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    //! `TcpStream` split support.
    //!
    //! A `TcpStream` can be split into a `ReadHalf` and a
    //! `WriteHalf` with the `TcpStream::split` method. `ReadHalf`
    //! implements `AsyncRead` while `WriteHalf` implements `AsyncWrite`.
    //!
    //! Compared to the generic split of `AsyncRead + AsyncWrite`, this specialized
    //! split has no associated overhead and enforces all invariants at the type
    //! level.

    use crate::future::poll_fn;
    use crate::io::{AsyncRead, AsyncWrite, ReadBuf};
    use crate::net::TcpStream;

    use std::io;
    use std::net::Shutdown;
    use std::pin::Pin;
    use std::task::{Context, Poll};

    /// Borrowed read half of a [`TcpStream`], created by [`split`].
    ///
    /// Reading from a `ReadHalf` is usually done using the convenience methods found on the
    /// [`AsyncReadExt`] trait.
    ///
    /// [`TcpStream`]: TcpStream
    /// [`split`]: TcpStream::split()
    /// [`AsyncReadExt`]: trait@crate::io::AsyncReadExt
    #[derive(Debug)]
    pub struct ReadHalf<'a>(&'a TcpStream);

    /// Borrowed write half of a [`TcpStream`], created by [`split`].
    ///
    /// Note that in the [`AsyncWrite`] implemenation of this type, [`poll_shutdown`] will
    /// shut down the TCP stream in the write direction.
    ///
    /// Writing to an `WriteHalf` is usually done using the convenience methods found
    /// on the [`AsyncWriteExt`] trait.
    ///
    /// [`TcpStream`]: TcpStream
    /// [`split`]: TcpStream::split()
    /// [`AsyncWrite`]: trait@crate::io::AsyncWrite
    /// [`poll_shutdown`]: fn@crate::io::AsyncWrite::poll_shutdown
    /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
    #[derive(Debug)]
    pub struct WriteHalf<'a>(&'a TcpStream);

    pub(crate) fn split(stream: &mut TcpStream) -> (ReadHalf<'_>, WriteHalf<'_>) {
        (ReadHalf(&*stream), WriteHalf(&*stream))
    }

    impl ReadHalf<'_> {
        /// Attempt to receive data on the socket, without removing that data from
        /// the queue, registering the current task for wakeup if data is not yet
        /// available.
        ///
        /// Note that on multiple calls to `poll_peek` or `poll_read`, only the
        /// `Waker` from the `Context` passed to the most recent call is scheduled
        /// to receive a wakeup.
        ///
        /// See the [`TcpStream::poll_peek`] level documenation for more details.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::io::{self, ReadBuf};
        /// use tokio::net::TcpStream;
        ///
        /// use futures::future::poll_fn;
        ///
        /// #[tokio::main]
        /// async fn main() -> io::Result<()> {
        ///     let mut stream = TcpStream::connect("127.0.0.1:8000").await?;
        ///     let (mut read_half, _) = stream.split();
        ///     let mut buf = [0; 10];
        ///     let mut buf = ReadBuf::new(&mut buf);
        ///
        ///     poll_fn(|cx| {
        ///         read_half.poll_peek(cx, &mut buf)
        ///     }).await?;
        ///
        ///     Ok(())
        /// }
        /// ```
        ///
        /// [`TcpStream::poll_peek`]: TcpStream::poll_peek
        pub fn poll_peek(
            &mut self,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<usize>> {
            self.0.poll_peek(cx, buf)
        }

        /// Receives data on the socket from the remote address to which it is
        /// connected, without removing that data from the queue. On success,
        /// returns the number of bytes peeked.
        ///
        /// See the [`TcpStream::peek`] level documenation for more details.
        ///
        /// [`TcpStream::peek`]: TcpStream::peek
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::net::TcpStream;
        /// use tokio::io::AsyncReadExt;
        /// use std::error::Error;
        ///
        /// #[tokio::main]
        /// async fn main() -> Result<(), Box<dyn Error>> {
        ///     // Connect to a peer
        ///     let mut stream = TcpStream::connect("127.0.0.1:8080").await?;
        ///     let (mut read_half, _) = stream.split();
        ///
        ///     let mut b1 = [0; 10];
        ///     let mut b2 = [0; 10];
        ///
        ///     // Peek at the data
        ///     let n = read_half.peek(&mut b1).await?;
        ///
        ///     // Read the data
        ///     assert_eq!(n, read_half.read(&mut b2[..n]).await?);
        ///     assert_eq!(&b1[..n], &b2[..n]);
        ///
        ///     Ok(())
        /// }
        /// ```
        ///
        /// The [`read`] method is defined on the [`AsyncReadExt`] trait.
        ///
        /// [`read`]: fn@crate::io::AsyncReadExt::read
        /// [`AsyncReadExt`]: trait@crate::io::AsyncReadExt
        pub async fn peek(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            let mut buf = ReadBuf::new(buf);
            poll_fn(|cx| self.poll_peek(cx, &mut buf)).await
        }
    }

    impl AsyncRead for ReadHalf<'_> {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            self.0.poll_read_priv(cx, buf)
        }
    }

    impl AsyncWrite for WriteHalf<'_> {
        fn poll_write(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &[u8],
        ) -> Poll<io::Result<usize>> {
            self.0.poll_write_priv(cx, buf)
        }

        fn poll_write_vectored(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            bufs: &[io::IoSlice<'_>],
        ) -> Poll<io::Result<usize>> {
            self.0.poll_write_vectored_priv(cx, bufs)
        }

        fn is_write_vectored(&self) -> bool {
            self.0.is_write_vectored()
        }

        #[inline]
        fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
            // tcp flush is a no-op
            Poll::Ready(Ok(()))
        }

        // `poll_shutdown` on a write half shutdowns the stream in the "write" direction.
        fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
            self.0.shutdown_std(Shutdown::Write).into()
        }
    }

    impl AsRef<TcpStream> for ReadHalf<'_> {
        fn as_ref(&self) -> &TcpStream {
            self.0
        }
    }

    impl AsRef<TcpStream> for WriteHalf<'_> {
        fn as_ref(&self) -> &TcpStream {
            self.0
        }
    }