Handle weak type aliases by immediately resolving them to their aliased type

Author: oli-obk

compiler/rustc_ty_utils/src/opaque_types.rs

@@ -107,6 +107,12 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
                     }
                 }
             }
+            ty::Alias(ty::Weak, alias_ty) if alias_ty.def_id.is_local() => {
+                self.tcx
+                    .type_of(alias_ty.def_id)
+                    .subst(self.tcx, alias_ty.substs)
+                    .visit_with(self)?;
+            }
             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
@@ -136,24 +142,23 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
                                 ty::InternalSubsts::identity_for_item(self.tcx, parent),
                             );
 
-                            if !check_substs_compatible(self.tcx, assoc, impl_substs) {
+                            if check_substs_compatible(self.tcx, assoc, impl_substs) {
+                                return self
+                                    .tcx
+                                    .type_of(assoc.def_id)
+                                    .subst(self.tcx, impl_substs)
+                                    .visit_with(self);
+                            } else {
                                 self.tcx.sess.delay_span_bug(
                                     self.tcx.def_span(assoc.def_id),
                                     "item had incorrect substs",
                                 );
-                                return ControlFlow::Continue(());
                             }
-
-                            return self
-                                .tcx
-                                .type_of(assoc.def_id)
-                                .subst(self.tcx, impl_substs)
-                                .visit_with(self);
                         }
                     }
                 }
             }
-            _ => {}
+            _ => trace!(kind=?t.kind()),
         }
         ControlFlow::Continue(())
     }

tests/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr

@@ -20,11 +20,6 @@ LL |         fn eq(&self, _other: &(Foo, i32)) -> bool {
    |
    = note: expected signature `fn(&a::Bar, &(a::Bar, i32)) -> _`
               found signature `fn(&a::Bar, &(a::Foo, i32)) -> _`
-note: this item must have the opaque type in its signature in order to be able to register hidden types
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:10:12
-   |
-LL |         fn eq(&self, _other: &(Foo, i32)) -> bool {
-   |            ^^
 
 error: unconstrained opaque type
   --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:18:16

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

@@ -25,11 +25,6 @@ LL |         fn dont_define_this(_private: Private) {}
    |                                       ^^^^^^^
    = note: expected signature `fn(Private)`
               found signature `fn(MyPrivate)`
-note: this item must have the opaque type in its signature in order to be able to register hidden types
-  --> $DIR/unnameable_type.rs:20:8
-   |
-LL |     fn dont_define_this(_private: MyPrivate) {}
-   |        ^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors