1 #![stable(feature = "futures_api", since = "1.36.0")]
3 use crate::marker::Unpin;
6 use crate::task::{Context, Poll};
8 /// A future represents an asynchronous computation obtained by use of [`async`].
10 /// A future is a value that might not have finished computing yet. This kind of
11 /// "asynchronous value" makes it possible for a thread to continue doing useful
12 /// work while it waits for the value to become available.
14 /// # The `poll` method
16 /// The core method of future, `poll`, *attempts* to resolve the future into a
17 /// final value. This method does not block if the value is not ready. Instead,
18 /// the current task is scheduled to be woken up when it's possible to make
19 /// further progress by `poll`ing again. The `context` passed to the `poll`
20 /// method can provide a [`Waker`], which is a handle for waking up the current
23 /// When using a future, you generally won't call `poll` directly, but instead
24 /// `.await` the value.
26 /// [`async`]: ../../std/keyword.async.html
27 /// [`Waker`]: crate::task::Waker
29 #[must_use = "futures do nothing unless you `.await` or poll them"]
30 #[stable(feature = "futures_api", since = "1.36.0")]
31 #[lang = "future_trait"]
32 #[rustc_on_unimplemented(
33 label = "`{Self}` is not a future",
34 message = "`{Self}` is not a future",
35 note = "{Self} must be a future or must implement `IntoFuture` to be awaited"
38 /// The type of value produced on completion.
39 #[stable(feature = "futures_api", since = "1.36.0")]
42 /// Attempt to resolve the future to a final value, registering
43 /// the current task for wakeup if the value is not yet available.
47 /// This function returns:
49 /// - [`Poll::Pending`] if the future is not ready yet
50 /// - [`Poll::Ready(val)`] with the result `val` of this future if it
51 /// finished successfully.
53 /// Once a future has finished, clients should not `poll` it again.
55 /// When a future is not ready yet, `poll` returns `Poll::Pending` and
56 /// stores a clone of the [`Waker`] copied from the current [`Context`].
57 /// This [`Waker`] is then woken once the future can make progress.
58 /// For example, a future waiting for a socket to become
59 /// readable would call `.clone()` on the [`Waker`] and store it.
60 /// When a signal arrives elsewhere indicating that the socket is readable,
61 /// [`Waker::wake`] is called and the socket future's task is awoken.
62 /// Once a task has been woken up, it should attempt to `poll` the future
63 /// again, which may or may not produce a final value.
65 /// Note that on multiple calls to `poll`, only the [`Waker`] from the
66 /// [`Context`] passed to the most recent call should be scheduled to
69 /// # Runtime characteristics
71 /// Futures alone are *inert*; they must be *actively* `poll`ed to make
72 /// progress, meaning that each time the current task is woken up, it should
73 /// actively re-`poll` pending futures that it still has an interest in.
75 /// The `poll` function is not called repeatedly in a tight loop -- instead,
76 /// it should only be called when the future indicates that it is ready to
77 /// make progress (by calling `wake()`). If you're familiar with the
78 /// `poll(2)` or `select(2)` syscalls on Unix it's worth noting that futures
79 /// typically do *not* suffer the same problems of "all wakeups must poll
80 /// all events"; they are more like `epoll(4)`.
82 /// An implementation of `poll` should strive to return quickly, and should
83 /// not block. Returning quickly prevents unnecessarily clogging up
84 /// threads or event loops. If it is known ahead of time that a call to
85 /// `poll` may end up taking awhile, the work should be offloaded to a
86 /// thread pool (or something similar) to ensure that `poll` can return
91 /// Once a future has completed (returned `Ready` from `poll`), calling its
92 /// `poll` method again may panic, block forever, or cause other kinds of
93 /// problems; the `Future` trait places no requirements on the effects of
94 /// such a call. However, as the `poll` method is not marked `unsafe`,
95 /// Rust's usual rules apply: calls must never cause undefined behavior
96 /// (memory corruption, incorrect use of `unsafe` functions, or the like),
97 /// regardless of the future's state.
99 /// [`Poll::Ready(val)`]: Poll::Ready
100 /// [`Waker`]: crate::task::Waker
101 /// [`Waker::wake`]: crate::task::Waker::wake
103 #[stable(feature = "futures_api", since = "1.36.0")]
104 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
107 #[stable(feature = "futures_api", since = "1.36.0")]
108 impl<F: ?Sized + Future + Unpin> Future for &mut F {
109 type Output = F::Output;
111 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
112 F::poll(Pin::new(&mut **self), cx)
116 #[stable(feature = "futures_api", since = "1.36.0")]
117 impl<P> Future for Pin<P>
119 P: ops::DerefMut<Target: Future>,
121 type Output = <<P as ops::Deref>::Target as Future>::Output;
123 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
124 <P::Target as Future>::poll(self.as_deref_mut(), cx)