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use crate::io::AsyncBufRead;
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};
pin_project! {
/// Future for the [`fill_buf`](crate::io::AsyncBufReadExt::fill_buf) method.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct FillBuf<'a, R: ?Sized> {
reader: Option<&'a mut R>,
#[pin]
_pin: PhantomPinned,
}
}
pub(crate) fn fill_buf<R>(reader: &mut R) -> FillBuf<'_, R>
where
R: AsyncBufRead + ?Sized + Unpin,
{
FillBuf {
reader: Some(reader),
_pin: PhantomPinned,
}
}
impl<'a, R: AsyncBufRead + ?Sized + Unpin> Future for FillBuf<'a, R> {
type Output = io::Result<&'a [u8]>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let me = self.project();
let reader = me.reader.take().expect("Polled after completion.");
match Pin::new(&mut *reader).poll_fill_buf(cx) {
Poll::Ready(Ok(slice)) => unsafe {
// Safety: This is necessary only due to a limitation in the
// borrow checker. Once Rust starts using the polonius borrow
// checker, this can be simplified.
//
// The safety of this transmute relies on the fact that the
// value of `reader` is `None` when we return in this branch.
// Otherwise the caller could poll us again after
// completion, and access the mutable reference while the
// returned immutable reference still exists.
let slice = std::mem::transmute::<&[u8], &'a [u8]>(slice);
Poll::Ready(Ok(slice))
},
Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
Poll::Pending => {
*me.reader = Some(reader);
Poll::Pending
}
}
}
}