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    //! 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>, position: SeekFrom) -> Result<()> {
            delegate_call!(self.start_seek(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};
        use tokio_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]);
        }
    }