Remove some noise from opaque type errors around associated types

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -551,6 +551,20 @@ impl<'a, 'tcx> Instantiator<'a, 'tcx> {
             let predicate = predicate.subst(tcx, substs);
             debug!(?predicate);
 
+            // Replace all other mentions of the same opaque type with the hidden type,
+            // as the bounds must hold on the hidden type after all.
+            let predicate = predicate.fold_with(&mut BottomUpFolder {
+                tcx,
+                ty_op: |ty| match *ty.kind() {
+                    ty::Opaque(def_id2, substs2) if def_id == def_id2 && substs == substs2 => {
+                        ty_var
+                    }
+                    _ => ty,
+                },
+                lt_op: |lt| lt,
+                ct_op: |ct| ct,
+            });
+
             // We can't normalize associated types from `rustc_infer`, but we can eagerly register inference variables for them.
             let predicate = predicate.fold_with(&mut BottomUpFolder {
                 tcx,

src/test/ui/type-alias-impl-trait/bound_reduction2.rs

@@ -15,9 +15,5 @@ impl<W> Trait<W> for () {}
 
 fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {
     //~^ ERROR non-defining opaque type use in defining scope
-    //~| ERROR non-defining opaque type use in defining scope
-    //~| ERROR non-defining opaque type use in defining scope
-    //~| ERROR `T` is part of concrete type but not used in parameter list
-    //~| ERROR `T` is part of concrete type but not used in parameter list
     ()
 }

src/test/ui/type-alias-impl-trait/bound_reduction2.stderr

@@ -1,34 +1,8 @@
-error: type parameter `T` is part of concrete type but not used in parameter list for the `impl Trait` type alias
-  --> $DIR/bound_reduction2.rs:16:60
-   |
-LL |   fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {
-   |  ____________________________________________________________^
-LL | |
-LL | |
-LL | |
-...  |
-LL | |     ()
-LL | | }
-   | |_^
-
-error: type parameter `T` is part of concrete type but not used in parameter list for the `impl Trait` type alias
-  --> $DIR/bound_reduction2.rs:16:60
-   |
-LL |   fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {
-   |  ____________________________________________________________^
-LL | |
-LL | |
-LL | |
-...  |
-LL | |     ()
-LL | | }
-   | |_^
-
 error: non-defining opaque type use in defining scope
-  --> $DIR/bound_reduction2.rs:16:1
+  --> $DIR/bound_reduction2.rs:16:46
    |
 LL | fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                                              ^^^^^^^^^^^^^
    |
 note: used non-generic type `<T as TraitWithAssoc>::Assoc` for generic parameter
   --> $DIR/bound_reduction2.rs:9:10
@@ -36,35 +10,11 @@ note: used non-generic type `<T as TraitWithAssoc>::Assoc` for generic parameter
 LL | type Foo<V> = impl Trait<V>;
    |          ^
 
-error: non-defining opaque type use in defining scope
-  --> $DIR/bound_reduction2.rs:16:1
-   |
-LL | fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: used non-generic type `_` for generic parameter
-  --> $DIR/bound_reduction2.rs:9:10
-   |
-LL | type Foo<V> = impl Trait<V>;
-   |          ^
-
-error: non-defining opaque type use in defining scope
-  --> $DIR/bound_reduction2.rs:16:1
-   |
-LL | fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: used non-generic type `_` for generic parameter
-  --> $DIR/bound_reduction2.rs:9:10
-   |
-LL | type Foo<V> = impl Trait<V>;
-   |          ^
-
 error: could not find defining uses
   --> $DIR/bound_reduction2.rs:9:15
    |
 LL | type Foo<V> = impl Trait<V>;
    |               ^^^^^^^^^^^^^
 
-error: aborting due to 6 previous errors
+error: aborting due to 2 previous errors