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

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

    /// A future which can be used to easily read exactly enough bytes to fill
    /// a buffer.
    ///
    /// Created by the [`AsyncReadExt::read_exact`][read_exact].
    /// [read_exact]: [crate::io::AsyncReadExt::read_exact]
    pub(crate) fn read_exact<'a, A>(reader: &'a mut A, buf: &'a mut [u8]) -> ReadExact<'a, A>
    where
        A: AsyncRead + Unpin + ?Sized,
    {
        ReadExact {
            reader,
            buf: ReadBuf::new(buf),
            _pin: PhantomPinned,
        }
    }

    pin_project! {
        /// Creates a future which will read exactly enough bytes to fill `buf`,
        /// returning an error if EOF is hit sooner.
        ///
        /// On success the number of bytes is returned
        #[derive(Debug)]
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        pub struct ReadExact<'a, A: ?Sized> {
            reader: &'a mut A,
            buf: ReadBuf<'a>,
            // Make this future `!Unpin` for compatibility with async trait methods.
            #[pin]
            _pin: PhantomPinned,
        }
    }

    fn eof() -> io::Error {
        io::Error::new(io::ErrorKind::UnexpectedEof, "early eof")
    }

    impl<A> Future for ReadExact<'_, A>
    where
        A: AsyncRead + Unpin + ?Sized,
    {
        type Output = io::Result<usize>;

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

            loop {
                // if our buffer is empty, then we need to read some data to continue.
                let rem = me.buf.remaining();
                if rem != 0 {
                    ready!(Pin::new(&mut *me.reader).poll_read(cx, &mut me.buf))?;
                    if me.buf.remaining() == rem {
                        return Err(eof()).into();
                    }
                } else {
                    return Poll::Ready(Ok(me.buf.capacity()));
                }
            }
        }
    }