blob: 7611424e6e42138c1b20de0a71ab2b3b90bd1fff [file] [log] [blame]
//! Definition of the MaybeDone combinator
use core::marker::Unpin;
use core::mem::{self, PinMut};
use futures_core::future::Future;
use futures_core::task::{self, Poll};
/// A future that may have completed.
///
/// This is created by the [`maybe_done()`] function.
#[derive(Debug)]
pub enum MaybeDone<Fut: Future> {
/// A not-yet-completed future
Future(Fut),
/// The output of the completed future
Done(Fut::Output),
/// The empty variant after the result of a [`MaybeDone`] has been
/// taken using the [`take_output`](MaybeDone::take_output) method.
Gone,
}
// Safe because we never generate `PinMut<Fut::Output>`
impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}
/// Wraps a future into a `MaybeDone`
///
/// # Examples
///
/// ```
/// #![feature(async_await, await_macro, futures_api, use_extern_macros, pin)]
/// # futures::executor::block_on(async {
/// use futures::future;
/// use pin_utils::pin_mut;
///
/// let future = future::maybe_done(future::ready(5));
/// pin_mut!(future);
/// assert_eq!(future.reborrow().take_output(), None);
/// let () = await!(future.reborrow());
/// assert_eq!(future.reborrow().take_output(), Some(5));
/// assert_eq!(future.reborrow().take_output(), None);
/// # });
/// ```
pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
MaybeDone::Future(future)
}
impl<Fut: Future> MaybeDone<Fut> {
/// Returns an [`Option`] containing a mutable reference to the output of the future.
/// The output of this method will be [`Some`] if and only if the inner
/// future has been completed and [`take_output`](MaybeDone::take_output)
/// has not yet been called.
#[inline]
#[allow(needless_lifetimes)] // https://github.com/rust-lang/rust/issues/52675
pub fn output_mut<'a>(self: PinMut<'a, Self>) -> Option<&'a mut Fut::Output> {
unsafe {
let this = PinMut::get_mut_unchecked(self);
match this {
MaybeDone::Done(res) => Some(res),
_ => None,
}
}
}
/// Attempt to take the output of a `MaybeDone` without driving it
/// towards completion.
#[inline]
pub fn take_output(self: PinMut<Self>) -> Option<Fut::Output> {
unsafe {
let this = PinMut::get_mut_unchecked(self);
match this {
MaybeDone::Done(_) => {},
MaybeDone::Future(_) | MaybeDone::Gone => return None,
};
if let MaybeDone::Done(output) = mem::replace(this, MaybeDone::Gone) {
Some(output)
} else {
unreachable!()
}
}
}
}
impl<Fut: Future> Future for MaybeDone<Fut> {
type Output = ();
fn poll(mut self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
let res = unsafe {
match PinMut::get_mut_unchecked(self.reborrow()) {
MaybeDone::Future(a) => {
if let Poll::Ready(res) = PinMut::new_unchecked(a).poll(cx) {
res
} else {
return Poll::Pending
}
}
MaybeDone::Done(_) => return Poll::Ready(()),
MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
}
};
PinMut::set(self, MaybeDone::Done(res));
Poll::Ready(())
}
}