util: add futures-io/tokio::io compatibility layer (#2117)

* util: add futures-io/tokio::io compatibility layer

This PR adds a compatibility layer with conversions between the
`tokio::io` and `futures-io` versions of the `AsyncRead` and
`AsyncWrite` traits.

I initially opened this PR against `tokio-compat`, but we decided that
a compatibility layer for current versions of the `tokio` and
`futures-io` crates (rather than for compatibility with legacy code)
ought to go in `tokio-util` instead. See:
https://github.com/tokio-rs/tokio-compat/pull/2#issuecomment-551310953

This is based on code originally written by @Nemo157 as part of the
`futures-tokio-compat` crate, and is contributed on behalf of the
original author:
https://github.com/Nemo157/futures-tokio-compat/issues/2#issuecomment-544118866

Closes tokio-rs/tokio-compat#2

Co-authored-by: Wim Looman <wim@nemo157.com>
Signed-off-by: Eliza Weisman <eliza@buoyant.io>

3 files changed, 215 insertions, 0 deletions

diff --git a/tokio-util/src/cfg.rs b/tokio-util/src/cfg.rs
index 13fabd36..27e8c66a 100644
--- a/tokio-util/src/cfg.rs
+++ b/tokio-util/src/cfg.rs
@@ -8,6 +8,16 @@ macro_rules! cfg_codec {
     }
 }
 
