Limit impl trait in assoc type defining scope

compiler/rustc_ty_utils/src/opaque_types.rs

@@ -275,11 +275,89 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
     }
 }
 
+struct ImplTraitInAssocTypeCollector<'tcx>(OpaqueTypeCollector<'tcx>);
+
+impl<'tcx> super::sig_types::SpannedTypeVisitor<'tcx> for ImplTraitInAssocTypeCollector<'tcx> {
+    #[instrument(skip(self), ret, level = "trace")]
+    fn visit(&mut self, span: Span, value: impl TypeVisitable<TyCtxt<'tcx>>) -> ControlFlow<!> {
+        let old = self.0.span;
+        self.0.span = Some(span);
+        value.visit_with(self);
+        self.0.span = old;
+
+        ControlFlow::Continue(())
+    }
+}
+
+impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ImplTraitInAssocTypeCollector<'tcx> {
+    #[instrument(skip(self), ret, level = "trace")]
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<!> {
+        t.super_visit_with(self)?;
+        match t.kind() {
+            ty::Alias(ty::Opaque, alias_ty) if alias_ty.def_id.is_local() => {
+                self.0.visit_opaque_ty(alias_ty);
+            }
+            ty::Alias(ty::Projection, alias_ty) => {
+                // This avoids having to do normalization of `Self::AssocTy` by only
+                // supporting the case of a method defining opaque types from assoc types
+                // in the same impl block.
+                let parent_trait_ref = self
+                    .0
+                    .parent_trait_ref()
+                    .expect("impl trait in assoc type collector used on non-assoc item");
+                // If the trait ref of the associated item and the impl differs,
+                // then we can't use the impl's identity substitutions below, so
+                // just skip.
+                if alias_ty.trait_ref(self.0.tcx) == parent_trait_ref {
+                    let parent = self.0.parent().expect("we should have a parent here");
+
+                    for &assoc in self.0.tcx.associated_items(parent).in_definition_order() {
+                        trace!(?assoc);
+                        if assoc.trait_item_def_id != Some(alias_ty.def_id) {
+                            continue;
+                        }
+
+                        // If the type is further specializable, then the type_of
+                        // is not actually correct below.
+                        if !assoc.defaultness(self.0.tcx).is_final() {
+                            continue;
+                        }
+
+                        let impl_args = alias_ty.args.rebase_onto(
+                            self.0.tcx,
+                            parent_trait_ref.def_id,
+                            ty::GenericArgs::identity_for_item(self.0.tcx, parent),
+                        );
+
+                        if check_args_compatible(self.0.tcx, assoc, impl_args) {
+                            return self
+                                .0
+                                .tcx
+                                .type_of(assoc.def_id)
+                                .instantiate(self.0.tcx, impl_args)
+                                .visit_with(self);
+                        } else {
+                            self.0.tcx.dcx().span_delayed_bug(
+                                self.0.tcx.def_span(assoc.def_id),
+                                "item had incorrect args",
+                            );
+                        }
+                    }
+                }
+            }
+            _ => trace!(kind=?t.kind()),
+        }
+        ControlFlow::Continue(())
+    }
+}
+
 fn impl_trait_in_assoc_types_defined_by<'tcx>(
     tcx: TyCtxt<'tcx>,
     item: LocalDefId,
 ) -> &'tcx ty::List<LocalDefId> {
-    opaque_types_defined_by(tcx, item)
+    let mut collector = ImplTraitInAssocTypeCollector(OpaqueTypeCollector::new(tcx, item));
+    super::sig_types::walk_types(tcx, item, &mut collector);
+    tcx.mk_local_def_ids(&collector.0.opaques)
 }
 
 fn opaque_types_defined_by<'tcx>(

tests/ui/type-alias-impl-trait/hidden_behind_struct_field2.rs

@@ -1,9 +1,8 @@
-//! This test shows that we can even follow projections
+//! This test shows that we do not treat opaque types
-//! into associated types of the same impl if they are
+//! as defined by a method if the opaque type is
-//! indirectly mentioned in a struct field.
+//! only indirectly mentioned in a struct field.
 
 #![feature(impl_trait_in_assoc_type)]
-// check-pass
 
 struct Bar;
 
@@ -16,6 +15,7 @@ impl Trait for Bar {
     type Assoc = impl std::fmt::Debug;
     fn foo() -> Foo {
         Foo { field: () }
+        //~^ ERROR: item constrains opaque type that is not in its signature
     }
 }
 

tests/ui/type-alias-impl-trait/hidden_behind_struct_field2.stderr

@@ -0,0 +1,15 @@
+error: item constrains opaque type that is not in its signature
+  --> $DIR/hidden_behind_struct_field2.rs:17:22
+   |
+LL |         Foo { field: () }
+   |                      ^^
+   |
+   = note: this item must mention the opaque type in its signature in order to be able to register hidden types
+note: this item must mention the opaque type in its signature in order to be able to register hidden types
+  --> $DIR/hidden_behind_struct_field2.rs:16:8
+   |
+LL |     fn foo() -> Foo {
+   |        ^^^
+
+error: aborting due to 1 previous error
+