Rollup merge of #100387 - cjgillot:hygiene-trait-impl, r=petrochenkov

Check uniqueness of impl items by trait item when applicable.

When checking uniqueness of item names in impl blocks, we currently use the same definition of hygiene as for toplevel items.  This means that a plain item and one generated by a macro 2.0 do not collide.

This hygiene rule does not match with how impl items resolve to associated trait items. As a consequence, we misdiagnose the trait impls.

This PR proposes to consider that trait impl items are uses of the corresponding trait items during resolution, instead of checking for duplicates later. An error is emitted when a trait impl item is used twice.

There should be no stable breakage, since macros 2.0 are still unstable.

r? ``@petrochenkov``
cc ``@RalfJung``

Fixes https://github.com/rust-lang/rust/issues/71614.

compiler/rustc_resolve/src/diagnostics.rs

@@ -1050,6 +1050,19 @@ impl<'a> Resolver<'a> {
                 err.span_label(trait_item_span, "item in trait");
                 err
             }
+            ResolutionError::TraitImplDuplicate { name, trait_item_span, old_span } => {
+                let mut err = struct_span_err!(
+                    self.session,
+                    span,
+                    E0201,
+                    "duplicate definitions with name `{}`:",
+                    name,
+                );
+                err.span_label(old_span, "previous definition here");
+                err.span_label(trait_item_span, "item in trait");
+                err.span_label(span, "duplicate definition");
+                err
+            }
             ResolutionError::InvalidAsmSym => {
                 let mut err = self.session.struct_span_err(span, "invalid `sym` operand");
                 err.span_label(span, "is a local variable");

compiler/rustc_resolve/src/late.rs

@@ -2618,8 +2618,9 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                                         this.with_current_self_type(self_type, |this| {
                                             this.with_self_rib_ns(ValueNS, Res::SelfCtor(item_def_id), |this| {
                                                 debug!("resolve_implementation with_self_rib_ns(ValueNS, ...)");
+                                                let mut seen_trait_items = Default::default();
                                                 for item in impl_items {
-                                                    this.resolve_impl_item(&**item);
+                                                    this.resolve_impl_item(&**item, &mut seen_trait_items);
                                                 }
                                             });
                                         });
@@ -2633,7 +2634,11 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         );
     }
 
-    fn resolve_impl_item(&mut self, item: &'ast AssocItem) {
+    fn resolve_impl_item(
+        &mut self,
+        item: &'ast AssocItem,
+        seen_trait_items: &mut FxHashMap<DefId, Span>,
+    ) {
         use crate::ResolutionError::*;
         match &item.kind {
             AssocItemKind::Const(_, ty, default) => {
@@ -2646,6 +2651,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                     &item.kind,
                     ValueNS,
                     item.span,
+                    seen_trait_items,
                     |i, s, c| ConstNotMemberOfTrait(i, s, c),
                 );
 
@@ -2686,6 +2692,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                             &item.kind,
                             ValueNS,
                             item.span,
+                            seen_trait_items,
                             |i, s, c| MethodNotMemberOfTrait(i, s, c),
                         );
 
@@ -2714,6 +2721,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                                 &item.kind,
                                 TypeNS,
                                 item.span,
+                                seen_trait_items,
                                 |i, s, c| TypeNotMemberOfTrait(i, s, c),
                             );
 
@@ -2735,6 +2743,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         kind: &AssocItemKind,
         ns: Namespace,
         span: Span,
+        seen_trait_items: &mut FxHashMap<DefId, Span>,
         err: F,
     ) where
         F: FnOnce(Ident, String, Option<Symbol>) -> ResolutionError<'a>,
@@ -2767,7 +2776,25 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         };
 
         let res = binding.res();
-        let Res::Def(def_kind, _) = res else { bug!() };
+        let Res::Def(def_kind, id_in_trait) = res else { bug!() };
+
+        match seen_trait_items.entry(id_in_trait) {
+            Entry::Occupied(entry) => {
+                self.report_error(
+                    span,
+                    ResolutionError::TraitImplDuplicate {
+                        name: ident.name,
+                        old_span: *entry.get(),
+                        trait_item_span: binding.span,
+                    },
+                );
+                return;
+            }
+            Entry::Vacant(entry) => {
+                entry.insert(span);
+            }
+        };
+
         match (def_kind, kind) {
             (DefKind::AssocTy, AssocItemKind::Type(..))
             | (DefKind::AssocFn, AssocItemKind::Fn(..))

compiler/rustc_resolve/src/lib.rs

@@ -235,6 +235,8 @@ enum ResolutionError<'a> {
         trait_item_span: Span,
         code: rustc_errors::DiagnosticId,
     },
+    /// Error E0201: multiple impl items for the same trait item.
+    TraitImplDuplicate { name: Symbol, trait_item_span: Span, old_span: Span },
     /// Inline asm `sym` operand must refer to a `fn` or `static`.
     InvalidAsmSym,
 }