You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: read_exact.rs
    path of file: ./repos/phaser/phaser/vendor/futures-util/src/io
    
    the code of the file until where you have to start completion: use crate::io::AsyncRead;
use futures_core::future::Future;
use futures_core::ready;
use futures_core::task::{Context, Poll};
use std::io;
use std::mem;
use std::pin::Pin;

/// Future for the [`read_exact`](super::AsyncReadExt::read_exact) method.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct ReadExact<'a, R: ?Sized> {
    reader: &'a mut R,
    buf: &
    
    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let this = &mut *self;
        while !this.buf.is_empty() {
            let n = ready!(Pin::new(&mut this.reader).poll_read(cx, this.buf))?;
            {
                let (_, rest) = mem::replace(&mut this.buf, &mut []).split_at_mut(n);
                this.buf = rest;
            }
            if n == 0 {
                return Poll::Ready(Err(io::ErrorKind::UnexpectedEof.into()));
            }
        }
        Poll::Ready(Ok(()))
    }