Don't cancel stashed OpaqueHiddenTypeMismatch errors.

This gives one extra error message on one test, but is necessary to fix
bigger problems caused by the cancellation of stashed errors.

(Note: why not just avoid stashing altogether? Because that resulted in
additional output changes.)

compiler/rustc_middle/src/ty/mod.rs

@@ -846,12 +846,17 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
  opaque_def_id: LocalDefId,
  tcx: TyCtxt<'tcx>,
  ) -> Result<DiagnosticBuilder<'tcx>, ErrorGuaranteed> {
+
  if let Some(diag) = tcx
  .sess
  .dcx()
  .steal_diagnostic(tcx.def_span(opaque_def_id), StashKey::OpaqueHiddenTypeMismatch)
  {
- diag.cancel();
+ // We used to cancel here for slightly better error messages, but
+ // cancelling stashed diagnostics is no longer allowed because it
+ // causes problems when tracking whether errors have actually
+ // occurred.
+ diag.emit();
  }
  (self.ty, other.ty).error_reported()?;
  // Found different concrete types for the opaque type.

tests/ui/type-alias-impl-trait/different_defining_uses_never_type-2.rs

@@ -11,6 +11,7 @@ fn foo<'a, 'b>() -> Tait<'a> {
  }
  let x: Tait<'a> = ();
  x
+ //~^ ERROR concrete type differs from previous defining opaque type use
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/different_defining_uses_never_type-2.stderr

@@ -1,3 +1,15 @@
+error: concrete type differs from previous defining opaque type use
+ --> $DIR/different_defining_uses_never_type-2.rs:13:5
+ |
+LL | x
+ | ^ expected `i32`, got `()`
+ |
+note: previous use here
+ --> $DIR/different_defining_uses_never_type-2.rs:8:31
+ |
+LL | let y: Tait<'b> = 1i32;
+ | ^^^^
+
 error: concrete type differs from previous defining opaque type use
  --> $DIR/different_defining_uses_never_type-2.rs:8:31
  |
@@ -10,5 +22,5 @@ note: previous use here
 LL | if { return } {
  | ^^^^^^
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors