Add warning when property has a backing field but a manually implemented accessor does not use it

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8008,4 +8008,10 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureFirstClassSpan" xml:space="preserve">
     <value>first-class Span types</value>
   </data>
+  <data name="WRN_AccessorDoesNotUseBackingField" xml:space="preserve">
+    <value>The '{0}' accessor of property '{1}' should use 'field' because the other accessor is using it.</value>
+  </data>
+  <data name="WRN_AccessorDoesNotUseBackingField_Title" xml:space="preserve">
+    <value>Property accessor should use 'field' because the other accessor is using it.</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2347,6 +2347,7 @@ internal enum ErrorCode
 
         WRN_UninitializedNonNullableBackingField = 9264,
         WRN_UnassignedInternalRefField = 9265,
+        WRN_AccessorDoesNotUseBackingField = 9266,
 
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -564,6 +564,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_PartialPropertySignatureDifference:
                 case ErrorCode.WRN_FieldIsAmbiguous:
                 case ErrorCode.WRN_UninitializedNonNullableBackingField:
+                case ErrorCode.WRN_AccessorDoesNotUseBackingField:
                     return 1;
                 default:
                     return 0;
@@ -2464,6 +2465,7 @@ or ErrorCode.ERR_CannotApplyOverloadResolutionPriorityToMember
                 or ErrorCode.ERR_PartialPropertyDuplicateInitializer
                 or ErrorCode.WRN_UninitializedNonNullableBackingField
                 or ErrorCode.WRN_UnassignedInternalRefField
+                or ErrorCode.WRN_AccessorDoesNotUseBackingField
                     => false,
             };
 #pragma warning restore CS8524 // The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value.

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs

@@ -43,11 +43,12 @@ private static SourcePropertySymbol Create(
                 out bool isExpressionBodied,
                 out bool hasGetAccessorImplementation,
                 out bool hasSetAccessorImplementation,
-                out bool usesFieldKeyword,
+                out bool getterUsesFieldKeyword,
+                out bool setterUsesFieldKeyword,
                 out var getSyntax,
                 out var setSyntax);
 
-            Debug.Assert(!usesFieldKeyword ||
+            Debug.Assert(!(getterUsesFieldKeyword || setterUsesFieldKeyword) ||
                 ((CSharpParseOptions)syntax.SyntaxTree.Options).IsFeatureEnabled(MessageID.IDS_FeatureFieldKeyword));
 
             bool accessorsHaveImplementation = hasGetAccessorImplementation || hasSetAccessorImplementation;
@@ -90,7 +91,8 @@ private static SourcePropertySymbol Create(
                 hasAutoPropertySet: hasAutoPropertySet,
                 isExpressionBodied: isExpressionBodied,
                 accessorsHaveImplementation: accessorsHaveImplementation,
-                usesFieldKeyword: usesFieldKeyword,
+                getterUsesFieldKeyword: getterUsesFieldKeyword,
+                setterUsesFieldKeyword: setterUsesFieldKeyword,
                 memberName,
                 location,
                 diagnostics);
@@ -110,7 +112,8 @@ private SourcePropertySymbol(
             bool hasAutoPropertySet,
             bool isExpressionBodied,
             bool accessorsHaveImplementation,
-            bool usesFieldKeyword,
+            bool getterUsesFieldKeyword,
+            bool setterUsesFieldKeyword,
             string memberName,
             Location location,
             BindingDiagnosticBag diagnostics)
@@ -129,7 +132,8 @@ private SourcePropertySymbol(
                 hasAutoPropertySet: hasAutoPropertySet,
                 isExpressionBodied: isExpressionBodied,
                 accessorsHaveImplementation: accessorsHaveImplementation,
-                usesFieldKeyword: usesFieldKeyword,
+                getterUsesFieldKeyword: getterUsesFieldKeyword,
+                setterUsesFieldKeyword: setterUsesFieldKeyword,
                 syntax.Type.SkipScoped(out _).GetRefKindInLocalOrReturn(diagnostics),
                 memberName,
                 syntax.AttributeLists,
@@ -214,7 +218,8 @@ private static void GetAccessorDeclarations(
             out bool isExpressionBodied,
             out bool hasGetAccessorImplementation,
             out bool hasSetAccessorImplementation,
-            out bool usesFieldKeyword,
+            out bool getterUsesFieldKeyword,
+            out bool setterUsesFieldKeyword,
             out AccessorDeclarationSyntax? getSyntax,
             out AccessorDeclarationSyntax? setSyntax)
         {
@@ -225,7 +230,8 @@ private static void GetAccessorDeclarations(
 
