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    use crate::io::util::DEFAULT_BUF_SIZE;
    use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite, ReadBuf};

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

    pin_project! {
        /// The `BufReader` struct adds buffering to any reader.
        ///
        /// It can be excessively inefficient to work directly with a [`AsyncRead`]
        /// instance. A `BufReader` performs large, infrequent reads on the underlying
        /// [`AsyncRead`] and maintains an in-memory buffer of the results.
        ///
        /// `BufReader` can improve the speed of programs that make *small* and
        /// *repeated* read calls to the same file or network socket. It does not
        /// help when reading very large amounts at once, or reading just one or a few
        /// times. It also provides no advantage when reading from a source that is
        /// already in memory, like a `Vec<u8>`.
        ///
        /// When the `BufReader` is dropped, the contents of its buffer will be
        /// discarded. Creating multiple instances of a `BufReader` on the same
        /// stream can cause data loss.
        #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
        pub struct BufReader<R> {
            #[pin]
            pub(super) inner: R,
            pub(super) buf: Box<[u8]>,
            pub(super) pos: usize,
            pub(super) cap: usize,
        }
    }

    impl<R: AsyncRead> BufReader<R> {
        /// Creates a new `BufReader` with a default buffer capacity. The default is currently 8 KB,
        /// but may change in the future.
        pub fn new(inner: R) -> Self {
            Self::with_capacity(DEFAULT_BUF_SIZE, inner)
        }

        /// Creates a new `BufReader` with the specified buffer capacity.
        pub fn with_capacity(capacity: usize, inner: R) -> Self {
            let buffer = vec![0; capacity];
            Self {
                inner,
                buf: buffer.into_boxed_slice(),
                pos: 0,
                cap: 0,
            }
        }

        /// Gets a reference to the underlying reader.
        ///
        /// It is inadvisable to directly read from the underlying reader.
        pub fn get_ref(&self) -> &R {
            &self.inner
        }

        /// Gets a mutable reference to the underlying reader.
        ///
        /// It is inadvisable to directly read from the underlying reader.
        pub fn get_mut(&mut self) -> &mut R {
            &mut self.inner
        }

        /// Gets a pinned mutable reference to the underlying reader.
        ///
        /// It is inadvisable to directly read from the underlying reader.
        pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut R> {
            self.project().inner
        }

        /// Consumes this `BufReader`, returning the underlying reader.
        ///
        /// Note that any leftover data in the internal buffer is lost.
        pub fn into_inner(self) -> R {
            self.inner
        }

        /// Returns a reference to the internally buffered data.
        ///
        /// Unlike `fill_buf`, this will not attempt to fill the buffer if it is empty.
        pub fn buffer(&self) -> &[u8] {
            &self.buf[self.pos..self.cap]
        }

        /// Invalidates all data in the internal buffer.
        #[inline]
        fn discard_buffer(self: Pin<&mut Self>) {
            let me = self.project();
            *me.pos = 0;
            *me.cap = 0;
        }
    }

    impl<R: AsyncRead> AsyncRead for BufReader<R> {
        fn poll_read(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            // If we don't have any buffered data and we're doing a massive read
            // (larger than our internal buffer), bypass our internal buffer
            // entirely.
            if self.pos == self.cap && buf.remaining() >= self.buf.len() {
                let res = ready!(self.as_mut().get_pin_mut().poll_read(cx, buf));
                self.discard_buffer();
                return Poll::Ready(res);
            }
            let rem = ready!(self.as_mut().poll_fill_buf(cx))?;
            let amt = std::cmp::min(rem.len(), buf.remaining());
            buf.put_slice(&rem[..amt]);
            self.consume(amt);
            Poll::Ready(Ok(()))
        }
    }

    impl<R: AsyncRead> AsyncBufRead for BufReader<R> {
        fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
            let me = self.project();

            // If we've reached the end of our internal buffer then we need to fetch
            // some more data from the underlying reader.
            // Branch using `>=` instead of the more correct `==`
            // to tell the compiler that the pos..cap slice is always valid.
            if *me.pos >= *me.cap {
                debug_assert!(*me.pos == *me.cap);
                let mut buf = ReadBuf::new(me.buf);
                ready!(me.inner.poll_read(cx, &mut buf))?;
                *me.cap = buf.filled().len();
                *me.pos = 0;
            }
            Poll::Ready(Ok(&me.buf[*me.pos..*me.cap]))
        }

        fn consume(self: Pin<&mut Self>, amt: usize) {
            let me = self.project();
            *me.pos = cmp::min(*me.pos + amt, *me.cap);
        }
    }

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

        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
            self.get_pin_mut().poll_flush(cx)
        }

        fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
            self.get_pin_mut().poll_shutdown(cx)
        }
    }

    impl<R: fmt::Debug> fmt::Debug for BufReader<R> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("BufReader")
                .field("reader", &self.inner)
                .field(
                    "buffer",
                    &format_args!("{}/{}", self.cap - self.pos, self.buf.len()),
                )
                .finish()
        }
    }

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

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