fs: use async fn instead of custom futures (#1381)

Also update all the doc examples.

tokio-fs/src/hard_link.rs

@@ -1,10 +1,7 @@
-use std::fs;
-use std::future::Future;
+use crate::asyncify;
+
 use std::io;
 use std::path::Path;
-use std::pin::Pin;
-use std::task::Context;
-use std::task::Poll;
 
 /// Creates a new hard link on the filesystem.
 ///
@@ -14,40 +11,6 @@ use std::task::Poll;
 /// This is an async version of [`std::fs::hard_link`][std]
 ///
 /// [std]: https://doc.rust-lang.org/std/fs/fn.hard_link.html
-pub fn hard_link<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> HardLinkFuture<P, Q> {
-    HardLinkFuture::new(src, dst)
-}
-
-/// Future returned by `hard_link`.
-#[derive(Debug)]
-#[must_use = "futures do nothing unless you `.await` or poll them"]
-pub struct HardLinkFuture<P, Q>
-where
-    P: AsRef<Path>,
-    Q: AsRef<Path>,
-{
-    src: P,
-    dst: Q,
-}
-
-impl<P, Q> HardLinkFuture<P, Q>
-where
-    P: AsRef<Path>,
-    Q: AsRef<Path>,
-{
-    fn new(src: P, dst: Q) -> HardLinkFuture<P, Q> {
-        HardLinkFuture { src, dst }
-    }
-}
-
-impl<P, Q> Future for HardLinkFuture<P, Q>
-where
-    P: AsRef<Path>,
-    Q: AsRef<Path>,
-{
-    type Output = io::Result<()>;
-
-    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
-        crate::blocking_io(|| fs::hard_link(&self.src, &self.dst))
-    }
+pub async fn hard_link<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<()> {
+    asyncify(|| std::fs::hard_link(&src, &dst)).await
 }