pub trait Future {
type Output;
// Required method
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}
prelude_2024
)Expand description
A future represents an asynchronous computation obtained by use of async
.
A future is a value that might not have finished computing yet. This kind of โasynchronous valueโ makes it possible for a thread to continue doing useful work while it waits for the value to become available.
ยงThe 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 itโs possible to make
further progress by poll
ing again. The context
passed to the poll
method can provide a Waker
, which is a handle for waking up the current
task.
When using a future, you generally wonโt call poll
directly, but instead
.await
the value.
Required Associated Typesยง
Required Methodsยง
1.36.0 ยท Sourcefn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>
Attempts to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.
ยงReturn value
This function returns:
Poll::Pending
if the future is not ready yetPoll::Ready(val)
with the resultval
of this future if it finished successfully.
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 futureโs 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 Waker
from the
Context
passed to the most recent call should be scheduled to
receive a wakeup.
ยงRuntime characteristics
Futures alone are inert; they must be actively poll
ed 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 โ instead,
it should only be called when the future indicates that it is ready to
make progress (by calling wake()
). If youโre familiar with the
poll(2)
or select(2)
syscalls on Unix itโs worth noting that futures
typically do not suffer the same problems of โall wakeups must poll
all eventsโ; 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 a while, the work should be offloaded to a
thread pool (or something similar) to ensure that poll
can return
quickly.
ยงPanics
Once a future has completed (returned Ready
from poll
), calling its
poll
method again may panic, block forever, or cause other kinds of
problems; the Future
trait places no requirements on the effects of
such a call. However, as the poll
method is not marked unsafe
,
Rustโs usual rules apply: calls must never cause undefined behavior
(memory corruption, incorrect use of unsafe
functions, or the like),
regardless of the futureโs state.
Trait Implementationsยง
Sourceยงimpl<'a, T> UnsafeFutureObj<'a, T> for &'a mut (dyn Future<Output = T> + Unpin)
impl<'a, T> UnsafeFutureObj<'a, T> for &'a mut (dyn Future<Output = T> + Unpin)
Sourceยงimpl<'a, T> UnsafeFutureObj<'a, T> for Box<dyn Future<Output = T> + 'a>where
T: 'a,
impl<'a, T> UnsafeFutureObj<'a, T> for Box<dyn Future<Output = T> + 'a>where
T: 'a,
Sourceยงimpl<'a, T> UnsafeFutureObj<'a, T> for Pin<&'a mut dyn Future<Output = T>>
impl<'a, T> UnsafeFutureObj<'a, T> for Pin<&'a mut dyn Future<Output = T>>
Implementorsยง
Sourceยงimpl Future for PushedResponseFuture
impl Future for PushedResponseFuture
Sourceยงimpl Future for h2::client::ResponseFuture
impl Future for h2::client::ResponseFuture
Sourceยงimpl Future for hyper_util::client::legacy::client::ResponseFuture
impl Future for hyper_util::client::legacy::client::ResponseFuture
Sourceยงimpl Future for rustmax::axum::middleware::future::FromFnResponseFuture
impl Future for rustmax::axum::middleware::future::FromFnResponseFuture
Sourceยงimpl Future for rustmax::axum::middleware::future::MapRequestResponseFuture
impl Future for rustmax::axum::middleware::future::MapRequestResponseFuture
Sourceยงimpl Future for rustmax::axum::middleware::future::MapResponseResponseFuture
