io: add missing utility functions (#1632)

The standard library's `io` module has small utilities such as `repeat`,
`empty`, and `sink`, which return `Read` and `Write` implementations.
These can come in handy in some circiumstances. `tokio::io` has no
equivalents that implement `AsyncRead`/`AsyncWrite`.

This commit adds `repeat`, `empty`, and `sink` helpers to `tokio::io`.

tokio-io/src/io/repeat.rs

@@ -0,0 +1,67 @@
+use crate::AsyncRead;
+
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// An async reader which yields one byte over and over and over and over and
+/// over and...
+///
+/// This struct is generally created by calling [`repeat`][repeat]. Please
+/// see the documentation of `repeat()` for more details.
+///
+/// This is an asynchronous version of [`std::io::Repeat`][std].
+///
+/// [repeat]: fn.repeat.html
+/// [std]: https://doc.rust-lang.org/std/io/struct.Repeat.html
+#[derive(Debug)]
+pub struct Repeat {
+    byte: u8,
+}
+
+/// Creates an instance of an async reader that infinitely repeats one byte.
+///
+/// All reads from this reader will succeed by filling the specified buffer with
+/// the given byte.
+///
+/// This is an asynchronous version of [`std::io::repeat`][std].
+///
+/// # Examples
+///
+/// ```
+/// # use tokio_io::{self as io, AsyncReadExt};
+/// # async fn dox() {
+/// let mut buffer = [0; 3];
+/// io::repeat(0b101).read_exact(&mut buffer).await.unwrap();
+/// assert_eq!(buffer, [0b101, 0b101, 0b101]);
+/// # }
+/// ```
+///
+/// [std]: https://doc.rust-lang.org/std/io/fn.repeat.html
+pub fn repeat(byte: u8) -> Repeat {
+    Repeat { byte }
+}
+
+impl AsyncRead for Repeat {
+    #[inline]
+    fn poll_read(
+        self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        for byte in &mut *buf {
+            *byte = self.byte;
+        }
+        Poll::Ready(Ok(buf.len()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Repeat>();
+    }
+}