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    //! Split a single value implementing `AsyncRead + AsyncWrite` into separate
    //! `AsyncRead` and `AsyncWrite` handles.
    //!
    //! To restore this read/write object from its `split::ReadHalf` and
    //! `split::WriteHalf` use `unsplit`.

    use crate::io::{AsyncRead, AsyncWrite, ReadBuf};

    use std::cell::UnsafeCell;
    use std::fmt;
    use std::io;
    use std::pin::Pin;
    use std::sync::atomic::AtomicBool;
    use std::sync::atomic::Ordering::{Acquire, Release};
    use std::sync::Arc;
    use std::task::{Context, Poll};

    cfg_io_util! {
        /// The readable half of a value returned from [`split`](split()).
        pub struct ReadHalf<T> {
            inner: Arc<Inner<T>>,
        }

        /// The writable half of a value returned from [`split`](split()).
        pub struct WriteHalf<T> {
            inner: Arc<Inner<T>>,
        }

        /// Splits a single value implementing `AsyncRead + AsyncWrite` into separate
        /// `AsyncRead` and `AsyncWrite` handles.
        ///
        /// To restore this read/write object from its `ReadHalf` and
        /// `WriteHalf` use [`unsplit`](ReadHalf::unsplit()).
        pub fn split<T>(stream: T) -> (ReadHalf<T>, WriteHalf<T>)
        where
            T: AsyncRead + AsyncWrite,
        {
            let inner = Arc::new(Inner {
                locked: AtomicBool::new(false),
                stream: UnsafeCell::new(stream),
            });

            let rd = ReadHalf {
                inner: inner.clone(),
            };

            let wr = WriteHalf { inner };

            (rd, wr)
        }
    }

    struct Inner<T> {
        locked: AtomicBool,
        stream: UnsafeCell<T>,
    }

    struct Guard<'a, T> {
        inner: &'a Inner<T>,
    }

    impl<T> ReadHalf<T> {
        /// Checks if this `ReadHalf` and some `WriteHalf` were split from the same
        /// stream.
        pub fn is_pair_of(&self, other: &WriteHalf<T>) -> bool {
            other.is_pair_of(&self)
        }

        /// Reunites with a previously split `WriteHalf`.
        ///
        /// # Panics
        ///
        /// If this `ReadHalf` and the given `WriteHalf` do not originate from the
        /// same `split` operation this method will panic.
        /// This can be checked ahead of time by comparing the stream ID
        /// of the two halves.
        pub fn unsplit(self, wr: WriteHalf<T>) -> T {
            if self.is_pair_of(&wr) {
                drop(wr);

                let inner = Arc::try_unwrap(self.inner)
                    .ok()
                    .expect("`Arc::try_unwrap` failed");

                inner.stream.into_inner()
            } else {
                panic!("Unrelated `split::Write` passed to `split::Read::unsplit`.")
            }
        }
    }

    impl<T> WriteHalf<T> {
        /// Check if this `WriteHalf` and some `ReadHalf` were split from the same
        /// stream.
        pub fn is_pair_of(&self, other: &ReadHalf<T>) -> bool {
            Arc::ptr_eq(&self.inner, &other.inner)
        }
    }

    impl<T: AsyncRead> AsyncRead for ReadHalf<T> {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            let mut inner = ready!(self.inner.poll_lock(cx));
            inner.stream_pin().poll_read(cx, buf)
        }
    }

    impl<T: AsyncWrite> AsyncWrite for WriteHalf<T> {
        fn poll_write(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &[u8],
        ) -> Poll<Result<usize, io::Error>> {
            let mut inner = ready!(self.inner.poll_lock(cx));
            inner.stream_pin().poll_write(cx, buf)
        }

        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
            let mut inner = ready!(self.inner.poll_lock(cx));
            inner.stream_pin().poll_flush(cx)
        }

        fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
            let mut inner = ready!(self.inner.poll_lock(cx));
            inner.stream_pin().poll_shutdown(cx)
        }
    }

    impl<T> Inner<T> {
        fn poll_lock(&self, cx: &mut Context<'_>) -> Poll<Guard<'_, T>> {
            if self
                .locked
                .compare_exchange(false, true, Acquire, Acquire)
                .is_ok()
            {
                Poll::Ready(Guard { inner: self })
            } else {
                // Spin... but investigate a better strategy

                std::thread::yield_now();
                cx.waker().wake_by_ref();

                Poll::Pending
            }
        }
    }

    impl<T> Guard<'_, T> {
        fn stream_pin(&mut self) -> Pin<&mut T> {
            // safety: the stream is pinned in `Arc` and the `Guard` ensures mutual
            // exclusion.
            unsafe { Pin::new_unchecked(&mut *self.inner.stream.get()) }
        }
    }

    impl<T> Drop for Guard<'_, T> {
        fn drop(&mut self) {
            self.inner.locked.store(false, Release);
        }
    }

    unsafe impl<T: Send> Send for ReadHalf<T> {}
    unsafe impl<T: Send> Send for WriteHalf<T> {}
    unsafe impl<T: Sync> Sync for ReadHalf<T> {}
    unsafe impl<T: Sync> Sync for WriteHalf<T> {}

    impl<T: fmt::Debug> fmt::Debug for ReadHalf<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("split::ReadHalf").finish()
        }
    }

    impl<T: fmt::Debug> fmt::Debug for WriteHalf<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("split::WriteHalf").finish()
        }
    }