#![allow(dead_code)]
//! Definition of the `PollFn` adapter combinator.
use std::fmt;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
// This struct is intentionally `!Unpin` when `F` is `!Unpin`. This is to
// mitigate the issue where rust puts noalias on mutable references to the
// `PollFn` type if it is `Unpin`. If the closure has ownership of a future,
// then this "leaks" and the future is affected by noalias too, which we don't
// want.
//
// See this thread for more information:
// <
https://internals.rust-lang.org/t/surprising-soundness-trouble-around-pollfn/17484>
//
// The fact that `PollFn` is not `Unpin` when it shouldn't be is tested in
// `tests/async_send_sync.rs`.
/// Future for the [`poll_fn`] function.
pub struct PollFn<F> {
f: F,
}
/// Creates a new future wrapping around a function returning [`Poll`].
pub fn poll_fn<T, F>(f: F) -> PollFn<F>
where
F: FnMut(&mut Context<'_>) -> Poll<T>,
{
PollFn { f }
}
impl<F> fmt::Debug for PollFn<F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("PollFn").finish()
}
}
impl<T, F> Future for PollFn<F>
where
F: FnMut(&mut Context<'_>) -> Poll<T>,
{
type Output = T;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
// Safety: We never construct a `Pin<&mut F>` anywhere, so accessing `f`
// mutably in an unpinned way is sound.
//
// This use of unsafe cannot be replaced with the pin-project macro
// because:
// * If we put `#[pin]` on the field, then it gives us a `Pin<&mut F>`,
// which we can't use to call the closure.
// * If we don't put `#[pin]` on the field, then it makes `PollFn` be
// unconditionally `Unpin`, which we also don't want.
let me = unsafe { Pin::into_inner_unchecked(self) };
(me.f)(cx)
}
}
