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use crate::io::AsyncRead;

use bytes::BufMut;
use std::future::Future;
use std::io;
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 }
}

cfg_io_util! {
    /// 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,
    }
}

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

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