async-await: fix build for latest nightly (#940)

Fixes: #936

src/async_await.rs

@@ -1,8 +1,28 @@
 use std::future::Future as StdFuture;
+use std::pin::Pin;
+use std::task::{Poll, Waker};
 
-async fn map_ok<T: StdFuture>(future: T) -> Result<(), ()> {
-    let _ = await!(future);
-    Ok(())
+fn map_ok<T: StdFuture>(future: T) -> impl StdFuture<Output = Result<(), ()>> {
+    MapOk(future)
+}
+
+struct MapOk<T>(T);
+
+impl<T> MapOk<T> {
+    fn future<'a>(self: Pin<&'a mut Self>) -> Pin<&'a mut T> {
+        unsafe { Pin::map_unchecked_mut(self, |x| &mut x.0) }
+    }
+}
+
+impl<T: StdFuture> StdFuture for MapOk<T> {
+    type Output = Result<(), ()>;
+
+    fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output> {
+        match self.future().poll(waker) {
+            Poll::Ready(_) => Poll::Ready(Ok(())),
+            Poll::Pending => Poll::Pending,
+        }
+    }
 }
 
 /// Like `tokio::run`, but takes an `async` block