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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::fmt;
    use std::io::{self, Write};
    use std::pin::Pin;
    use std::task::{Context, Poll};

    pin_project! {
        /// Wraps a writer and buffers its output.
        ///
        /// It can be excessively inefficient to work directly with something that
        /// implements [`AsyncWrite`]. A `BufWriter` keeps an in-memory buffer of data and
        /// writes it to an underlying writer in large, infrequent batches.
        ///
        /// `BufWriter` can improve the speed of programs that make *small* and
        /// *repeated* write calls to the same file or network socket. It does not
        /// help when writing very large amounts at once, or writing just one or a few
        /// times. It also provides no advantage when writing to a destination that is
        /// in memory, like a `Vec<u8>`.
        ///
        /// When the `BufWriter` is dropped, the contents of its buffer will be
        /// discarded. Creating multiple instances of a `BufWriter` on the same
        /// stream can cause data loss. If you need to write out the contents of its
        /// buffer, you must manually call flush before the writer is dropped.
        ///
        /// [`AsyncWrite`]: AsyncWrite
        /// [`flush`]: super::AsyncWriteExt::flush
        ///
        #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
        pub struct BufWriter<W> {
            #[pin]
            pub(super) inner: W,
            pub(super) buf: Vec<u8>,
            pub(super) written: usize,
        }
    }

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

        /// Creates a new `BufWriter` with the specified buffer capacity.
        pub fn with_capacity(cap: usize, inner: W) -> Self {
            Self {
                inner,
                buf: Vec::with_capacity(cap),
                written: 0,
            }
        }

        fn flush_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
            let mut me = self.project();

            let len = me.buf.len();
            let mut ret = Ok(());
            while *me.written < len {
                match ready!(me.inner.as_mut().poll_write(cx, &me.buf[*me.written..])) {
                    Ok(0) => {
                        ret = Err(io::Error::new(
                            io::ErrorKind::WriteZero,
                            "failed to write the buffered data",
                        ));
                        break;
                    }
                    Ok(n) => *me.written += n,
                    Err(e) => {
                        ret = Err(e);
                        break;
                    }
                }
            }
            if *me.written > 0 {
                me.buf.drain(..*me.written);
            }
            *me.written = 0;
            Poll::Ready(ret)
        }

        /// Gets a reference to the underlying writer.
        pub fn get_ref(&self) -> &W {
            &self.inner
        }

        /// Gets a mutable reference to the underlying writer.
        ///
        /// It is inadvisable to directly write to the underlying writer.
        pub fn get_mut(&mut self) -> &mut W {
            &mut self.inner
        }

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

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

        /// Returns a reference to the internally buffered data.
        pub fn buffer(&self) -> &[u8] {
            &self.buf
        }
    }

    impl<W: AsyncWrite> AsyncWrite for BufWriter<W> {
        fn poll_write(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &[u8],
        ) -> Poll<io::Result<usize>> {
            if self.buf.len() + buf.len() > self.buf.capacity() {
                ready!(self.as_mut().flush_buf(cx))?;
            }

            let me = self.project();
            if buf.len() >= me.buf.capacity() {
                me.inner.poll_write(cx, buf)
            } else {
                Poll::Ready(me.buf.write(buf))
            }
        }

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

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

    impl<W: AsyncWrite + AsyncRead> AsyncRead for BufWriter<W> {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            self.get_pin_mut().poll_read(cx, buf)
        }
    }

    impl<W: AsyncWrite + AsyncBufRead> AsyncBufRead for BufWriter<W> {
        fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
            self.get_pin_mut().poll_fill_buf(cx)
        }

        fn consume(self: Pin<&mut Self>, amt: usize) {
            self.get_pin_mut().consume(amt)
        }
    }

    impl<W: fmt::Debug> fmt::Debug for BufWriter<W> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("BufWriter")
                .field("writer", &self.inner)
                .field(
                    "buffer",
                    &format_args!("{}/{}", self.buf.len(), self.buf.capacity()),
                )
                .field("written", &self.written)
                .finish()
        }
    }

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

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