util: implement Either type (#2821)

tokio-util/src/either.rs

@@ -0,0 +1,194 @@
+//! Module defining an Either type.
+use std::{
+    future::Future,
+    io::SeekFrom,
+    pin::Pin,
+    task::{Context, Poll},
+};
+use tokio::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, ReadBuf, Result};
+
+/// Combines two different futures, streams, or sinks having the same associated types into a single type.
+///
+/// This type implements common asynchronous traits such as [`Future`] and those in Tokio.
+///
+/// [`Future`]: std::future::Future
+///
+/// # Example
+///
+/// The following code will not work:
+///
+/// ```compile_fail
+/// # fn some_condition() -> bool { true }
+/// # async fn some_async_function() -> u32 { 10 }
+/// # async fn other_async_function() -> u32 { 20 }
+/// #[tokio::main]
+/// async fn main() {
+///     let result = if some_condition() {
+///         some_async_function()
+///     } else {
+///         other_async_function() // <- Will print: "`if` and `else` have incompatible types"
+///     };
+///
+///     println!("Result is {}", result.await);
+/// }
+/// ```
+///
+// This is because although the output types for both futures is the same, the exact future
+// types are different, but the compiler must be able to choose a single type for the
+// `result` variable.
+///
+/// When the output type is the same, we can wrap each future in `Either` to avoid the
+/// issue:
+///
+/// ```
+/// use tokio_util::either::Either;
+/// # fn some_condition() -> bool { true }
+/// # async fn some_async_function() -> u32 { 10 }
+/// # async fn other_async_function() -> u32 { 20 }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let result = if some_condition() {
+///         Either::Left(some_async_function())
+///     } else {
+///         Either::Right(other_async_function())
+///     };
+///
+///     let value = result.await;
+///     println!("Result is {}", value);
+///     # assert_eq!(value, 10);
+/// }
+/// ```
+#[allow(missing_docs)] // Doc-comments for variants in this particular case don't make much sense.
+#[derive(Debug, Clone)]
+pub enum Either<L, R> {
+    Left(L),
+    Right(R),
+}
+
+/// A small helper macro which reduces amount of boilerplate in the actual trait method implementation.
+/// It takes an invokation of method as an argument (e.g. `self.poll(cx)`), and redirects it to either
+/// enum variant held in `self`.
+macro_rules! delegate_call {
+    ($self:ident.$method:ident($($args:ident),+)) => {
+        unsafe {
+            match $self.get_unchecked_mut() {
+                Self::Left(l) => Pin::new_unchecked(l).$method($($args),+),
+                Self::Right(r) => Pin::new_unchecked(r).$method($($args),+),
+            }
+        }
+    }
+}
+
+impl<L, R, O> Future for Either<L, R>
+where
+    L: Future<Output = O>,
+    R: Future<Output = O>,
+{
+    type Output = O;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        delegate_call!(self.poll(cx))
+    }
+}
+
+impl<L, R> AsyncRead for Either<L, R>
+where
+    L: AsyncRead,
+    R: AsyncRead,
+{
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut ReadBuf<'_>,
+    ) -> Poll<Result<()>> {
+        delegate_call!(self.poll_read(cx, buf))
+    }
+}
+
+impl<L, R> AsyncBufRead for Either<L, R>
+where
+    L: AsyncBufRead,
+    R: AsyncBufRead,
+{
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<&[u8]>> {
+        delegate_call!(self.poll_fill_buf(cx))
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        delegate_call!(self.consume(amt))
+    }
+}
+
+impl<L, R> AsyncSeek for Either<L, R>
+where
+    L: AsyncSeek,
+    R: AsyncSeek,
+{
+    fn start_seek(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        position: SeekFrom,
+    ) -> Poll<Result<()>> {
+        delegate_call!(self.start_seek(cx, position))
+    }
+
+    fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<u64>> {
+        delegate_call!(self.poll_complete(cx))
+    }
+}
+
+impl<L, R> AsyncWrite for Either<L, R>
+where
+    L: AsyncWrite,
+    R: AsyncWrite,
+{
+    fn poll_write(self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &[u8]) -> Poll<Result<usize>> {
+        delegate_call!(self.poll_write(cx, buf))
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<tokio::io::Result<()>> {
+        delegate_call!(self.poll_flush(cx))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<tokio::io::Result<()>> {
+        delegate_call!(self.poll_shutdown(cx))
+    }
+}
+
+impl<L, R> futures_core::stream::Stream for Either<L, R>
+where
+    L: futures_core::stream::Stream,
+    R: futures_core::stream::Stream<Item = L::Item>,
+{
+    type Item = L::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        delegate_call!(self.poll_next(cx))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tokio::{
+        io::{repeat, AsyncReadExt, Repeat},
+        stream::{once, Once, StreamExt},
+    };
+
+    #[tokio::test]
+    async fn either_is_stream() {
+        let mut either: Either<Once<u32>, Once<u32>> = Either::Left(once(1));
+
+        assert_eq!(Some(1u32), either.next().await);
+    }
+
+    #[tokio::test]
+    async fn either_is_async_read() {
+        let mut buffer = [0; 3];
+        let mut either: Either<Repeat, Repeat> = Either::Right(repeat(0b101));
+
+        either.read_exact(&mut buffer).await.unwrap();
+        assert_eq!(buffer, [0b101, 0b101, 0b101]);
+    }
+}

tokio-util/src/lib.rs

@@ -45,3 +45,5 @@ cfg_io! {
 pub mod context;
 
 pub mod sync;
+
+pub mod either;