+macro_rules! cfg_compat {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "compat")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "compat")))]
+            $item
+        )*
+    }
+}
+
 macro_rules! cfg_udp {
     ($($item:item)*) => {
         $(
diff --git a/tokio-util/src/compat.rs b/tokio-util/src/compat.rs
new file mode 100644
index 00000000..769e30c2
--- /dev/null
+++ b/tokio-util/src/compat.rs
@@ -0,0 +1,201 @@
+//! Compatibility between the `tokio::io` and `futures-io` versions of the
+//! `AsyncRead` and `AsyncWrite` traits.
+use pin_project_lite::pin_project;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// A compatibility layer that allows conversion between the
+    /// `tokio::io` and `futures-io` `AsyncRead` and `AsyncWrite` traits.
+    #[derive(Copy, Clone, Debug)]
+    pub struct Compat<T> {
+        #[pin]
+        inner: T,
+    }
+}
+
+/// Extension trait that allows converting a type implementing
+/// `futures_io::AsyncRead` to implement `tokio::io::AsyncRead`.
+pub trait FuturesAsyncReadCompatExt: futures_io::AsyncRead {
+    /// Wraps `self` with a compatibility layer that implements
+    /// `tokio_io::AsyncWrite`.
+    fn compat(self) -> Compat<Self>
+    where
+        Self: Sized,
+    {
+        Compat::new(self)
+    }
+}
+
+impl<T: futures_io::AsyncRead> FuturesAsyncReadCompatExt for T {}
+
+/// Extension trait that allows converting a type implementing
+/// `futures_io::AsyncWrite` to implement `tokio::io::AsyncWrite`.
+pub trait FuturesAsyncWriteCompatExt: futures_io::AsyncWrite {
+    /// Wraps `self` with a compatibility layer that implements
+    /// `tokio::io::AsyncWrite`.
+    fn compat_write(self) -> Compat<Self>
+    where
+        Self: Sized,
+    {
+        Compat::new(self)
+    }
+}
+
+impl<T: futures_io::AsyncWrite> FuturesAsyncWriteCompatExt for T {}
+
+/// Extension trait that allows converting a type implementing
+/// `tokio::io::AsyncRead` to implement `futures_io::AsyncRead`.
+pub trait Tokio02AsyncReadCompatExt: tokio::io::AsyncRead {
+    /// Wraps `self` with a compatibility layer that implements
+    /// `futures_io::AsyncRead`.
+    fn compat(self) -> Compat<Self>
+    where
+        Self: Sized,
+    {
+        Compat::new(self)
+    }
+}
+
+impl<T: tokio::io::AsyncRead> Tokio02AsyncReadCompatExt for T {}
+
+/// Extension trait that allows converting a type implementing
+/// `tokio::io::AsyncWrite` to implement `futures_io::AsyncWrite`.
+pub trait Tokio02AsyncWriteCompatExt: tokio::io::AsyncWrite {
+    /// Wraps `self` with a compatibility layer that implements
+    /// `futures_io::AsyncWrite`.
+    fn compat_write(self) -> Compat<Self>
+    where
+        Self: Sized,
+    {
+        Compat::new(self)
+    }
+}
+
+impl<T: tokio::io::AsyncWrite> Tokio02AsyncWriteCompatExt for T {}
+
+// === impl Compat ===
+
+impl<T> Compat<T> {
+    fn new(inner: T) -> Self {
+        Self { inner }
+    }
+
+    /// Get a reference to the `Future`, `Stream`, `AsyncRead`, or `AsyncWrite` object
+    /// contained within.
+    pub fn get_ref(&self) -> &T {
+        &self.inner
+    }
+
+    /// Get a mutable reference to the `Future`, `Stream`, `AsyncRead`, or `AsyncWrite` object
+    /// contained within.
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.inner
+    }
+
+    /// Returns the wrapped item.
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
+impl<T> tokio::io::AsyncRead for Compat<T>
+where
+    T: futures_io::AsyncRead,
+{
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        futures_io::AsyncRead::poll_read(self.project().inner, cx, buf)
+    }
+}
+
+impl<T> futures_io::AsyncRead for Compat<T>
+where
+    T: tokio::io::AsyncRead,
+{
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        tokio::io::AsyncRead::poll_read(self.project().inner, cx, buf)
+    }
+}
+
+impl<T> tokio::io::AsyncBufRead for Compat<T>
+where
+    T: futures_io::AsyncBufRead,
+{
+    fn poll_fill_buf<'a>(
+        self: Pin<&'a mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<&'a [u8]>> {
+        futures_io::AsyncBufRead::poll_fill_buf(self.project().inner, cx)
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        futures_io::AsyncBufRead::consume(self.project().inner, amt)
+    }
+}
+
+impl<T> futures_io::AsyncBufRead for Compat<T>
+where
+    T: tokio::io::AsyncBufRead,
+{
+    fn poll_fill_buf<'a>(
+        self: Pin<&'a mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<&'a [u8]>> {
+        tokio::io::AsyncBufRead::poll_fill_buf(self.project().inner, cx)
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        tokio::io::AsyncBufRead::consume(self.project().inner, amt)
+    }
+}
+
+impl<T> tokio::io::AsyncWrite for Compat<T>
+where
+    T: futures_io::AsyncWrite,
+{
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        futures_io::AsyncWrite::poll_write(self.project().inner, cx, buf)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        futures_io::AsyncWrite::poll_flush(self.project().inner, cx)
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        futures_io::AsyncWrite::poll_close(self.project().inner, cx)
+    }
+}
+
+impl<T> futures_io::AsyncWrite for Compat<T>
+where
+    T: tokio::io::AsyncWrite,
+{
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        tokio::io::AsyncWrite::poll_write(self.project().inner, cx, buf)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        tokio::io::AsyncWrite::poll_flush(self.project().inner, cx)
+    }
+
+    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        tokio::io::AsyncWrite::poll_shutdown(self.project().inner, cx)
+    }
+}
diff --git a/tokio-util/src/lib.rs b/tokio-util/src/lib.rs
index 48c0fd16..4554516e 100644
--- a/tokio-util/src/lib.rs
+++ b/tokio-util/src/lib.rs
@@ -25,3 +25,7 @@ cfg_codec! {
 cfg_udp! {
     pub mod udp;
 }
+
+cfg_compat! {
+    pub mod compat;
+}