Fix ICE from async closure variance

Author: chenyukang

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -698,6 +698,7 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for MakeSuggestableFolder<'tcx> {
             }
 
             Closure(..)
+            | CoroutineClosure(..)
             | FnDef(..)
             | Infer(..)
             | Coroutine(..)

tests/ui/async-await/async-closures/ice-async-closure-variance-issue-148488.rs

@@ -0,0 +1,12 @@
+//@ edition: 2024
+
+struct T<'g>();
+//~^ ERROR lifetime parameter `'g` is never used
+
+fn ord<a>() -> _ {
+    //~^ WARN type parameter `a` should have an upper camel case name
+    //~| ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    async || {}
+}
+
+fn main() {}

tests/ui/async-await/async-closures/ice-async-closure-variance-issue-148488.stderr

@@ -0,0 +1,26 @@
+warning: type parameter `a` should have an upper camel case name
+  --> $DIR/ice-async-closure-variance-issue-148488.rs:6:8
+   |
+LL | fn ord<a>() -> _ {
+   |        ^ help: convert the identifier to upper camel case: `A`
+   |
+   = note: `#[warn(non_camel_case_types)]` (part of `#[warn(nonstandard_style)]`) on by default
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/ice-async-closure-variance-issue-148488.rs:6:16
+   |
+LL | fn ord<a>() -> _ {
+   |                ^ not allowed in type signatures
+
+error[E0392]: lifetime parameter `'g` is never used
+  --> $DIR/ice-async-closure-variance-issue-148488.rs:3:10
+   |
+LL | struct T<'g>();
+   |          ^^ unused lifetime parameter
+   |
+   = help: consider removing `'g`, referring to it in a field, or using a marker such as `PhantomData`
+
+error: aborting due to 2 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0121, E0392.
+For more information about an error, try `rustc --explain E0121`.