 return this;
             }
 
-            return new ModifiedWhitespace(this.Options, this, line, indentation, elastic: false, language: this.Language);
+            return new ModifiedWhitespace(this.Options, this, line, indentation, elastic: false);
         }
 
         public override TriviaData WithIndentation(
@@ -74,7 +72,7 @@ public override TriviaData WithIndentation(
                 return this;
             }
 
-            return new ModifiedWhitespace(this.Options, this, this.LineBreaks, indentation, elastic: false, language: this.Language);
+            return new ModifiedWhitespace(this.Options, this, this.LineBreaks, indentation, elastic: false);
         }
 
         public override void Format(
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.cs
index 19b2882c81115..71a0fce380856 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/AbstractTriviaDataFactory.cs
@@ -2,6 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
+using System.Collections.Generic;
+using System.Security.Principal;
 using System.Threading;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Roslyn.Utilities;
@@ -14,23 +17,73 @@ internal abstract partial class AbstractTriviaDataFactory
     private const int LineBreakCacheSize = 5;
     private const int IndentationLevelCacheSize = 20;
 
+    private static readonly Dictionary<LineFormattingOptions, (Whitespace[] spaces, Whitespace[,] whitespaces)> s_optionsToWhitespace = new();
+    private static Tuple<LineFormattingOptions, (Whitespace[] spaces, Whitespace[,] whitespaces)>? s_lastOptionAndWhitespace;
+
     protected readonly TreeData TreeInfo;
-    protected readonly SyntaxFormattingOptions Options;
+    protected readonly LineFormattingOptions Options;
 
     private readonly Whitespace[] _spaces;
-    private readonly Whitespace?[,] _whitespaces = new Whitespace[LineBreakCacheSize, IndentationLevelCacheSize];
+    private readonly Whitespace[,] _whitespaces;
 
-    protected AbstractTriviaDataFactory(TreeData treeInfo, SyntaxFormattingOptions options)
+    protected AbstractTriviaDataFactory(TreeData treeInfo, LineFormattingOptions options)
     {
         Contract.ThrowIfNull(treeInfo);
 
         TreeInfo = treeInfo;
         Options = options;
 
-        _spaces = new Whitespace[SpaceCacheSize];
-        for (var i = 0; i < SpaceCacheSize; i++)
+        (_spaces, _whitespaces) = GetSpacesAndWhitespaces(options);
+    }
+
+    private static (Whitespace[] spaces, Whitespace[,] whitespaces) GetSpacesAndWhitespaces(LineFormattingOptions options)
+    {
+        // Fast path where we'er asking for the same options as last time
+        var lastOptionAndWhitespace = s_lastOptionAndWhitespace;
+        if (lastOptionAndWhitespace?.Item1 == options)
+            return lastOptionAndWhitespace.Item2;
+
+        // Otherwise, get from the dictionary, computing if necessary.
+        var (spaces, whitespaces) = ComputeAndCacheSpacesAndWhitespaces(options);
+
+        // Cache this result for the next time.
+        s_lastOptionAndWhitespace = Tuple.Create(options, (spaces, whitespaces));
+        return (spaces, whitespaces);
+
+        static (Whitespace[] spaces, Whitespace[,] whitespaces) ComputeAndCacheSpacesAndWhitespaces(LineFormattingOptions options)
         {
-            _spaces[i] = new Whitespace(Options, space: i, elastic: false, language: treeInfo.Root.Language);
+            // First check if it's already in the cache.
+            lock (s_optionsToWhitespace)
+            {
+                if (s_optionsToWhitespace.TryGetValue(options, out var result))
+                    return result;
+            }
+
+            // If not, compute it.
+            var spaces = new Whitespace[SpaceCacheSize];
+            for (var i = 0; i < SpaceCacheSize; i++)
+                spaces[i] = new Whitespace(options, space: i, elastic: false);
+
+            var whitespaces = new Whitespace[LineBreakCacheSize, IndentationLevelCacheSize];
+            for (var lineIndex = 0; lineIndex < LineBreakCacheSize; lineIndex++)
+            {
+                for (var indentationLevel = 0; indentationLevel < IndentationLevelCacheSize; indentationLevel++)
+                {
+                    var indentation = indentationLevel * options.IndentationSize;
+                    whitespaces[lineIndex, indentationLevel] = new Whitespace(
+                        options, lineBreaks: lineIndex + 1, indentation: indentation, elastic: false);
+                }
+            }
+
+            // Attempt to store in cache.  But defer to any other thread that may have already stored it.
+            lock (s_optionsToWhitespace)
+            {
+                if (s_optionsToWhitespace.TryGetValue(options, out var result))
+                    return result;
+
+                s_optionsToWhitespace[options] = (spaces, whitespaces);
+                return (spaces, whitespaces);
+            }
         }
     }
 
