Never say "Trait is implemented for {type error}"

When a trait bound error occurs, we look for alternative types that
would have made the bound succeed. For some reason `{type error}`
sometimes would appear as a type that would do so.

We now remove `{type error}` from the list in every case to avoid
nonsensical `note`s.

Author: estebank

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -1908,6 +1908,9 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             ct_op: |ct| ct.normalize(self.tcx, ty::ParamEnv::empty()),
                         },
                     );
+                    if cand.references_error() {
+                        return false;
+                    }
                     err.highlighted_help(vec![
                         StringPart::normal(format!("the trait `{}` ", cand.print_trait_sugared())),
                         StringPart::highlighted("is"),
@@ -1932,7 +1935,8 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
 
         let other = if other { "other " } else { "" };
-        let report = |candidates: Vec<TraitRef<'tcx>>, err: &mut Diag<'_>| {
+        let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diag<'_>| {
+            candidates.retain(|tr| !tr.references_error());
             if candidates.is_empty() {
                 return false;
             }

tests/ui/associated-consts/issue-105330.stderr

@@ -55,7 +55,6 @@ error[E0277]: the trait bound `Demo: TraitWAssocConst` is not satisfied
 LL |     foo::<Demo>()();
    |           ^^^^ the trait `TraitWAssocConst` is not implemented for `Demo`
    |
-   = help: the trait `TraitWAssocConst` is implemented for `{type error}`
 note: required by a bound in `foo`
   --> $DIR/issue-105330.rs:11:11
    |
@@ -92,7 +91,6 @@ error[E0277]: the trait bound `Demo: TraitWAssocConst` is not satisfied
 LL |     foo::<Demo>();
    |           ^^^^ the trait `TraitWAssocConst` is not implemented for `Demo`
    |
-   = help: the trait `TraitWAssocConst` is implemented for `{type error}`
 note: required by a bound in `foo`
   --> $DIR/issue-105330.rs:11:11
    |