Revert "Make PROC_MACRO_DERIVE_RESOLUTION_FALLBACK a hard error"

This reverts commit 7d82cad.

I am doing this to buy us some time with respect to issue #106337 w.r.t. the
1.67 release.

compiler/rustc_lint_defs/src/builtin.rs

@@ -2023,6 +2023,73 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `proc_macro_derive_resolution_fallback` lint detects proc macro
+    /// derives using inaccessible names from parent modules.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (proc-macro)
+    /// // foo.rs
+    /// #![crate_type = "proc-macro"]
+    ///
+    /// extern crate proc_macro;
+    ///
+    /// use proc_macro::*;
+    ///
+    /// #[proc_macro_derive(Foo)]
+    /// pub fn foo1(a: TokenStream) -> TokenStream {
+    ///     drop(a);
+    ///     "mod __bar { static mut BAR: Option<Something> = None; }".parse().unwrap()
+    /// }
+    /// ```
+    ///
+    /// ```rust,ignore (needs-dependency)
+    /// // bar.rs
+    /// #[macro_use]
+    /// extern crate foo;
+    ///
+    /// struct Something;
+    ///
+    /// #[derive(Foo)]
+    /// struct Another;
+    ///
+    /// fn main() {}
+    /// ```
+    ///
+    /// This will produce:
+    ///
+    /// ```text
+    /// warning: cannot find type `Something` in this scope
+    ///  --> src/main.rs:8:10
+    ///   |
+    /// 8 | #[derive(Foo)]
+    ///   |          ^^^ names from parent modules are not accessible without an explicit import
+    ///   |
+    ///   = note: `#[warn(proc_macro_derive_resolution_fallback)]` on by default
+    ///   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    ///   = note: for more information, see issue #50504 <https://github.com/rust-lang/rust/issues/50504>
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// If a proc-macro generates a module, the compiler unintentionally
+    /// allowed items in that module to refer to items in the crate root
+    /// without importing them. This is a [future-incompatible] lint to
+    /// transition this to a hard error in the future. See [issue #50504] for
+    /// more details.
+    ///
+    /// [issue #50504]: https://github.com/rust-lang/rust/issues/50504
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    pub PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
+    Deny,
+    "detects proc macro derives using inaccessible names from parent modules",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #83583 <https://github.com/rust-lang/rust/issues/83583>",
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportNow,
+    };
+}
+
 declare_lint! {
     /// The `macro_use_extern_crate` lint detects the use of the
     /// [`macro_use` attribute].
@@ -3261,6 +3328,7 @@ declare_lint_pass! {
         UNSTABLE_NAME_COLLISIONS,
         IRREFUTABLE_LET_PATTERNS,
         WHERE_CLAUSES_OBJECT_SAFETY,
+        PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
         MACRO_USE_EXTERN_CRATE,
         MACRO_EXPANDED_MACRO_EXPORTS_ACCESSED_BY_ABSOLUTE_PATHS,
         ILL_FORMED_ATTRIBUTE_INPUT,

compiler/rustc_resolve/src/diagnostics.rs

@@ -1033,7 +1033,7 @@ impl<'a> Resolver<'a> {
                     let root_module = this.resolve_crate_root(root_ident);
                     this.add_module_candidates(root_module, &mut suggestions, filter_fn, None);
                 }
-                Scope::Module(module) => {
+                Scope::Module(module, _) => {
                     this.add_module_candidates(module, &mut suggestions, filter_fn, None);
                 }
                 Scope::MacroUsePrelude => {

compiler/rustc_resolve/src/ident.rs

@@ -1,9 +1,11 @@
-use rustc_ast as ast;
+use rustc_ast::{self as ast, NodeId};
 use rustc_feature::is_builtin_attr_name;
 use rustc_hir::def::{DefKind, Namespace, NonMacroAttrKind, PartialRes, PerNS};
 use rustc_hir::PrimTy;
 use rustc_middle::bug;
 use rustc_middle::ty;
+use rustc_session::lint::builtin::PROC_MACRO_DERIVE_RESOLUTION_FALLBACK;
+use rustc_session::lint::BuiltinLintDiagnostics;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::edition::Edition;
 use rustc_span::hygiene::{ExpnId, ExpnKind, LocalExpnId, MacroKind, SyntaxContext};
@@ -99,7 +101,7 @@ impl<'a> Resolver<'a> {
         };
         let mut scope = match ns {
             _ if is_absolute_path => Scope::CrateRoot,
-            TypeNS | ValueNS => Scope::Module(module),
+            TypeNS | ValueNS => Scope::Module(module, None),
             MacroNS => Scope::DeriveHelpers(parent_scope.expansion),
         };
         let mut ctxt = ctxt.normalize_to_macros_2_0();
@@ -163,7 +165,7 @@ impl<'a> Resolver<'a> {
                     MacroRulesScope::Invocation(invoc_id) => {
                         Scope::MacroRules(self.invocation_parent_scopes[&invoc_id].macro_rules)
                     }
-                    MacroRulesScope::Empty => Scope::Module(module),
+                    MacroRulesScope::Empty => Scope::Module(module, None),
                 },
                 Scope::CrateRoot => match ns {
                     TypeNS => {
@@ -172,10 +174,16 @@ impl<'a> Resolver<'a> {
                     }
                     ValueNS | MacroNS => break,
                 },
-                Scope::Module(module) => {
+                Scope::Module(module, prev_lint_id) => {
                     use_prelude = !module.no_implicit_prelude;
-                    match self.hygienic_lexical_parent(module, &mut ctxt) {
-                        Some(parent_module) => Scope::Module(parent_module),
+                    let derive_fallback_lint_id = match scope_set {
+                        ScopeSet::Late(.., lint_id) => lint_id,
+                        _ => None,
+                    };
+                    match self.hygienic_lexical_parent(module, &mut ctxt, derive_fallback_lint_id) {
+                        Some((parent_module, lint_id)) => {
+                            Scope::Module(parent_module, lint_id.or(prev_lint_id))
+                        }
                         None => {
                             ctxt.adjust(ExpnId::root());
                             match ns {
@@ -207,13 +215,45 @@ impl<'a> Resolver<'a> {
         &mut self,
         module: Module<'a>,
         ctxt: &mut SyntaxContext,
-    ) -> Option<Module<'a>> {
+        derive_fallback_lint_id: Option<NodeId>,
+    ) -> Option<(Module<'a>, Option<NodeId>)> {
         if !module.expansion.outer_expn_is_descendant_of(*ctxt) {
-            return Some(self.expn_def_scope(ctxt.remove_mark()));
+            return Some((self.expn_def_scope(ctxt.remove_mark()), None));
         }
 
