1.36.0[][src]Trait futures::future::Future

#[must_use = "futures do nothing unless you `.await` or poll them"]
#[lang = "future_trait"]
pub trait Future {
    type Output;
    fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output>;
}

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 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 won't call poll directly, but instead .await the value.

Associated Types

type Output

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>

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 Waker from the Context passed to the most recent call 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.

Panics

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, Rust's usual rules apply: calls must never cause undefined behavior (memory corruption, incorrect use of unsafe functions, or the like), regardless of the future's state.

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Trait Implementations

impl<'a, T> UnsafeFutureObj<'a, T> for &'a mut (dyn Future<Output = T> + 'a + Unpin)[src]

Implementations on Foreign Types

impl<F> Future for AssertUnwindSafe<F> where
    F: Future
[src]

type Output = <F as Future>::Output

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

type Output = <F as Future>::Output

impl<P> Future for Pin<P> where
    P: Unpin + DerefMut,
    <P as Deref>::Target: Future
[src]

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

impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized
[src]

type Output = <F as Future>::Output

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Implementors

impl<'_, A> Future for ReadToEnd<'_, A> where
    A: AsyncRead + Unpin + ?Sized
[src]

type Output = Result<usize, Error>

impl<'_, A> Future for ReadToString<'_, A> where
    A: AsyncRead + Unpin + ?Sized
[src]

type Output = Result<usize, Error>

impl<'_, R> Future for Read<'_, R> where
    R: Unpin + AsyncRead + ?Sized
[src]

type Output = Result<usize, Error>

impl<'_, R> Future for ReadExact<'_, R> where
    R: Unpin + AsyncRead + ?Sized
[src]

type Output = Result<(), Error>

impl<'_, R> Future for ReadLine<'_, R> where
    R: Unpin + AsyncBufRead + ?Sized
[src]

type Output = Result<usize, Error>

impl<'_, R> Future for ReadUntil<'_, R> where
    R: Unpin + AsyncBufRead + ?Sized
[src]

type Output = Result<usize, Error>

impl<'_, R> Future for ReadVectored<'_, R> where
    R: Unpin + AsyncRead + ?Sized
[src]

type Output = Result<usize, Error>

impl<'_, R, W> Future for CopyBufInto<'_, R, W> where
    R: AsyncBufRead,
    W: AsyncWrite + Unpin + ?Sized
[src]

type Output = Result<u64, Error>

impl<'_, R, W> Future for CopyInto<'_, R, W> where
    R: AsyncRead,
    W: Unpin + AsyncWrite + ?Sized
[src]

type Output = Result<u64, Error>

impl<'_, S> Future for Seek<'_, S> where
    S: Unpin + AsyncSeek + ?Sized
[src]

type Output = Result<u64, Error>

impl<'_, Si, Item> Future for futures::sink::Close<'_, Si, Item> where
    Si: Unpin + Sink<Item> + ?Sized
[src]

type Output = Result<(), <Si as Sink<Item>>::Error>

impl<'_, Si, Item> Future for futures::sink::Flush<'_, Si, Item> where
    Si: Unpin + Sink<Item> + ?Sized
[src]

type Output = Result<(), <Si as Sink<Item>>::Error>

impl<'_, Si, Item> Future for Send<'_, Si, Item> where
    Si: Unpin + Sink<Item> + ?Sized
[src]

type Output = Result<(), <Si as Sink<Item>>::Error>

impl<'_, Si, St> Future for SendAll<'_, Si, St> where
    Si: Sink<<St as Stream>::Item> + Unpin + ?Sized,
    St: Stream + Unpin + ?Sized
[src]

type Output = Result<(), <Si as Sink<<St as Stream>::Item>>::Error>

impl<'_, St> Future for Next<'_, St> where
    St: Unpin + Stream + ?Sized
[src]

type Output = Option<<St as Stream>::Item>

impl<'_, St> Future for SelectNextSome<'_, St> where
    St: Unpin + FusedStream + ?Sized
[src]

type Output = <St as Stream>::Item

impl<'_, St> Future for TryNext<'_, St> where
    St: Unpin + TryStream + ?Sized
[src]

type Output = Result<Option<<St as TryStream>::Ok>, <St as TryStream>::Error>

impl<'_, T> Future for FutureObj<'_, T>[src]

type Output = T

impl<'_, T> Future for LocalFutureObj<'_, T>[src]

type Output = T

impl<'_, W> Future for futures::io::Close<'_, W> where
    W: Unpin + AsyncWrite + ?Sized
[src]

type Output = Result<(), Error>

impl<'_, W> Future for futures::io::Flush<'_, W> where
    W: AsyncWrite + Unpin + ?Sized
[src]

