Fix async closures in CTFE

Author: compiler-errors

tests/pass/async-closure.rs

@@ -0,0 +1,40 @@
+#![feature(async_closure, noop_waker, async_fn_traits)]
+
+use std::future::Future;
+use std::pin::pin;
+use std::task::*;
+
+pub fn block_on<T>(fut: impl Future<Output = T>) -> T {
+    let mut fut = pin!(fut);
+    let ctx = &mut Context::from_waker(Waker::noop());
+
+    loop {
+        match fut.as_mut().poll(ctx) {
+            Poll::Pending => {}
+            Poll::Ready(t) => break t,
+        }
+    }
+}
+
+async fn call_once(f: impl async FnOnce(DropMe)) {
+    f(DropMe("world")).await;
+}
+
+#[derive(Debug)]
+struct DropMe(&'static str);
+
+impl Drop for DropMe {
+    fn drop(&mut self) {
+        println!("{}", self.0);
+    }
+}
+
+pub fn main() {
+    block_on(async {
+        let b = DropMe("hello");
+        let async_closure = async move |a: DropMe| {
+            println!("{a:?} {b:?}");
+        };
+        call_once(async_closure).await;
+    });
+}

tests/pass/async-closure.stdout

@@ -0,0 +1,3 @@
+DropMe("world") DropMe("hello")
+world
+hello