@@ -41,7 +94,7 @@ protected TriviaData GetSpaceTriviaData(int space, bool elastic = false)
         // if result has elastic trivia in them, never use cache
         if (elastic)
         {
-            return new Whitespace(this.Options, space, elastic: true, language: this.TreeInfo.Root.Language);
+            return new Whitespace(this.Options, space, elastic: true);
         }
 
         if (space < SpaceCacheSize)
@@ -50,7 +103,7 @@ protected TriviaData GetSpaceTriviaData(int space, bool elastic = false)
         }
 
         // create a new space
-        return new Whitespace(this.Options, space, elastic: false, language: this.TreeInfo.Root.Language);
+        return new Whitespace(this.Options, space, elastic: false);
     }
 
     protected TriviaData GetWhitespaceTriviaData(int lineBreaks, int indentation, bool useTriviaAsItIs, bool elastic)
@@ -75,29 +128,13 @@ protected TriviaData GetWhitespaceTriviaData(int lineBreaks, int indentation, bo
             if (indentationLevel < IndentationLevelCacheSize)
             {
                 var lineIndex = lineBreaks - 1;
-                EnsureWhitespaceTriviaInfo(lineIndex, indentationLevel);
                 return _whitespaces[lineIndex, indentationLevel]!;
             }
         }
 
-        return
-            useTriviaAsItIs
-                ? new Whitespace(this.Options, lineBreaks, indentation, elastic, language: this.TreeInfo.Root.Language)
-                : new ModifiedWhitespace(this.Options, lineBreaks, indentation, elastic, language: this.TreeInfo.Root.Language);
-    }
-
-    private void EnsureWhitespaceTriviaInfo(int lineIndex, int indentationLevel)
-    {
-        Contract.ThrowIfFalse(lineIndex is >= 0 and < LineBreakCacheSize);
-        Contract.ThrowIfFalse(indentationLevel >= 0 && indentationLevel < _whitespaces.Length / _whitespaces.Rank);
-
-        // set up caches
-        if (_whitespaces[lineIndex, indentationLevel] == null)
-        {
-            var indentation = indentationLevel * Options.IndentationSize;
-            var triviaInfo = new Whitespace(Options, lineBreaks: lineIndex + 1, indentation: indentation, elastic: false, language: this.TreeInfo.Root.Language);
-            Interlocked.CompareExchange(ref _whitespaces[lineIndex, indentationLevel], triviaInfo, null);
-        }
+        return useTriviaAsItIs
+            ? new Whitespace(this.Options, lineBreaks, indentation, elastic)
+            : new ModifiedWhitespace(this.Options, lineBreaks, indentation, elastic);
     }
 
     public abstract TriviaData CreateLeadingTrivia(SyntaxToken token);
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TokenData.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TokenData.cs
index e50090dda0738..1bc2f1c870430 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TokenData.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TokenData.cs
@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Roslyn.Utilities;
 
@@ -63,50 +64,47 @@ public int CompareTo(TokenData other)
         Contract.ThrowIfFalse(this.TokenStream == other.TokenStream);
 
         if (this.IndexInStream >= 0 && other.IndexInStream >= 0)
-        {
             return this.IndexInStream - other.IndexInStream;
-        }
+
+        if (this.Token == other.Token)
+            return 0;
 
         var start = this.Token.SpanStart - other.Token.SpanStart;
         if (start != 0)
-        {
             return start;
-        }
 
         var end = this.Token.Span.End - other.Token.Span.End;
         if (end != 0)
-        {
             return end;
-        }
 
-        // this is expansive check. but there is no other way to check.
+        // We have two different tokens, which are at the same location.  This can happen with things like empty/missing
+        // tokens.  In order to give a strict ordering, we need to walk up the tree to find the first common ancestor
+        // and see which token we hit first in that ancestor.
         var commonRoot = this.Token.GetCommonRoot(other.Token);
-        RoslynDebug.Assert(commonRoot != null);
+        Contract.ThrowIfNull(commonRoot);
 
-        var tokens = commonRoot.DescendantTokens();
+        // Now, figure out the ancestor of each token parented by the common root.
+        var thisTokenAncestor = GetAncestorUnderRoot(this.Token, commonRoot);
+        var otherTokenAncestor = GetAncestorUnderRoot(other.Token, commonRoot);
 
