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    use crate::io::AsyncRead;

    use bytes::BufMut;
    use pin_project_lite::pin_project;
    use std::future::Future;
    use std::io;
    use std::marker::PhantomPinned;
    use std::pin::Pin;
    use std::task::{Context, Poll};

    pub(crate) fn read_buf<'a, R, B>(reader: &'a mut R, buf: &'a mut B) -> ReadBuf<'a, R, B>
    where
        R: AsyncRead + Unpin,
        B: BufMut,
    {
        ReadBuf {
            reader,
            buf,
            _pin: PhantomPinned,
        }
    }

    pin_project! {
        /// Future returned by [`read_buf`](crate::io::AsyncReadExt::read_buf).
        #[derive(Debug)]
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        pub struct ReadBuf<'a, R, B> {
            reader: &'a mut R,
            buf: &'a mut B,
            #[pin]
            _pin: PhantomPinned,
        }
    }

    impl<R, B> Future for ReadBuf<'_, R, B>
    where
        R: AsyncRead + Unpin,
        B: BufMut,
    {
        type Output = io::Result<usize>;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
            use crate::io::ReadBuf;
            use std::mem::MaybeUninit;

            let me = self.project();

            if !me.buf.has_remaining_mut() {
                return Poll::Ready(Ok(0));
            }

            let n = {
                let dst = me.buf.chunk_mut();
                let dst = unsafe { &mut *(dst as *mut _ as *mut [MaybeUninit<u8>]) };
                let mut buf = ReadBuf::uninit(dst);
                let ptr = buf.filled().as_ptr();
                ready!(Pin::new(me.reader).poll_read(cx, &mut buf)?);

                // Ensure the pointer does not change from under us
                assert_eq!(ptr, buf.filled().as_ptr());
                buf.filled().len()
            };

            // Safety: This is guaranteed to be the number of initialized (and read)
            // bytes due to the invariants provided by `ReadBuf::filled`.
            unsafe {
                me.buf.advance_mut(n);
            }

            Poll::Ready(Ok(n))
        }
    }