1use crate::future::Future;
2use crate::pin::Pin;
3use crate::task::{Context, Poll};
4
5/// A future that is immediately ready with a value.
6///
7/// This `struct` is created by [`ready()`]. See its
8/// documentation for more.
9#[stable(feature = "future_readiness_fns", since = "1.48.0")]
10#[derive(Debug, Clone)]
11#[must_use = "futures do nothing unless you `.await` or poll them"]
12pub struct Ready<T>(Option<T>);
13
14#[stable(feature = "future_readiness_fns", since = "1.48.0")]
15impl<T> Unpin for Ready<T> {}
16
17#[stable(feature = "future_readiness_fns", since = "1.48.0")]
18impl<T> Future for Ready<T> {
19 type Output = T;
20
21 #[inline]
22 fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<T> {
23 Poll::Ready(self.0.take().expect(msg:"`Ready` polled after completion"))
24 }
25}
26
27impl<T> Ready<T> {
28 /// Consumes the `Ready`, returning the wrapped value.
29 ///
30 /// # Panics
31 ///
32 /// Will panic if this [`Ready`] was already polled to completion.
33 ///
34 /// # Examples
35 ///
36 /// ```
37 /// #![feature(ready_into_inner)]
38 /// use std::future;
39 ///
40 /// let a = future::ready(1);
41 /// assert_eq!(a.into_inner(), 1);
42 /// ```
43 #[unstable(feature = "ready_into_inner", issue = "101196")]
44 #[must_use]
45 #[inline]
46 pub fn into_inner(self) -> T {
47 self.0.expect(msg:"Called `into_inner()` on `Ready` after completion")
48 }
49}
50
51/// Creates a future that is immediately ready with a value.
52///
53/// Futures created through this function are functionally similar to those
54/// created through `async {}`. The main difference is that futures created
55/// through this function are named and implement `Unpin`.
56///
57/// # Examples
58///
59/// ```
60/// use std::future;
61///
62/// # async fn run() {
63/// let a = future::ready(1);
64/// assert_eq!(a.await, 1);
65/// # }
66/// ```
67#[stable(feature = "future_readiness_fns", since = "1.48.0")]
68pub fn ready<T>(t: T) -> Ready<T> {
69 Ready(Some(t))
70}
71