Rollup merge of rust-lang#122449 - compiler-errors:stranded-opaque, r=oli-obk

Delay a bug for stranded opaques r? oli-obk Fixes rust-lang#122445

Author: matthiaskrgr

compiler/rustc_hir_analysis/src/check/check.rs

@@ -381,11 +381,17 @@ fn check_opaque_meets_bounds<'tcx>(
  match ocx.eq(&misc_cause, param_env, opaque_ty, hidden_ty) {
  Ok(()) => {}
  Err(ty_err) => {
+ // Some types may be left "stranded" if they can't be reached
+ // from an astconv'd bound but they're mentioned in the HIR. This
+ // will happen, e.g., when a nested opaque is inside of a non-
+ // existent associated type, like `impl Trait<Missing = impl Trait>`.
+ // See <tests/ui/impl-trait/stranded-opaque.rs>.
  let ty_err = ty_err.to_string(tcx);
- tcx.dcx().span_bug(
+ let guar = tcx.dcx().span_delayed_bug(
  span,
  format!("could not unify `{hidden_ty}` with revealed type:\n{ty_err}"),
  );
+ return Err(guar);
  }
  }
 

tests/ui/impl-trait/stranded-opaque.rs

@@ -0,0 +1,13 @@
+trait Trait {}
+
+impl Trait for i32 {}
+
+// Since `Assoc` doesn't actually exist, it's "stranded", and won't show up in
+// the list of opaques that may be defined by the function. Make sure we don't
+// ICE in this case.
+fn produce<T>() -> impl Trait<Assoc = impl Trait> {
+ //~^ ERROR associated type `Assoc` not found for `Trait`
+ 16
+}
+
+fn main () {}

tests/ui/impl-trait/stranded-opaque.stderr

@@ -0,0 +1,9 @@
+error[E0220]: associated type `Assoc` not found for `Trait`
+ --> $DIR/stranded-opaque.rs:8:31
+ |
+LL | fn produce<T>() -> impl Trait<Assoc = impl Trait> {
+ | ^^^^^ associated type `Assoc` not found
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0220`.