blob: 6860d666a4970b24002dd2d75e1f55c11012069d [file] [log] [blame]
//! Definition of the MaybeDone combinator
use core::marker::Unpin;
use core::mem;
use core::pin::Pin;
use futures_core::future::{FusedFuture, Future};
use futures_core::task::{LocalWaker, 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 `Pin<&mut Fut::Output>`
impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}
/// Wraps a future into a `MaybeDone`
///
/// # Examples
///
/// ```
/// #![feature(async_await, await_macro, futures_api)]
/// # 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.as_mut().take_output(), None);
/// let () = await!(future.as_mut());
/// assert_eq!(future.as_mut().take_output(), Some(5));
/// assert_eq!(future.as_mut().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(clippy::needless_lifetimes)] // https://github.com/rust-lang/rust/issues/52675
pub fn output_mut<'a>(self: Pin<&'a mut Self>) -> Option<&'a mut Fut::Output> {
unsafe {
let this = Pin::get_unchecked_mut(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: Pin<&mut Self>) -> Option<Fut::Output> {
unsafe {
let this = Pin::get_unchecked_mut(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> FusedFuture for MaybeDone<Fut> {
fn is_terminated(&self) -> bool {
match self {
MaybeDone::Future(_) => false,
MaybeDone::Done(_) | MaybeDone::Gone => true,
}
}
}
impl<Fut: Future> Future for MaybeDone<Fut> {
type Output = ();
fn poll(mut self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output> {
let res = unsafe {
match Pin::get_unchecked_mut(self.as_mut()) {
MaybeDone::Future(a) => {
if let Poll::Ready(res) = Pin::new_unchecked(a).poll(lw) {
res
} else {
return Poll::Pending
}
}
MaybeDone::Done(_) => return Poll::Ready(()),
MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
}
};
Pin::set(self, MaybeDone::Done(res));
Poll::Ready(())
}
}