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

    use pin_project_lite::pin_project;
    use std::fmt;
    use std::io;
    use std::pin::Pin;
    use std::task::{Context, Poll};

    pin_project! {
        /// Stream for the [`chain`](super::AsyncReadExt::chain) method.
        #[must_use = "streams do nothing unless polled"]
        #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
        pub struct Chain<T, U> {
            #[pin]
            first: T,
            #[pin]
            second: U,
            done_first: bool,
        }
    }

    pub(super) fn chain<T, U>(first: T, second: U) -> Chain<T, U>
    where
        T: AsyncRead,
        U: AsyncRead,
    {
        Chain {
            first,
            second,
            done_first: false,
        }
    }

    impl<T, U> Chain<T, U>
    where
        T: AsyncRead,
        U: AsyncRead,
    {
        /// Gets references to the underlying readers in this `Chain`.
        pub fn get_ref(&self) -> (&T, &U) {
            (&self.first, &self.second)
        }

        /// Gets mutable references to the underlying readers in this `Chain`.
        ///
        /// Care should be taken to avoid modifying the internal I/O state of the
        /// underlying readers as doing so may corrupt the internal state of this
        /// `Chain`.
        pub fn get_mut(&mut self) -> (&mut T, &mut U) {
            (&mut self.first, &mut self.second)
        }

        /// Gets pinned mutable references to the underlying readers in this `Chain`.
        ///
        /// Care should be taken to avoid modifying the internal I/O state of the
        /// underlying readers as doing so may corrupt the internal state of this
        /// `Chain`.
        pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut T>, Pin<&mut U>) {
            let me = self.project();
            (me.first, me.second)
        }

        /// Consumes the `Chain`, returning the wrapped readers.
        pub fn into_inner(self) -> (T, U) {
            (self.first, self.second)
        }
    }

    impl<T, U> fmt::Debug for Chain<T, U>
    where
        T: fmt::Debug,
        U: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Chain")
                .field("t", &self.first)
                .field("u", &self.second)
                .finish()
        }
    }

    impl<T, U> AsyncRead for Chain<T, U>
    where
        T: AsyncRead,
        U: AsyncRead,
    {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            let me = self.project();

            if !*me.done_first {
                let rem = buf.remaining();
                ready!(me.first.poll_read(cx, buf))?;
                if buf.remaining() == rem {
                    *me.done_first = true;
                } else {
                    return Poll::Ready(Ok(()));
                }
            }
            me.second.poll_read(cx, buf)
        }
    }

    impl<T, U> AsyncBufRead for Chain<T, U>
    where
        T: AsyncBufRead,
        U: AsyncBufRead,
    {
        fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
            let me = self.project();

            if !*me.done_first {
                match ready!(me.first.poll_fill_buf(cx)?) {
                    buf if buf.is_empty() => {
                        *me.done_first = true;
                    }
                    buf => return Poll::Ready(Ok(buf)),
                }
            }
            me.second.poll_fill_buf(cx)
        }

        fn consume(self: Pin<&mut Self>, amt: usize) {
            let me = self.project();
            if !*me.done_first {
                me.first.consume(amt)
            } else {
                me.second.consume(amt)
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn assert_unpin() {
            crate::is_unpin::<Chain<(), ()>>();
        }
    }