         if let ModuleKind::Block = module.kind {
-            return Some(module.parent.unwrap().nearest_item_scope());
+            return Some((module.parent.unwrap().nearest_item_scope(), None));
+        }
+
+        // We need to support the next case under a deprecation warning
+        // ```
+        // struct MyStruct;
+        // ---- begin: this comes from a proc macro derive
+        // mod implementation_details {
+        //     // Note that `MyStruct` is not in scope here.
+        //     impl SomeTrait for MyStruct { ... }
+        // }
+        // ---- end
+        // ```
+        // So we have to fall back to the module's parent during lexical resolution in this case.
+        if derive_fallback_lint_id.is_some() {
+            if let Some(parent) = module.parent {
+                // Inner module is inside the macro, parent module is outside of the macro.
+                if module.expansion != parent.expansion
+                    && module.expansion.is_descendant_of(parent.expansion)
+                {
+                    // The macro is a proc macro derive
+                    if let Some(def_id) = module.expansion.expn_data().macro_def_id {
+                        let ext = self.get_macro_by_def_id(def_id).ext;
+                        if ext.builtin_name.is_none()
+                            && ext.macro_kind() == MacroKind::Derive
+                            && parent.expansion.outer_expn_is_descendant_of(*ctxt)
+                        {
+                            return Some((parent, derive_fallback_lint_id));
+                        }
+                    }
+                }
+            }
         }
 
         None
@@ -470,7 +510,7 @@ impl<'a> Resolver<'a> {
                             Err((Determinacy::Determined, _)) => Err(Determinacy::Determined),
                         }
                     }
-                    Scope::Module(module) => {
+                    Scope::Module(module, derive_fallback_lint_id) => {
                         let adjusted_parent_scope = &ParentScope { module, ..*parent_scope };
                         let binding = this.resolve_ident_in_module_unadjusted_ext(
                             ModuleOrUniformRoot::Module(module),
@@ -483,6 +523,21 @@ impl<'a> Resolver<'a> {
                         );
                         match binding {
                             Ok(binding) => {
+                                if let Some(lint_id) = derive_fallback_lint_id {
+                                    this.lint_buffer.buffer_lint_with_diagnostic(
+                                        PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
+                                        lint_id,
+                                        orig_ident.span,
+                                        &format!(
+                                            "cannot find {} `{}` in this scope",
+                                            ns.descr(),
+                                            ident
+                                        ),
+                                        BuiltinLintDiagnostics::ProcMacroDeriveResolutionFallback(
+                                            orig_ident.span,
+                                        ),
+                                    );
+                                }
                                 let misc_flags = if ptr::eq(module, this.graph_root) {
                                     Flags::MISC_SUGGEST_CRATE
                                 } else if module.is_normal() {

compiler/rustc_resolve/src/lib.rs

@@ -105,7 +105,9 @@ enum Scope<'a> {
     DeriveHelpersCompat,
     MacroRules(MacroRulesScopeRef<'a>),
     CrateRoot,
-    Module(Module<'a>),
+    // The node ID is for reporting the `PROC_MACRO_DERIVE_RESOLUTION_FALLBACK`
+    // lint if it should be reported.
+    Module(Module<'a>, Option<NodeId>),
     MacroUsePrelude,
     BuiltinAttrs,
     ExternPrelude,
@@ -1591,7 +1593,7 @@ impl<'a> Resolver<'a> {
 
         self.visit_scopes(ScopeSet::All(TypeNS, false), parent_scope, ctxt, |this, scope, _, _| {
             match scope {
-                Scope::Module(module) => {
+                Scope::Module(module, _) => {
                     this.traits_in_module(module, assoc_item, &mut found_traits);
                 }
                 Scope::StdLibPrelude => {

tests/ui/proc-macro/generate-mod.rs

@@ -15,16 +15,19 @@ struct S;
 
 #[derive(generate_mod::CheckDerive)] //~ ERROR cannot find type `FromOutside` in this scope
                                      //~| ERROR cannot find type `OuterDerive` in this scope
+                                     //~| WARN this was previously accepted
+                                     //~| WARN this was previously accepted
 struct Z;
 
 fn inner_block() {
     #[derive(generate_mod::CheckDerive)] //~ ERROR cannot find type `FromOutside` in this scope
                                         //~| ERROR cannot find type `OuterDerive` in this scope
+                                        //~| WARN this was previously accepted
+                                        //~| WARN this was previously accepted
     struct InnerZ;
 }
 
-#[derive(generate_mod::CheckDeriveLint)] //~  ERROR cannot find type `OuterDeriveLint` in this scope
-                                         //~| ERROR cannot find type `FromOutside` in this scope
+#[derive(generate_mod::CheckDeriveLint)] // OK, lint is suppressed
 struct W;
 
 fn main() {}