-        var index1 = Index(tokens, this.Token);
-        var index2 = Index(tokens, other.Token);
-        Contract.ThrowIfFalse(index1 >= 0 && index2 >= 0);
-
-        return index1 - index2;
-    }
+        foreach (var child in commonRoot.ChildNodesAndTokens())
+        {
+            if (child == thisTokenAncestor)
+                return -1;
+            else if (child == otherTokenAncestor)
+                return 1;
+        }
 
-    private static int Index(IEnumerable<SyntaxToken> tokens, SyntaxToken token)
-    {
-        var index = 0;
+        throw ExceptionUtilities.Unreachable();
 
-        foreach (var current in tokens)
+        static SyntaxNodeOrToken GetAncestorUnderRoot(SyntaxNodeOrToken start, SyntaxNode root)
         {
-            if (current == token)
-            {
-                return index;
-            }
+            var current = start;
+            while (current.Parent != root)
+                current = current.Parent;
 
-            index++;
+            return current;
         }
-
-        return -1;
     }
 
     public static bool operator <(TokenData left, TokenData right)
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TriviaData.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TriviaData.cs
index 5d39109a8d864..4f62d0df988a8 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TriviaData.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TriviaData.cs
@@ -18,16 +18,12 @@ internal abstract class TriviaData
 {
     protected const int TokenPairIndexNotNeeded = int.MinValue;
 
-    private readonly string _language;
-
-    protected TriviaData(SyntaxFormattingOptions options, string language)
+    protected TriviaData(LineFormattingOptions options)
     {
         Options = options;
-        _language = language;
     }
 
-    protected SyntaxFormattingOptions Options { get; }
-    protected string Language => _language;
+    protected LineFormattingOptions Options { get; }
 
     public int LineBreaks { get; protected set; }
     public int Spaces { get; protected set; }
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TriviaDataWithList.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TriviaDataWithList.cs
index dff9d8703a24c..9cc7dc29fce5e 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TriviaDataWithList.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/Engine/TriviaDataWithList.cs
@@ -3,11 +3,10 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Threading;
-using Microsoft.CodeAnalysis.Diagnostics;
 
 namespace Microsoft.CodeAnalysis.Formatting;
 
-internal abstract class TriviaDataWithList(SyntaxFormattingOptions options, string language) : TriviaData(options, language)
+internal abstract class TriviaDataWithList(LineFormattingOptions options) : TriviaData(options)
 {
     public abstract SyntaxTriviaList GetTriviaList(CancellationToken cancellationToken);
 }
diff --git a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.AbstractLineBreakTrivia.vb b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.AbstractLineBreakTrivia.vb
index d75d120ab0a90..4b543b4e32ee7 100644
--- a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.AbstractLineBreakTrivia.vb
+++ b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.AbstractLineBreakTrivia.vb
@@ -15,12 +15,12 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
             Protected ReadOnly _original As String
             Protected ReadOnly _newString As String
 
-            Public Sub New(options As SyntaxFormattingOptions,
+            Public Sub New(options As LineFormattingOptions,
                            original As String,
                            lineBreaks As Integer,
                            indentation As Integer,
                            elastic As Boolean)
-                MyBase.New(options, lineBreaks, indentation, elastic, LanguageNames.VisualBasic)
+                MyBase.New(options, lineBreaks, indentation, elastic)
 
                 Me._original = original
                 Me._newString = CreateStringFromState()
diff --git a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.ComplexTrivia.vb b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.ComplexTrivia.vb
index 7644e750dd40b..71c51db59f694 100644
--- a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.ComplexTrivia.vb
+++ b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.ComplexTrivia.vb
@@ -17,7 +17,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
         Private Class ComplexTrivia
             Inherits AbstractComplexTrivia
 
-            Public Sub New(options As SyntaxFormattingOptions, treeInfo As TreeData, token1 As SyntaxToken, token2 As SyntaxToken)
+            Public Sub New(options As LineFormattingOptions, treeInfo As TreeData, token1 As SyntaxToken, token2 As SyntaxToken)
                 MyBase.New(options, treeInfo, token1, token2)
                 Contract.ThrowIfNull(treeInfo)
             End Sub
diff --git a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.FormattedComplexTrivia.vb b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.FormattedComplexTrivia.vb
index ba33b5201885f..07fc99fba20f8 100644
--- a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.FormattedComplexTrivia.vb
+++ b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.FormattedComplexTrivia.vb
@@ -8,7 +8,7 @@ Imports Microsoft.CodeAnalysis.Text
 
 Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
     Partial Friend Class TriviaDataFactory
-        Private Class FormattedComplexTrivia
+        Private NotInheritable Class FormattedComplexTrivia
             Inherits TriviaDataWithList
 
             Private ReadOnly _formatter As VisualBasicTriviaFormatter
@@ -22,7 +22,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
                            spaces As Integer,
                            originalString As String,
                            cancellationToken As CancellationToken)
-                MyBase.New(context.Options, LanguageNames.VisualBasic)
+                MyBase.New(context.Options.LineFormatting)
 
                 Contract.ThrowIfNull(context)
                 Contract.ThrowIfNull(formattingRules)
diff --git a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.LineContinuationTrivia.vb b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.LineContinuationTrivia.vb
index 83e0d3bf3d7dd..52007299afb51 100644
--- a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.LineContinuationTrivia.vb
+++ b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.LineContinuationTrivia.vb
@@ -16,7 +16,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
         Private Class LineContinuationTrivia
             Inherits AbstractLineBreakTrivia
 
-            Public Sub New(options As SyntaxFormattingOptions,
+            Public Sub New(options As LineFormattingOptions,
                            originalString As String,
                            indentation As Integer)
                 MyBase.New(options, originalString, 1, indentation, False)
diff --git a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.ModifiedComplexTrivia.vb b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.ModifiedComplexTrivia.vb
index 2062779935531..4dcd37bdaca7e 100644
--- a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.ModifiedComplexTrivia.vb
+++ b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.ModifiedComplexTrivia.vb
@@ -3,19 +3,18 @@
 ' See the LICENSE file in the project root for more information.
 
 Imports System.Threading
-Imports Microsoft.CodeAnalysis.Diagnostics
 Imports Microsoft.CodeAnalysis.Formatting
 Imports Microsoft.CodeAnalysis.Text
 
 Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
     Partial Friend Class TriviaDataFactory
-        Private Class ModifiedComplexTrivia
+        Private NotInheritable Class ModifiedComplexTrivia
             Inherits TriviaDataWithList
 
             Private ReadOnly _original As ComplexTrivia
 
-            Public Sub New(options As SyntaxFormattingOptions, original As ComplexTrivia, lineBreaks As Integer, space As Integer)
-                MyBase.New(options, LanguageNames.VisualBasic)
+            Public Sub New(options As LineFormattingOptions, original As ComplexTrivia, lineBreaks As Integer, space As Integer)
+                MyBase.New(options)
                 Contract.ThrowIfNull(original)
 
                 Me._original = original
diff --git a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.vb b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.vb
index 11bdd60ff1700..3475c8aad1dc2 100644
--- a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.vb
+++ b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/Trivia/TriviaDataFactory.vb
@@ -20,7 +20,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
 
         Private ReadOnly _lineContinuations(s_lineContinuationCacheSize) As LineContinuationTrivia
 
-        Public Sub New(treeInfo As TreeData, options As SyntaxFormattingOptions)
+        Public Sub New(treeInfo As TreeData, options As LineFormattingOptions)
             MyBase.New(treeInfo, options)
         End Sub
 
@@ -160,7 +160,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
             If result.LineBreaks = 0 AndAlso result.Tab > 0 Then
                 ' calculate actual space size from tab
                 Dim spaces = CalculateSpaces(token1, token2)
-                Return New ModifiedWhitespace(Me.Options, result.LineBreaks, indentation:=spaces, elastic:=result.TreatAsElastic, language:=LanguageNames.VisualBasic)
+                Return New ModifiedWhitespace(Me.Options, result.LineBreaks, indentation:=spaces, elastic:=result.TreatAsElastic)
             End If
 
             ' check whether we can cache trivia info for current indentation
diff --git a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/VisualBasicFormatEngine.vb b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/VisualBasicFormatEngine.vb
index d6b00a74ecdb5..4e89985b11d94 100644
--- a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/VisualBasicFormatEngine.vb
+++ b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/VisualBasicFormatEngine.vb
@@ -27,7 +27,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
         Friend Overrides ReadOnly Property HeaderFacts As IHeaderFacts = VisualBasicHeaderFacts.Instance
 
         Protected Overrides Function CreateTriviaFactory() As AbstractTriviaDataFactory
-            Return New TriviaDataFactory(Me.TreeData, Me.Options)
+            Return New TriviaDataFactory(Me.TreeData, Me.Options.LineFormatting)
         End Function
 
