fs: update to use std::future (#1269)

tokio-fs/src/stdin.rs

@@ -1,4 +1,7 @@
 use std::io::{self, Read, Stdin as StdStdin};
+use std::pin::Pin;
+use std::task::Context;
+use std::task::Poll;
 use tokio_io::AsyncRead;
 
 /// A handle to the standard input stream of a process.
@@ -37,7 +40,14 @@ impl Read for Stdin {
 }
 
 impl AsyncRead for Stdin {
-    unsafe fn prepare_uninitialized_buffer(&self, _: &mut [u8]) -> bool {
-        false
+    fn poll_read(
+        self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        match Pin::get_mut(self).read(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => Poll::Pending,
+            other => Poll::Ready(other),
+        }
     }
 }