Also support fnptr(): async Fn in codegen

tests/ui/async-await/async-fn/higher-ranked-async-fn.rs

@@ -0,0 +1,31 @@
+//@ aux-build:block-on.rs
+//@ edition:2018
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ build-pass (since it ICEs during mono)
+
+#![feature(async_closure)]
+
+extern crate block_on;
+
+use std::future::Future;
+
+async fn f(arg: &i32) {}
+
+async fn func<F>(f: F)
+where
+    F: async for<'a> Fn(&'a i32),
+{
+    let x: i32 = 0;
+    f(&x).await;
+}
+
+fn main() {
+    block_on::block_on(async {
+        // Function
+        func(f).await;
+
+        // Regular closure (doesn't capture)
+        func(|x: &i32| async {});
+    });
+}