         Protected Overrides Function CreateFormattingResult(tokenStream As TokenStream) As AbstractFormattingResult
diff --git a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/VisualBasicStructuredTriviaFormatEngine.vb b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/VisualBasicStructuredTriviaFormatEngine.vb
index ac7baf64c323a..e683b19beacec 100644
--- a/src/Workspaces/VisualBasic/Portable/Formatting/Engine/VisualBasicStructuredTriviaFormatEngine.vb
+++ b/src/Workspaces/VisualBasic/Portable/Formatting/Engine/VisualBasicStructuredTriviaFormatEngine.vb
@@ -35,7 +35,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Formatting
         Friend Overrides ReadOnly Property HeaderFacts As IHeaderFacts = VisualBasicHeaderFacts.Instance
 
         Protected Overrides Function CreateTriviaFactory() As AbstractTriviaDataFactory
-            Return New TriviaDataFactory(Me.TreeData, Me.Options)
+            Return New TriviaDataFactory(Me.TreeData, Me.Options.LineFormatting)
         End Function
 
         Protected Overrides Function CreateFormattingContext(tokenStream As TokenStream, cancellationToken As CancellationToken) As FormattingContext
diff --git a/src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.NodesAndTokensToReduceComputer.vb b/src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.NodesAndTokensToReduceComputer.vb
index 12ab6a61cad72..c313960a29eed 100644
--- a/src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.NodesAndTokensToReduceComputer.vb
+++ b/src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.NodesAndTokensToReduceComputer.vb
@@ -9,10 +9,7 @@ Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
 Imports Microsoft.CodeAnalysis.VisualBasic.Utilities
 
 Namespace Microsoft.CodeAnalysis.VisualBasic.Simplification
-
     Partial Friend Class VisualBasicSimplificationService
-        Inherits AbstractSimplificationService(Of ExpressionSyntax, ExecutableStatementSyntax, CrefReferenceSyntax)
-
         Private Class NodesAndTokensToReduceComputer
             Inherits VisualBasicSyntaxRewriter
 
@@ -98,7 +95,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Simplification
                 Me._simplifyAllDescendants = Me._simplifyAllDescendants OrElse token.HasAnnotation(Simplifier.Annotation)
 
                 If Me._simplifyAllDescendants AndAlso Not Me._insideSpeculatedNode AndAlso token.Kind <> SyntaxKind.None Then
-                    Me._nodesAndTokensToReduce.Add(New NodeOrTokenToReduce(token, simplifyAllDescendants:=True, originalNodeOrToken:=token))
+                    Me._nodesAndTokensToReduce.Add(New NodeOrTokenToReduce(token, SimplifyAllDescendants:=True, OriginalNodeOrToken:=token))
                 End If
 
                 If token.ContainsAnnotations OrElse savedSimplifyAllDescendants Then
diff --git a/src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.vb b/src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.vb
index e4d912a910015..9246a9f11d22e 100644
--- a/src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.vb
+++ b/src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.vb
@@ -6,10 +6,10 @@ Imports System.Collections.Immutable
 Imports System.Composition
 Imports System.Threading
 Imports Microsoft.CodeAnalysis
-Imports Microsoft.CodeAnalysis.Diagnostics
 Imports Microsoft.CodeAnalysis.Host.Mef
 Imports Microsoft.CodeAnalysis.Internal.Log
 Imports Microsoft.CodeAnalysis.Options
+Imports Microsoft.CodeAnalysis.PooledObjects
 Imports Microsoft.CodeAnalysis.Simplification
 Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
 Imports Microsoft.CodeAnalysis.VisualBasic.Utilities
@@ -17,7 +17,7 @@ Imports Microsoft.CodeAnalysis.VisualBasic.Utilities
 Namespace Microsoft.CodeAnalysis.VisualBasic.Simplification
     <ExportLanguageService(GetType(ISimplificationService), LanguageNames.VisualBasic), [Shared]>
     Partial Friend Class VisualBasicSimplificationService
-        Inherits AbstractSimplificationService(Of ExpressionSyntax, ExecutableStatementSyntax, CrefReferenceSyntax)
+        Inherits AbstractSimplificationService(Of CompilationUnitSyntax, ExpressionSyntax, ExecutableStatementSyntax, CrefReferenceSyntax)
 
         Private Shared ReadOnly s_reducers As ImmutableArray(Of AbstractReducer) =
             ImmutableArray.Create(Of AbstractReducer)(
@@ -177,5 +177,8 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Simplification
             Next
         End Sub
 
+        Protected Overrides Sub AddImportDeclarations(root As CompilationUnitSyntax, importDeclarations As ArrayBuilder(Of SyntaxNode))
+            importDeclarations.AddRange(root.Imports)
+        End Sub
     End Class
 End Namespace