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