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

Also update all the doc examples.

tokio-fs/src/read.rs

@@ -1,13 +1,8 @@
-use crate::{file, File};
+use crate::File;
 
-use tokio_io::AsyncRead;
+use tokio_io::AsyncReadExt;
 
-use futures_core::ready;
-use std::future::Future;
-use std::pin::Pin;
-use std::task::Context;
-use std::task::Poll;
-use std::{io, mem, path::Path};
+use std::{io, path::Path};
 
 /// Creates a future which will open a file for reading and read the entire
 /// contents into a buffer and return said buffer.
@@ -17,74 +12,24 @@ use std::{io, mem, path::Path};
 /// # Examples
 ///
 /// ```no_run
-/// use tokio::prelude::Future;
+/// #![feature(async_await)]
 ///
-/// let task = tokio::fs::read("foo.txt").map(|data| {
-///     // do something with the contents of the file ...
-///     println!("foo.txt contains {} bytes", data.len());
-/// }).map_err(|e| {
-///     // handle errors
-///     eprintln!("IO error: {:?}", e);
-/// });
+/// use tokio::fs;
 ///
-/// tokio::run(task);
+/// # async fn dox() -> std::io::Result<()> {
+/// let contents = fs::read("foo.txt").await?;
+/// println!("foo.txt contains {} bytes", contents.len());
+/// # Ok(())
+/// # }
 /// ```
-pub fn read<P>(path: P) -> ReadFile<P>
+pub async fn read<P>(path: P) -> io::Result<Vec<u8>>
 where
     P: AsRef<Path> + Send + Unpin + 'static,
 {
-    ReadFile {
-        state: State::Open(File::open(path)),
-    }
-}
+    let mut file = File::open(path).await?;
+    let metadata = file.metadata().await?;
 
-/// A future used to open a file and read its entire contents into a buffer.
-#[derive(Debug)]
-#[must_use = "futures do nothing unless you `.await` or poll them"]
-pub struct ReadFile<P: AsRef<Path> + Send + Unpin + 'static> {
-    state: State<P>,
-}
-
-#[derive(Debug)]
-enum State<P: AsRef<Path> + Send + Unpin + 'static> {
-    Open(file::OpenFuture<P>),
-    Metadata(file::MetadataFuture),
-    Reading(Vec<u8>, usize, File),
-    Empty,
-}
-
-impl<P: AsRef<Path> + Send + Unpin + 'static> Future for ReadFile<P> {
-    type Output = io::Result<Vec<u8>>;
-
-    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let inner = Pin::get_mut(self);
-        match &mut inner.state {
-            State::Open(ref mut open_file) => {
-                let file = ready!(Pin::new(open_file).poll(cx))?;
-                let new_state = State::Metadata(file.metadata());
-                mem::replace(&mut inner.state, new_state);
-                Pin::new(inner).poll(cx)
-            }
-            State::Metadata(read_metadata) => {
-                let (file, metadata) = ready!(Pin::new(read_metadata).poll(cx))?;
-                let buf = Vec::with_capacity(metadata.len() as usize + 1);
-                let new_state = State::Reading(buf, 0, file);
-                mem::replace(&mut inner.state, new_state);
-                Pin::new(inner).poll(cx)
-            }
-            State::Reading(buf, ref mut pos, file) => {
-                let n = ready!(Pin::new(file).poll_read_buf(cx, buf))?;
-                *pos += n;
-                if *pos >= buf.len() {
-                    match mem::replace(&mut inner.state, State::Empty) {
-                        State::Reading(buf, _, _) => Poll::Ready(Ok(buf)),
-                        _ => panic!(),
-                    }
-                } else {
-                    Poll::Pending
-                }
-            }
-            State::Empty => panic!("poll a WriteFile after it's done"),
-        }
-    }
+    let mut contents = Vec::with_capacity(metadata.len() as usize + 1);
+    file.read_to_end(&mut contents).await?;
+    Ok(contents)
 }