impl Future for rustmax::axum::middleware::future::MapResponseResponseFuture
Sourceยงimpl<'a, T> Future for MutexLockFuture<'a, T>where
T: ?Sized,
impl<'a, T> Future for MutexLockFuture<'a, T>where
T: ?Sized,
type Output = MutexGuard<'a, T>
Sourceยงimpl<'a, T, Request> Future for rustmax::tower::util::Ready<'a, T, Request>where
T: Service<Request>,
impl<'a, T, Request> Future for rustmax::tower::util::Ready<'a, T, Request>where
T: Service<Request>,
Sourceยงimpl<A> Future for ReadToString<'_, A>
impl<A> Future for ReadToString<'_, A>
Sourceยงimpl<A, B> Future for EitherResponseFuture<A, B>
impl<A, B> Future for EitherResponseFuture<A, B>
Sourceยงimpl<B> Future for ReadySendRequest<B>where
B: Buf,
impl<B> Future for ReadySendRequest<B>where
B: Buf,
Sourceยงimpl<B, T, E, S> Future for rustmax::axum::middleware::future::FromExtractorResponseFuture<B, T, E, S>where
E: FromRequestParts<S>,
T: Service<Request<B>>,
<T as Service<Request<B>>>::Response: IntoResponse,
impl<B, T, E, S> Future for rustmax::axum::middleware::future::FromExtractorResponseFuture<B, T, E, S>where
E: FromRequestParts<S>,
T: Service<Request<B>>,
<T as Service<Request<B>>>::Response: IntoResponse,
Sourceยงimpl<F1, F2, N> Future for AndThenFuture<F1, F2, N>
impl<F1, F2, N> Future for AndThenFuture<F1, F2, N>
Sourceยงimpl<F1, F2, N> Future for ThenFuture<F1, F2, N>
impl<F1, F2, N> Future for ThenFuture<F1, F2, N>
Sourceยงimpl<F> Future for IntoServiceFuture<F>
impl<F> Future for IntoServiceFuture<F>
Sourceยงimpl<F> Future for OptionFuture<F>where
F: Future,
impl<F> Future for OptionFuture<F>where
F: Future,
Sourceยงimpl<F> Future for TryJoinAll<F>where
F: TryFuture,
impl<F> Future for TryJoinAll<F>where
F: TryFuture,
1.36.0 ยท Sourceยงimpl<F> Future for AssertUnwindSafe<F>where
F: Future,
impl<F> Future for AssertUnwindSafe<F>where
F: Future,
Sourceยงimpl<F> Future for Unconstrained<F>where
F: Future,
impl<F> Future for Unconstrained<F>where
F: Future,
Sourceยงimpl<F, C, H, T, E> Future for TrackCompletionFuture<F, C, H>
impl<F, C, H, T, E> Future for TrackCompletionFuture<F, C, H>
Sourceยงimpl<F, E> Future for rustmax::tower::spawn_ready::future::ResponseFuture<F, E>
impl<F, E> Future for rustmax::tower::spawn_ready::future::ResponseFuture<F, E>
Sourceยงimpl<F, N> Future for MapErrFuture<F, N>
impl<F, N> Future for MapErrFuture<F, N>
Sourceยงimpl<F, N> Future for MapResponseFuture<F, N>
impl<F, N> Future for MapResponseFuture<F, N>
Sourceยงimpl<F, N> Future for MapResultFuture<F, N>
impl<F, N> Future for MapResultFuture<F, N>
Sourceยงimpl<F, T, E> Future for rustmax::tower::buffer::future::ResponseFuture<F>
impl<F, T, E> Future for rustmax::tower::buffer::future::ResponseFuture<F>
Sourceยงimpl<F, T, E> Future for rustmax::tower::limit::concurrency::future::ResponseFuture<F>
impl<F, T, E> Future for rustmax::tower::limit::concurrency::future::ResponseFuture<F>
Sourceยงimpl<F, T, E> Future for rustmax::tower::load_shed::future::ResponseFuture<F>
impl<F, T, E> Future for rustmax::tower::load_shed::future::ResponseFuture<F>
Sourceยงimpl<F, T, E> Future for rustmax::tower::timeout::future::ResponseFuture<F>
impl<F, T, E> Future for rustmax::tower::timeout::future::ResponseFuture<F>
Sourceยงimpl<F, T, E> Future for rustmax::tower::util::future::optional::ResponseFuture<F>
impl<F, T, E> Future for rustmax::tower::util::future::optional::ResponseFuture<F>
Sourceยงimpl<F, T, E, ME> Future for rustmax::tower::reconnect::ResponseFuture<F, ME>
impl<F, T, E, ME> Future for rustmax::tower::reconnect::ResponseFuture<F, ME>
Sourceยงimpl<F, T, E, Req> Future for MakeFuture<F, Req>
