1//! Futures.
2
3use core::ops::DerefMut;
4use core::pin::Pin;
5use core::task::{Context, Poll};
6
7#[doc(no_inline)]
8pub use core::future::Future;
9
10/// An owned dynamically typed [`Future`] for use in cases where you can't
11/// statically type your result or need to add some indirection.
12///
13/// This type is often created by the [`boxed`] method on [`FutureExt`]. See its documentation for more.
14///
15/// [`boxed`]: https://docs.rs/futures/latest/futures/future/trait.FutureExt.html#method.boxed
16/// [`FutureExt`]: https://docs.rs/futures/latest/futures/future/trait.FutureExt.html
17#[cfg(feature = "alloc")]
18pub type BoxFuture<'a, T> = Pin<alloc::boxed::Box<dyn Future<Output = T> + Send + 'a>>;
19
20/// `BoxFuture`, but without the `Send` requirement.
21///
22/// This type is often created by the [`boxed_local`] method on [`FutureExt`]. See its documentation for more.
23///
24/// [`boxed_local`]: https://docs.rs/futures/latest/futures/future/trait.FutureExt.html#method.boxed_local
25/// [`FutureExt`]: https://docs.rs/futures/latest/futures/future/trait.FutureExt.html
26#[cfg(feature = "alloc")]
27pub type LocalBoxFuture<'a, T> = Pin<alloc::boxed::Box<dyn Future<Output = T> + 'a>>;
28
29/// A future which tracks whether or not the underlying future
30/// should no longer be polled.
31///
32/// `is_terminated` will return `true` if a future should no longer be polled.
33/// Usually, this state occurs after `poll` (or `try_poll`) returned
34/// `Poll::Ready`. However, `is_terminated` may also return `true` if a future
35/// has become inactive and can no longer make progress and should be ignored
36/// or dropped rather than being `poll`ed again.
37pub trait FusedFuture: Future {
38 /// Returns `true` if the underlying future should no longer be polled.
39 fn is_terminated(&self) -> bool;
40}
41
42impl<F: FusedFuture + ?Sized + Unpin> FusedFuture for &mut F {
43 fn is_terminated(&self) -> bool {
44 <F as FusedFuture>::is_terminated(&**self)
45 }
46}
47
48impl<P> FusedFuture for Pin<P>
49where
50 P: DerefMut + Unpin,
51 P::Target: FusedFuture,
52{
53 fn is_terminated(&self) -> bool {
54 <P::Target as FusedFuture>::is_terminated(&**self)
55 }
56}
57
58mod private_try_future {
59 use super::Future;
60
61 pub trait Sealed {}
62
63 impl<F, T, E> Sealed for F where F: ?Sized + Future<Output = Result<T, E>> {}
64}
65
66/// A convenience for futures that return `Result` values that includes
67/// a variety of adapters tailored to such futures.
68pub trait TryFuture: Future + private_try_future::Sealed {
69 /// The type of successful values yielded by this future
70 type Ok;
71
72 /// The type of failures yielded by this future
73 type Error;
74
75 /// Poll this `TryFuture` as if it were a `Future`.
76 ///
77 /// This method is a stopgap for a compiler limitation that prevents us from
78 /// directly inheriting from the `Future` trait; in the future it won't be
79 /// needed.
80 fn try_poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<Self::Ok, Self::Error>>;
81}
82
83impl<F, T, E> TryFuture for F
84where
85 F: ?Sized + Future<Output = Result<T, E>>,
86{
87 type Ok = T;
88 type Error = E;
89
90 #[inline]
91 fn try_poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
92 self.poll(cx)
93 }
94}
95
96#[cfg(feature = "alloc")]
97mod if_alloc {
98 use super::*;
99 use alloc::boxed::Box;
100
101 impl<F: FusedFuture + ?Sized + Unpin> FusedFuture for Box<F> {
102 fn is_terminated(&self) -> bool {
103 <F as FusedFuture>::is_terminated(&**self)
104 }
105 }
106
107 #[cfg(feature = "std")]
108 impl<F: FusedFuture> FusedFuture for std::panic::AssertUnwindSafe<F> {
109 fn is_terminated(&self) -> bool {
110 <F as FusedFuture>::is_terminated(&**self)
111 }
112 }
113}
114