Fix drop shim for AsyncFnOnce closure, AsyncFnMut shim for AsyncFn closure

tests/ui/async-await/async-closures/async-fn-mut-for-async-fn.rs

@@ -0,0 +1,23 @@
+// aux-build:block-on.rs
+// edition:2021
+// run-pass
+
+// FIXME(async_closures): When `fn_sig_for_fn_abi` is fixed, remove this.
+// ignore-pass (test emits codegen-time warnings)
+
+#![feature(async_closure, async_fn_traits)]
+
+extern crate block_on;
+
+use std::ops::AsyncFnMut;
+
+fn main() {
+    block_on::block_on(async {
+        let x = async || {};
+
+        async fn needs_async_fn_mut(mut x: impl AsyncFnMut()) {
+            x().await;
+        }
+        needs_async_fn_mut(x).await;
+    });
+}