             if (!isExpressionBodied)
             {
-                usesFieldKeyword = false;
+                getterUsesFieldKeyword = false;
+                setterUsesFieldKeyword = false;
                 hasGetAccessorImplementation = false;
                 hasSetAccessorImplementation = false;
                 foreach (var accessor in syntax.AccessorList!.Accessors)
@@ -237,6 +243,7 @@ private static void GetAccessorDeclarations(
                             {
                                 getSyntax = accessor;
                                 hasGetAccessorImplementation = hasImplementation(accessor);
+                                getterUsesFieldKeyword = containsFieldExpressionInAccessor(accessor);
                             }
                             else
                             {
@@ -249,6 +256,7 @@ private static void GetAccessorDeclarations(
                             {
                                 setSyntax = accessor;
                                 hasSetAccessorImplementation = hasImplementation(accessor);
+                                setterUsesFieldKeyword = containsFieldExpressionInAccessor(accessor);
                             }
                             else
                             {
@@ -266,16 +274,15 @@ private static void GetAccessorDeclarations(
                         default:
                             throw ExceptionUtilities.UnexpectedValue(accessor.Kind());
                     }
-
-                    usesFieldKeyword = usesFieldKeyword || containsFieldExpressionInAccessor(accessor);
                 }
             }
             else
             {
                 var body = GetArrowExpression(syntax);
                 hasGetAccessorImplementation = body is object;
                 hasSetAccessorImplementation = false;
-                usesFieldKeyword = body is { } && containsFieldExpressionInGreenNode(body.Green);
+                getterUsesFieldKeyword = body is { } && containsFieldExpressionInGreenNode(body.Green);
+                setterUsesFieldKeyword = false;
                 Debug.Assert(hasGetAccessorImplementation); // it's not clear how this even parsed as a property if it has no accessor list and no arrow expression.
             }
 

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs

@@ -31,12 +31,13 @@ private enum Flags : ushort
             IsExpressionBodied = 1 << 0,
             HasAutoPropertyGet = 1 << 1,
             HasAutoPropertySet = 1 << 2,
-            UsesFieldKeyword = 1 << 3,
-            IsExplicitInterfaceImplementation = 1 << 4,
-            HasInitializer = 1 << 5,
-            AccessorsHaveImplementation = 1 << 6,
-            HasExplicitAccessModifier = 1 << 7,
-            RequiresBackingField = 1 << 8,
+            GetterUsesFieldKeyword = 1 << 3,
+            SetterUsesFieldKeyword = 1 << 4,
+            IsExplicitInterfaceImplementation = 1 << 5,
+            HasInitializer = 1 << 6,
+            AccessorsHaveImplementation = 1 << 7,
+            HasExplicitAccessModifier = 1 << 8,
+            RequiresBackingField = 1 << 9,
         }
 
         // TODO (tomat): consider splitting into multiple subclasses/rare data.
@@ -90,7 +91,8 @@ protected SourcePropertySymbolBase(
             bool hasAutoPropertySet,
             bool isExpressionBodied,
             bool accessorsHaveImplementation,
-            bool usesFieldKeyword,
+            bool getterUsesFieldKeyword,
+            bool setterUsesFieldKeyword,
             RefKind refKind,
             string memberName,
             SyntaxList<AttributeListSyntax> indexerNameAttributeLists,
@@ -133,9 +135,14 @@ protected SourcePropertySymbolBase(
                 _propertyFlags |= Flags.HasAutoPropertySet;
             }
 
-            if (usesFieldKeyword)
+            if (getterUsesFieldKeyword)
             {
-                _propertyFlags |= Flags.UsesFieldKeyword;
+                _propertyFlags |= Flags.GetterUsesFieldKeyword;
+            }
+
+            if (setterUsesFieldKeyword)
+            {
+                _propertyFlags |= Flags.SetterUsesFieldKeyword;
             }
 
             if (hasInitializer)
@@ -171,7 +178,7 @@ protected SourcePropertySymbolBase(
                 _name = _lazySourceName = memberName;
             }
 
-            if (usesFieldKeyword || hasAutoPropertyGet || hasAutoPropertySet || hasInitializer)
+            if (getterUsesFieldKeyword || setterUsesFieldKeyword || hasAutoPropertyGet || hasAutoPropertySet || hasInitializer)
             {
                 Debug.Assert(!IsIndexer);
                 _propertyFlags |= Flags.RequiresBackingField;
@@ -305,6 +312,48 @@ protected void CheckInitializerIfNeeded(BindingDiagnosticBag diagnostics)
             }
         }
 
