Fix issue 78654.

compiler/rustc_typeck/src/check/closure.rs

@@ -605,6 +605,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
         let ret_vid = match *ret_ty.kind() {
             ty::Infer(ty::TyVar(ret_vid)) => ret_vid,
+            ty::Error(_) => return None,
             _ => span_bug!(
                 self.tcx.def_span(expr_def_id),
                 "async fn generator return type not an inference variable"

src/test/ui/async-await/issues/issue-78654.full.stderr

@@ -0,0 +1,19 @@
+error[E0573]: expected type, found built-in attribute `feature`
+  --> $DIR/issue-78654.rs:10:15
+   |
+LL | impl<const H: feature> Foo {
+   |               ^^^^^^^ not a type
+
+error[E0207]: the const parameter `H` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/issue-78654.rs:10:12
+   |
+LL | impl<const H: feature> Foo {
+   |            ^ unconstrained const parameter
+   |
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0207, E0573.
+For more information about an error, try `rustc --explain E0207`.

src/test/ui/async-await/issues/issue-78654.min.stderr

@@ -0,0 +1,19 @@
+error[E0573]: expected type, found built-in attribute `feature`
+  --> $DIR/issue-78654.rs:10:15
+   |
+LL | impl<const H: feature> Foo {
+   |               ^^^^^^^ not a type
+
+error[E0207]: the const parameter `H` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/issue-78654.rs:10:12
+   |
+LL | impl<const H: feature> Foo {
+   |            ^ unconstrained const parameter
+   |
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0207, E0573.
+For more information about an error, try `rustc --explain E0207`.

src/test/ui/async-await/issues/issue-78654.rs

@@ -0,0 +1,16 @@
+// edition:2018
+// revisions: full min
+
+#![cfg_attr(full, feature(const_generics))]
+#![cfg_attr(full, allow(incomplete_features))]
+#![cfg_attr(min, feature(min_const_generics))]
+
+struct Foo;
+
+impl<const H: feature> Foo {
+//~^ ERROR: expected type, found built-in attribute `feature`
+//~^^ ERROR: the const parameter `H` is not constrained by the impl trait, self type, or predicates
+    async fn biz() {}
+}
+
+fn main() {}