Support AsyncFnX traits

Only in calls, because to support them in bounds we need support from Chalk. However we don't yet report error from bounds anyway, so this is less severe.

The returned future is shown in its name within inlay hints instead of as a nicer `impl Future`, but that can wait for another PR.

crates/ide-diagnostics/src/handlers/expected_function.rs

@@ -37,4 +37,25 @@ fn foo() {
 "#,
         );
     }
+
+    #[test]
+    fn no_error_for_async_fn_traits() {
+        check_diagnostics(
+            r#"
+//- minicore: async_fn
+async fn f(it: impl AsyncFn(u32) -> i32) {
+    let fut = it(0);
+    let _: i32 = fut.await;
+}
+async fn g(mut it: impl AsyncFnMut(u32) -> i32) {
+    let fut = it(0);
+    let _: i32 = fut.await;
+}
+async fn h(it: impl AsyncFnOnce(u32) -> i32) {
+    let fut = it(0);
+    let _: i32 = fut.await;
+}
+        "#,
+        );
+    }
 }