tokio/io/util/
fill_buf.rs

Help
1use crate::io::AsyncBufRead;
2
3use pin_project_lite::pin_project;
4use std::future::Future;
5use std::io;
6use std::marker::PhantomPinned;
7use std::pin::Pin;
8use std::task::{Context, Poll};
9
10pin_project! {
11    /// Future for the [`fill_buf`](crate::io::AsyncBufReadExt::fill_buf) method.
12    #[derive(Debug)]
13    #[must_use = "futures do nothing unless you `.await` or poll them"]
14    pub struct FillBuf<'a, R: ?Sized> {
15        reader: Option<&'a mut R>,
16        #[pin]
17        _pin: PhantomPinned,
18    }
19}
20
21pub(crate) fn fill_buf<R>(reader: &mut R) -> FillBuf<'_, R>
22where
23    R: AsyncBufRead + ?Sized + Unpin,
24{
25    FillBuf {
26        reader: Some(reader),
27        _pin: PhantomPinned,
28    }
29}
30
31impl<'a, R: AsyncBufRead + ?Sized + Unpin> Future for FillBuf<'a, R> {
32    type Output = io::Result<&'a [u8]>;
33
34    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
35        let me = self.project();
36
37        let reader = me.reader.take().expect("Polled after completion.");
38        match Pin::new(&mut *reader).poll_fill_buf(cx) {
39            Poll::Ready(Ok(slice)) => unsafe {
40                // Safety: This is necessary only due to a limitation in the
41                // borrow checker. Once Rust starts using the polonius borrow
42                // checker, this can be simplified.
43                //
44                // The safety of this transmute relies on the fact that the
45                // value of `reader` is `None` when we return in this branch.
46                // Otherwise the caller could poll us again after
47                // completion, and access the mutable reference while the
48                // returned immutable reference still exists.
49                let slice = std::mem::transmute::<&[u8], &'a [u8]>(slice);
50                Poll::Ready(Ok(slice))
51            },
52            Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
53            Poll::Pending => {
54                *me.reader = Some(reader);
55                Poll::Pending
56            }
57        }
58    }
59}
tokio/io/util/fill_buf.rs

tokio/io/util/
fill_buf.rs
