futures_util/future/
option.rs

1//! Definition of the `Option` (optional step) combinator
2
3use core::pin::Pin;
4use futures_core::future::{FusedFuture, Future};
5use futures_core::task::{Context, Poll};
6use pin_project_lite::pin_project;
7
8pin_project! {
9    /// A future representing a value which may or may not be present.
10    ///
11    /// Created by the [`From`] implementation for [`Option`](std::option::Option).
12    ///
13    /// # Examples
14    ///
15    /// ```
16    /// # futures::executor::block_on(async {
17    /// use futures::future::OptionFuture;
18    ///
19    /// let mut a: OptionFuture<_> = Some(async { 123 }).into();
20    /// assert_eq!(a.await, Some(123));
21    ///
22    /// a = None.into();
23    /// assert_eq!(a.await, None);
24    /// # });
25    /// ```
26    #[derive(Debug, Clone)]
27    #[must_use = "futures do nothing unless you `.await` or poll them"]
28    pub struct OptionFuture<F> {
29        #[pin]
30        inner: Option<F>,
31    }
32}
33
34impl<F> Default for OptionFuture<F> {
35    fn default() -> Self {
36        Self { inner: None }
37    }
38}
39
40impl<F: Future> Future for OptionFuture<F> {
41    type Output = Option<F::Output>;
42
43    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
44        match self.project().inner.as_pin_mut() {
45            Some(x) => x.poll(cx).map(Some),
46            None => Poll::Ready(None),
47        }
48    }
49}
50
51impl<F: FusedFuture> FusedFuture for OptionFuture<F> {
52    fn is_terminated(&self) -> bool {
53        match &self.inner {
54            Some(x) => x.is_terminated(),
55            None => true,
56        }
57    }
58}
59
60impl<T> From<Option<T>> for OptionFuture<T> {
61    fn from(option: Option<T>) -> Self {
62        Self { inner: option }
63    }
64}
futures_util/future/option.rs

futures_util/future/
option.rs
