blob: b421063a3f1ecef09cfb126718d02b800aa8431c [file] [log] [blame]
use core::pin::PinMut;
use futures_core::future::Future;
use futures_core::task::{self, Poll};
use pin_utils::unsafe_pinned;
/// A future which "fuses" a future once it's been resolved.
///
/// Normally futures can behave unpredictable once they're used after a future
/// has been resolved, but `Fuse` is always defined to return `Async::Pending`
/// from `poll` after it has resolved successfully or returned an error.
///
/// This is created by the `Future::fuse` method.
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct Fuse<Fut: Future> {
future: Option<Fut>,
}
impl<Fut: Future> Fuse<Fut> {
unsafe_pinned!(future: Option<Fut>);
pub(super) fn new(f: Fut) -> Fuse<Fut> {
Fuse {
future: Some(f),
}
}
}
impl<Fut: Future> Future for Fuse<Fut> {
type Output = Fut::Output;
fn poll(mut self: PinMut<Self>, cx: &mut task::Context) -> Poll<Fut::Output> {
// safety: we use this &mut only for matching, not for movement
let v = match self.future().as_pin_mut() {
Some(fut) => {
// safety: this re-pinned future will never move before being dropped
match fut.poll(cx) {
Poll::Pending => return Poll::Pending,
Poll::Ready(v) => v
}
}
None => return Poll::Pending,
};
PinMut::set(self.future(), None);
Poll::Ready(v)
}
}