+#nullable enable
+        private static void CheckFieldKeywordUsage(SourcePropertySymbolBase property, BindingDiagnosticBag diagnostics)
+        {
+            Debug.Assert(property.PartialImplementationPart is null);
+            if (!property.DeclaringCompilation.IsFeatureEnabled(MessageID.IDS_FeatureFieldKeyword))
+            {
+                return;
+            }
+
+            SourcePropertyAccessorSymbol? accessorToBlame = null;
+            var propertyFlags = property._propertyFlags;
+            var getterUsesFieldKeyword = (propertyFlags & Flags.GetterUsesFieldKeyword) != 0;
+            var setterUsesFieldKeyword = (propertyFlags & Flags.SetterUsesFieldKeyword) != 0;
+            if (property._setMethod is { IsAutoPropertyAccessor: false } setMethod
+                && !setterUsesFieldKeyword
+                && !property.IsSetOnEitherPart(Flags.HasInitializer)
+                && (property.HasAutoPropertyGet || getterUsesFieldKeyword))
+            {
+                accessorToBlame = setMethod;
+            }
+            else if (property._getMethod is { IsAutoPropertyAccessor: false } getMethod
+                && !getterUsesFieldKeyword
+                && (property.HasAutoPropertySet || setterUsesFieldKeyword))
+            {
+                accessorToBlame = getMethod;
+            }
+
+            if (accessorToBlame is not null)
+            {
+                var accessorName = accessorToBlame switch
+                {
+                    { MethodKind: MethodKind.PropertyGet, IsInitOnly: false } => SyntaxFacts.GetText(SyntaxKind.GetKeyword),
+                    { MethodKind: MethodKind.PropertySet, IsInitOnly: false } => SyntaxFacts.GetText(SyntaxKind.SetKeyword),
+                    { MethodKind: MethodKind.PropertySet, IsInitOnly: true } => SyntaxFacts.GetText(SyntaxKind.InitKeyword),
+                    _ => throw ExceptionUtilities.UnexpectedValue(accessorToBlame)
+                };
+
+                diagnostics.Add(ErrorCode.WRN_AccessorDoesNotUseBackingField, accessorToBlame.GetFirstLocation(), accessorName, property);
+            }
+        }
+#nullable disable
+
         public sealed override RefKind RefKind
         {
             get
@@ -648,10 +697,10 @@ public bool HasSkipLocalsInitAttribute
         }
 
         internal bool IsAutoPropertyOrUsesFieldKeyword
-            => IsSetOnEitherPart(Flags.HasAutoPropertyGet | Flags.HasAutoPropertySet | Flags.UsesFieldKeyword);
+            => IsSetOnEitherPart(Flags.HasAutoPropertyGet | Flags.HasAutoPropertySet | Flags.GetterUsesFieldKeyword | Flags.SetterUsesFieldKeyword);
 
         internal bool UsesFieldKeyword
-            => IsSetOnEitherPart(Flags.UsesFieldKeyword);
+            => IsSetOnEitherPart(Flags.GetterUsesFieldKeyword | Flags.SetterUsesFieldKeyword);
 
         protected bool HasExplicitAccessModifier
             => (_propertyFlags & Flags.HasExplicitAccessModifier) != 0;
@@ -811,6 +860,7 @@ internal override void AfterAddingTypeMembersChecks(ConversionsBase conversions,
             this.CheckModifiers(isExplicitInterfaceImplementation, Location, IsIndexer, diagnostics);
 
             CheckInitializerIfNeeded(diagnostics);
+            CheckFieldKeywordUsage((SourcePropertySymbolBase?)PartialImplementationPart ?? this, diagnostics);
 
             if (RefKind != RefKind.None && IsRequired)
             {

src/Compilers/CSharp/Portable/Symbols/Synthesized/Records/SynthesizedRecordEqualityContractProperty.cs

@@ -36,7 +36,8 @@ public SynthesizedRecordEqualityContractProperty(SourceMemberContainerTypeSymbol
                 hasAutoPropertySet: false,
                 isExpressionBodied: false,
                 accessorsHaveImplementation: true,
-                usesFieldKeyword: false,
+                getterUsesFieldKeyword: false,
+                setterUsesFieldKeyword: false,
                 RefKind.None,
                 PropertyName,
                 indexerNameAttributeLists: new SyntaxList<AttributeListSyntax>(),

src/Compilers/CSharp/Portable/Symbols/Synthesized/Records/SynthesizedRecordPropertySymbol.cs

@@ -34,7 +34,8 @@ public SynthesizedRecordPropertySymbol(
                 hasAutoPropertySet: true,
                 isExpressionBodied: false,
                 accessorsHaveImplementation: true,
-                usesFieldKeyword: false,
+                getterUsesFieldKeyword: false,
+                setterUsesFieldKeyword: false,
                 RefKind.None,
                 backingParameter.Name,
                 indexerNameAttributeLists: new SyntaxList<AttributeListSyntax>(),