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//! Definition of the `Option` (optional step) combinator
use core::pin::Pin;
use futures_core::future::{FusedFuture, Future};
use futures_core::task::{Context, Poll};
use pin_project_lite::pin_project;
pin_project! {
/// A future representing a value which may or may not be present.
///
/// Created by the [`From`] implementation for [`Option`](std::option::Option).
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::OptionFuture;
///
/// let mut a: OptionFuture<_> = Some(async { 123 }).into();
/// assert_eq!(a.await, Some(123));
///
/// a = None.into();
/// assert_eq!(a.await, None);
/// # });
/// ```
#[derive(Debug, Clone)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct OptionFuture<F> {
#[pin]
inner: Option<F>,
}
}
impl<F> Default for OptionFuture<F> {
fn default() -> Self {
Self { inner: None }
}
}
impl<F: Future> Future for OptionFuture<F> {
type Output = Option<F::Output>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
match self.project().inner.as_pin_mut() {
Some(x) => x.poll(cx).map(Some),
None => Poll::Ready(None),
}
}
}
impl<F: FusedFuture> FusedFuture for OptionFuture<F> {
fn is_terminated(&self) -> bool {
match &self.inner {
Some(x) => x.is_terminated(),
None => true,
}
}
}
impl<T> From<Option<T>> for OptionFuture<T> {
fn from(option: Option<T>) -> Self {
Self { inner: option }
}
}