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