Auto merge of #71108 - estebank:suggest-proj-type-mismatch-constraint, r=oli-obk

On type mismatch involving associated type, suggest constraint

When an associated type is found when a specific type was expected, if
possible provide a structured suggestion constraining the associated
type in a bound.

```
error[E0271]: type mismatch resolving `<T as Foo>::Y == i32`
  --> $DIR/associated-types-multiple-types-one-trait.rs:13:5
   |
LL |     want_y(t);
   |     ^^^^^^ expected `i32`, found associated type
...
LL | fn want_y<T:Foo<Y=i32>>(t: &T) { }
   |                 ----- required by this bound in `want_y`
   |
   = note:         expected type `i32`
           found associated type `<T as Foo>::Y`
help: consider constraining the associated type `<T as Foo>::Y` to `i32`
   |
LL | fn have_x_want_y<T:Foo<X=u32, Y = i32>>(t: &T)
   |                             ^^^^^^^^^
```

```
error[E0308]: mismatched types
  --> $DIR/trait-with-missing-associated-type-restriction.rs:12:9
   |
LL |     qux(x.func())
   |         ^^^^^^^^ expected `usize`, found associated type
   |
   = note:         expected type `usize`
           found associated type `<impl Trait as Trait>::A`
help: consider constraining the associated type `<impl Trait as Trait>::A` to `usize`
   |
LL | fn foo(x: impl Trait<A = usize>) {
   |                     ^^^^^^^^^^
```

Fix #71035. Related to #70908.

Author: bors

src/librustc_infer/infer/error_reporting/mod.rs

@@ -1388,6 +1388,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         terr: &TypeError<'tcx>,
     ) {
         let span = cause.span(self.tcx);
+        debug!("note_type_err cause={:?} values={:?}, terr={:?}", cause, values, terr);
 
         // For some types of errors, expected-found does not make
         // sense, so just ignore the values we were given.
@@ -1599,11 +1600,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 self.tcx.hir().body_owner_def_id(hir::BodyId { hir_id: cause.body_id })
             });
         self.check_and_note_conflicting_crates(diag, terr);
-        self.tcx.note_and_explain_type_err(diag, terr, span, body_owner_def_id.to_def_id());
+        self.tcx.note_and_explain_type_err(diag, terr, cause, span, body_owner_def_id.to_def_id());
 
         // It reads better to have the error origin as the final
         // thing.
-        self.note_error_origin(diag, &cause, exp_found);
+        self.note_error_origin(diag, cause, exp_found);
     }
 
     /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,

src/librustc_middle/traits/mod.rs

@@ -193,6 +193,9 @@ pub enum ObligationCauseCode<'tcx> {
 
     DerivedObligation(DerivedObligationCause<'tcx>),
 
+    /// Error derived when matching traits/impls; see ObligationCause for more details
+    CompareImplConstObligation,
+
     /// Error derived when matching traits/impls; see ObligationCause for more details
     CompareImplMethodObligation {
         item_name: ast::Name,

src/librustc_middle/traits/structural_impls.rs

@@ -164,6 +164,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
                 tcx.lift(cause).map(super::ImplDerivedObligation)
             }
             super::DerivedObligation(ref cause) => tcx.lift(cause).map(super::DerivedObligation),
+            super::CompareImplConstObligation => Some(super::CompareImplConstObligation),
             super::CompareImplMethodObligation {
                 item_name,
                 impl_item_def_id,