async-await: track nightly changes (#661)

The `tokio-async-await` crate is no longer a facade. Instead, the `tokio` crate
provides a feature flag to enable async/await support.

tokio-async-await/src/io/write.rs

@@ -0,0 +1,38 @@
+use tokio_io::AsyncWrite;
+
+use std::future::Future;
+use std::task::{self, Poll};
+
+use std::io;
+use std::marker::Unpin;
+use std::pin::Pin;
+
+/// A future used to write data.
+#[derive(Debug)]
+pub struct Write<'a, T: 'a + ?Sized> {
+    writer: &'a mut T,
+    buf: &'a [u8],
+}
+
+// Pinning is never projected to fields
+impl<'a, T: ?Sized> Unpin for Write<'a, T> {}
+
+impl<'a, T: AsyncWrite + ?Sized> Write<'a, T> {
+    pub(super) fn new(writer: &'a mut T, buf: &'a [u8]) -> Write<'a, T> {
+        Write {
+            writer,
+            buf,
+        }
+    }
+}
+
+impl<'a, T: AsyncWrite + ?Sized> Future for Write<'a, T> {
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, _lw: &task::LocalWaker) -> Poll<io::Result<usize>> {
+        use crate::compat::forward::convert_poll;
+
+        let this = &mut *self;
+        convert_poll(this.writer.poll_write(this.buf))
+    }
+}