[][src]Trait core::future::Future

#[must_use = "futures do nothing unless polled"]
pub trait Future { type Output; fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output>; }
🔬 This is a nightly-only experimental API. (futures_api #50547)

futures in libcore are unstable

A future represents an asynchronous computation.

A future is a value that may not have finished computing yet. This kind of "asynchronous value" makes it possible for a thread to continue doing useful work while it waits for the value to become available.

The poll method

The core method of future, poll, attempts to resolve the future into a final value. This method does not block if the value is not ready. Instead, the current task is scheduled to be woken up when it's possible to make further progress by polling again. The wake up is performed using the waker argument of the poll() method, which is a handle for waking up the current task.

When using a future, you generally won't call poll directly, but instead await! the value.

Associated Types

type Output

🔬 This is a nightly-only experimental API. (futures_api #50547)

futures in libcore are unstable

The type of value produced on completion.

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Required methods

fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output>

🔬 This is a nightly-only experimental API. (futures_api #50547)

futures in libcore are unstable

Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.

Return value

This function returns:

Once a future has finished, clients should not poll it again.

When a future is not ready yet, poll returns Poll::Pending and stores a clone of the Waker copied from the current Context. This Waker is then woken once the future can make progress. For example, a future waiting for a socket to become readable would call .clone() on the Waker and store it. When a signal arrives elsewhere indicating that the socket is readable, [Waker::wake] is called and the socket future's task is awoken. Once a task has been woken up, it should attempt to poll the future again, which may or may not produce a final value.

Note that on multiple calls to poll, only the most recent Waker passed to poll should be scheduled to receive a wakeup.

Runtime characteristics

Futures alone are inert; they must be actively polled to make progress, meaning that each time the current task is woken up, it should actively re-poll pending futures that it still has an interest in.

The poll function is not called repeatedly in a tight loop -- instead, it should only be called when the future indicates that it is ready to make progress (by calling wake()). If you're familiar with the poll(2) or select(2) syscalls on Unix it's worth noting that futures typically do not suffer the same problems of "all wakeups must poll all events"; they are more like epoll(4).

An implementation of poll should strive to return quickly, and should not block. Returning quickly prevents unnecessarily clogging up threads or event loops. If it is known ahead of time that a call to poll may end up taking awhile, the work should be offloaded to a thread pool (or something similar) to ensure that poll can return quickly.

An implementation of poll may also never cause memory unsafety.


Once a future has completed (returned Ready from poll), then any future calls to poll may panic, block forever, or otherwise cause any kind of bad behavior except causing memory unsafety. The Future trait itself provides no guarantees about the behavior of poll after a future has completed.

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impl<F: ?Sized + Future + Unpin, '_> Future for &'_ mut F[src]

type Output = F::Output

🔬 This is a nightly-only experimental API. (futures_api #50547)

futures in libcore are unstable

impl<P> Future for Pin<P> where
    P: Unpin + DerefMut,
    P::Target: Future

type Output = <<P as Deref>::Target as Future>::Output

🔬 This is a nightly-only experimental API. (futures_api #50547)

futures in libcore are unstable

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