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.
10 /// A future is a value that may 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 #[must_use = "futures do nothing unless you `.await` or poll them"]
27 #[stable(feature = "futures_api", since = "1.36.0")]
28 #[lang = "future_trait"]
30 /// The type of value produced on completion.
31 #[stable(feature = "futures_api", since = "1.36.0")]
34 /// Attempt to resolve the future to a final value, registering
35 /// the current task for wakeup if the value is not yet available.
39 /// This function returns:
41 /// - [`Poll::Pending`] if the future is not ready yet
42 /// - [`Poll::Ready(val)`] with the result `val` of this future if it
43 /// finished successfully.
45 /// Once a future has finished, clients should not `poll` it again.
47 /// When a future is not ready yet, `poll` returns `Poll::Pending` and
48 /// stores a clone of the [`Waker`] copied from the current [`Context`].
49 /// This [`Waker`] is then woken once the future can make progress.
50 /// For example, a future waiting for a socket to become
51 /// readable would call `.clone()` on the [`Waker`] and store it.
52 /// When a signal arrives elsewhere indicating that the socket is readable,
53 /// [`Waker::wake`] is called and the socket future's task is awoken.
54 /// Once a task has been woken up, it should attempt to `poll` the future
55 /// again, which may or may not produce a final value.
57 /// Note that on multiple calls to `poll`, only the [`Waker`] from the
58 /// [`Context`] passed to the most recent call should be scheduled to
61 /// # Runtime characteristics
63 /// Futures alone are *inert*; they must be *actively* `poll`ed to make
64 /// progress, meaning that each time the current task is woken up, it should
65 /// actively re-`poll` pending futures that it still has an interest in.
67 /// The `poll` function is not called repeatedly in a tight loop -- instead,
68 /// it should only be called when the future indicates that it is ready to
69 /// make progress (by calling `wake()`). If you're familiar with the
70 /// `poll(2)` or `select(2)` syscalls on Unix it's worth noting that futures
71 /// typically do *not* suffer the same problems of "all wakeups must poll
72 /// all events"; they are more like `epoll(4)`.
74 /// An implementation of `poll` should strive to return quickly, and should
75 /// not block. Returning quickly prevents unnecessarily clogging up
76 /// threads or event loops. If it is known ahead of time that a call to
77 /// `poll` may end up taking awhile, the work should be offloaded to a
78 /// thread pool (or something similar) to ensure that `poll` can return
83 /// Once a future has completed (returned `Ready` from `poll`), calling its
84 /// `poll` method again may panic, block forever, or cause other kinds of
85 /// problems; the `Future` trait places no requirements on the effects of
86 /// such a call. However, as the `poll` method is not marked `unsafe`,
87 /// Rust's usual rules apply: calls must never cause undefined behavior
88 /// (memory corruption, incorrect use of `unsafe` functions, or the like),
89 /// regardless of the future's state.
91 /// [`Poll::Pending`]: ../task/enum.Poll.html#variant.Pending
92 /// [`Poll::Ready(val)`]: ../task/enum.Poll.html#variant.Ready
93 /// [`Context`]: ../task/struct.Context.html
94 /// [`Waker`]: ../task/struct.Waker.html
95 /// [`Waker::wake`]: ../task/struct.Waker.html#method.wake
96 #[stable(feature = "futures_api", since = "1.36.0")]
97 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
100 #[stable(feature = "futures_api", since = "1.36.0")]
101 impl<F: ?Sized + Future + Unpin> Future for &mut F {
102 type Output = F::Output;
104 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
105 F::poll(Pin::new(&mut **self), cx)
109 #[stable(feature = "futures_api", since = "1.36.0")]
110 impl<P> Future for Pin<P>
112 P: Unpin + ops::DerefMut,
115 type Output = <<P as ops::Deref>::Target as Future>::Output;
117 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
118 Pin::get_mut(self).as_mut().poll(cx)