core/future/
future.rs

1#![stable(feature = "futures_api", since = "1.36.0")]
2
3use crate::ops;
4use crate::pin::Pin;
5use crate::task::{Context, Poll};
6
7/// A future represents an asynchronous computation obtained by use of [`async`].
8///
9/// A future is a value that might not have finished computing yet. This kind of
10/// "asynchronous value" makes it possible for a thread to continue doing useful
11/// work while it waits for the value to become available.
12///
13/// # The `poll` method
14///
15/// The core method of future, `poll`, *attempts* to resolve the future into a
16/// final value. This method does not block if the value is not ready. Instead,
17/// the current task is scheduled to be woken up when it's possible to make
18/// further progress by `poll`ing again. The `context` passed to the `poll`
19/// method can provide a [`Waker`], which is a handle for waking up the current
20/// task.
21///
22/// When using a future, you generally won't call `poll` directly, but instead
23/// `.await` the value.
24///
25/// [`async`]: ../../std/keyword.async.html
26/// [`Waker`]: crate::task::Waker
27#[doc(notable_trait)]
28#[doc(search_unbox)]
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#[diagnostic::on_unimplemented(
33    label = "`{Self}` is not a future",
34    message = "`{Self}` is not a future"
35)]
36pub trait Future {
37    /// The type of value produced on completion.
38    #[stable(feature = "futures_api", since = "1.36.0")]
39    #[lang = "future_output"]
40    type Output;
41
42    /// Attempts to resolve the future to a final value, registering
43    /// the current task for wakeup if the value is not yet available.
44    ///
45    /// # Return value
46    ///
47    /// This function returns:
48    ///
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.
52    ///
53    /// Once a future has finished, clients should not `poll` it again.
54    ///
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.
64    ///
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
67    /// receive a wakeup.
68    ///
69    /// # Runtime characteristics
70    ///
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.
74    ///
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)`.
81    ///
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 a while, the work should be offloaded to a
86    /// thread pool (or something similar) to ensure that `poll` can return
87    /// quickly.
88    ///
89    /// # Panics
90    ///
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.
98    ///
99    /// [`Poll::Ready(val)`]: Poll::Ready
100    /// [`Waker`]: crate::task::Waker
101    /// [`Waker::wake`]: crate::task::Waker::wake
102    #[lang = "poll"]
103    #[stable(feature = "futures_api", since = "1.36.0")]
104    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
105}
106
107#[stable(feature = "futures_api", since = "1.36.0")]
108impl<F: ?Sized + Future + Unpin> Future for &mut F {
109    type Output = F::Output;
110
111    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
112        F::poll(Pin::new(&mut **self), cx)
113    }
114}
115
116#[stable(feature = "futures_api", since = "1.36.0")]
117impl<P> Future for Pin<P>
118where
119    P: ops::DerefMut<Target: Future>,
120{
121    type Output = <<P as ops::Deref>::Target as Future>::Output;
122
123    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
124        <P::Target as Future>::poll(self.as_deref_mut(), cx)
125    }
126}
core/future/future.rs

core/future/
future.rs