impl<F, T, E, Req> Future for MakeFuture<F, Req>
Sourceยงimpl<Fut1, Fut2> Future for TryFlatten<Fut1, Fut2>where
TryFlatten<Fut1, Fut2>: Future,
impl<Fut1, Fut2> Future for TryFlatten<Fut1, Fut2>where
TryFlatten<Fut1, Fut2>: Future,
Sourceยงimpl<Fut> Future for TryMaybeDone<Fut>where
Fut: TryFuture,
impl<Fut> Future for TryMaybeDone<Fut>where
Fut: TryFuture,
Sourceยงimpl<Fut> Future for CatchUnwind<Fut>where
Fut: Future + UnwindSafe,
impl<Fut> Future for CatchUnwind<Fut>where
Fut: Future + UnwindSafe,
Sourceยงimpl<Fut> Future for IntoFuture<Fut>where
Fut: TryFuture,
impl<Fut> Future for IntoFuture<Fut>where
Fut: TryFuture,
Sourceยงimpl<Fut> Future for NeverError<Fut>
impl<Fut> Future for NeverError<Fut>
Sourceยงimpl<Fut, F> Future for InspectErr<Fut, F>
impl<Fut, F> Future for InspectErr<Fut, F>
Sourceยงimpl<Fut, F> Future for UnwrapOrElse<Fut, F>
impl<Fut, F> Future for UnwrapOrElse<Fut, F>
Sourceยงimpl<Fut, F, G> Future for MapOkOrElse<Fut, F, G>
impl<Fut, F, G> Future for MapOkOrElse<Fut, F, G>
Sourceยงimpl<I, B, S> Future for rustmax::hyper::server::conn::http1::Connection<I, S>
impl<I, B, S> Future for rustmax::hyper::server::conn::http1::Connection<I, S>
Sourceยงimpl<I, B, S> Future for rustmax::hyper::server::conn::http1::UpgradeableConnection<I, S>
impl<I, B, S> Future for rustmax::hyper::server::conn::http1::UpgradeableConnection<I, S>
Sourceยงimpl<I, B, S, E> Future for rustmax::hyper::server::conn::http2::Connection<I, S, E>
impl<I, B, S, E> Future for rustmax::hyper::server::conn::http2::Connection<I, S, E>
Sourceยงimpl<I, S, E, B> Future for hyper_util::server::conn::auto::Connection<'_, I, S, E>where
S: Service<Request<Incoming>, Response = Response<B>>,
<S as Service<Request<Incoming>>>::Future: 'static,
<S as Service<Request<Incoming>>>::Error: Into<Box<dyn Error + Send + Sync>>,
B: Body + 'static,
<B as Body>::Error: Into<Box<dyn Error + Send + Sync>>,
I: Read + Write + Unpin + 'static,
E: HttpServerConnExec<<S as Service<Request<Incoming>>>::Future, B>,
impl<I, S, E, B> Future for hyper_util::server::conn::auto::Connection<'_, I, S, E>where
S: Service<Request<Incoming>, Response = Response<B>>,
<S as Service<Request<Incoming>>>::Future: 'static,
<S as Service<Request<Incoming>>>::Error: Into<Box<dyn Error + Send + Sync>>,
B: Body + 'static,
<B as Body>::Error: Into<Box<dyn Error + Send + Sync>>,
I: Read + Write + Unpin + 'static,
E: HttpServerConnExec<<S as Service<Request<Incoming>>>::Future, B>,
Sourceยงimpl<I, S, E, B> Future for hyper_util::server::conn::auto::UpgradeableConnection<'_, I, S, E>where
S: Service<Request<Incoming>, Response = Response<B>>,
<S as Service<Request<Incoming>>>::Future: 'static,
<S as Service<Request<Incoming>>>::Error: Into<Box<dyn Error + Send + Sync>>,
B: Body + 'static,
<B as Body>::Error: Into<Box<dyn Error + Send + Sync>>,
I: Read + Write + Unpin + Send + 'static,
E: HttpServerConnExec<<S as Service<Request<Incoming>>>::Future, B>,
impl<I, S, E, B> Future for hyper_util::server::conn::auto::UpgradeableConnection<'_, I, S, E>where
S: Service<Request<Incoming>, Response = Response<B>>,
<S as Service<Request<Incoming>>>::Future: 'static,
<S as Service<Request<Incoming>>>::Error: Into<Box<dyn Error + Send + Sync>>,
B: Body + 'static,
<B as Body>::Error: Into<Box<dyn Error + Send + Sync>>,
I: Read + Write + Unpin + Send + 'static,
E: HttpServerConnExec<<S as Service<Request<Incoming>>>::Future, B>,
Sourceยงimpl<L, R> Future for rustmax::rayon::iter::Either<L, R>
impl<L, R> Future for rustmax::rayon::iter::Either<L, R>
Either<L, R>
is a future if both L
and R
are futures.