Don't ICE in has_self_borrows when coroutine captures-by-ref ty is still inferred

`coroutine_captures_by_ref_ty` is a fresh `next_ty_var` until upvar
inference unifies it to an `FnPtr`. If `has_self_borrows` is reached
before that — e.g. when the body contains an unresolved `as _` cast —
the `_ => panic!()` arm fires.

Treat the `Infer` case like `Error`: report self-borrows defensively
so the closure falls back to `FnOnce`-only and the caller surfaces a
normal type-inference error instead of an ICE.

Closes #155999.

Author: swjng

compiler/rustc_type_ir/src/ty_kind/closure.rs

@@ -322,7 +322,9 @@ impl<I: Interner> CoroutineClosureArgs<I> {
                 .skip_binder()
                 .visit_with(&mut HasRegionsBoundAt { binder: ty::INNERMOST })
                 .is_break(),
-            ty::Error(_) => true,
+            // The captures-by-ref ty is a fresh inference variable until
+            // upvar inference unifies it; treat that case like `Error`.
+            ty::Error(_) | ty::Infer(_) => true,
             _ => panic!(),
         }
     }

tests/ui/async-await/async-closures/cast-as-infer-self-borrows.rs

@@ -0,0 +1,10 @@
+//@ edition:2021
+
+// Regression test for #155999.
+
+fn needs_fn_mut<T>(x: impl FnMut() -> T) {
+    needs_fn_mut(async || x as _)
+    //~^ ERROR type annotations needed
+}
+
+fn main() {}

tests/ui/async-await/async-closures/cast-as-infer-self-borrows.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/cast-as-infer-self-borrows.rs:6:32
+   |
+LL |     needs_fn_mut(async || x as _)
+   |                                ^ cannot infer type
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.