type Output = Result<(), Error>

impl<'_, W> Future for Write<'_, W> where
    W: Unpin + AsyncWrite + ?Sized
[src]

type Output = Result<usize, Error>

impl<'_, W> Future for WriteAll<'_, W> where
    W: Unpin + AsyncWrite + ?Sized
[src]

type Output = Result<(), Error>

impl<'_, W> Future for WriteVectored<'_, W> where
    W: Unpin + AsyncWrite + ?Sized
[src]

type Output = Result<usize, Error>

impl<'a, T> Future for MutexLockFuture<'a, T> where
    T: ?Sized
[src]

type Output = MutexGuard<'a, T>

impl<A, B> Future for Either<A, B> where
    A: Future,
    B: Future<Output = <A as Future>::Output>, 
[src]

type Output = <A as Future>::Output

impl<A, B> Future for Select<A, B> where
    A: Future + Unpin,
    B: Future + Unpin
[src]

type Output = Either<(<A as Future>::Output, B), (<B as Future>::Output, A)>

impl<A, B> Future for TrySelect<A, B> where
    A: Unpin + TryFuture,
    B: Unpin + TryFuture
[src]

type Output = Result<Either<(<A as TryFuture>::Ok, B), (<B as TryFuture>::Ok, A)>, Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::Error, A)>>

impl<F> Future for JoinAll<F> where
    F: Future
[src]

type Output = Vec<<F as Future>::Output>

impl<F> Future for OptionFuture<F> where
    F: Future
[src]

type Output = Option<<F as Future>::Output>

impl<F> Future for TryJoinAll<F> where
    F: TryFuture
[src]

type Output = Result<Vec<<F as TryFuture>::Ok>, <F as TryFuture>::Error>

impl<F, R> Future for Lazy<F> where
    F: FnOnce(&mut Context) -> R, 
[src]

type Output = R

impl<Fut> Future for MaybeDone<Fut> where
    Fut: Future
[src]

type Output = ()

impl<Fut> Future for Compat01As03<Fut> where
    Fut: Future
[src]

type Output = Result<<Fut as Future>::Item, <Fut as Future>::Error>

impl<Fut> Future for Abortable<Fut> where
    Fut: Future
[src]

type Output = Result<<Fut as Future>::Output, Aborted>

impl<Fut> Future for CatchUnwind<Fut> where
    Fut: Future + UnwindSafe
[src]

type Output = Result<<Fut as Future>::Output, Box<dyn Any + 'static + Send>>

impl<Fut> Future for Flatten<Fut> where
    Fut: Future,
    <Fut as Future>::Output: Future
[src]

type Output = <<Fut as Future>::Output as Future>::Output

impl<Fut> Future for Fuse<Fut> where
    Fut: Future
[src]

type Output = <Fut as Future>::Output

impl<Fut> Future for IntoFuture<Fut> where
    Fut: TryFuture
[src]

type Output = Result<<Fut as TryFuture>::Ok, <Fut as TryFuture>::Error>

impl<Fut> Future for Remote<Fut> where
    Fut: Future
[src]

type Output = ()

impl<Fut> Future for SelectAll<Fut> where
    Fut: Unpin + Future
[src]

type Output = (<Fut as Future>::Output, usize, Vec<Fut>)

impl<Fut> Future for SelectOk<Fut> where
    Fut: Unpin + TryFuture
[src]

type Output = Result<(<Fut as TryFuture>::Ok, Vec<Fut>), <Fut as TryFuture>::Error>

impl<Fut> Future for Shared<Fut> where
    Fut: Future,
    <Fut as Future>::Output: Clone
[src]

type Output = <Fut as Future>::Output

impl<Fut, E> Future for ErrInto<Fut, E> where
    Fut: TryFuture,
    <Fut as TryFuture>::Error: Into<E>, 
[src]

type Output = Result<<Fut as TryFuture>::Ok, E>

impl<Fut, F> Future for Inspect<Fut, F> where
    F: FnOnce(&<Fut as Future>::Output),
    Fut: Future
[src]

type Output = <Fut as Future>::Output

impl<Fut, F> Future for InspectErr<Fut, F> where
    F: FnOnce(&<Fut as TryFuture>::Error),
    Fut: TryFuture
[src]

type Output = Result<<Fut as TryFuture>::Ok, <Fut as TryFuture>::Error>

impl<Fut, F> Future for InspectOk<Fut, F> where
    F: FnOnce(&<Fut as TryFuture>::Ok),
    Fut: TryFuture
[src]

type Output = Result<<Fut as TryFuture>::Ok, <Fut as TryFuture>::Error>

impl<Fut, F> Future for UnwrapOrElse<Fut, F> where
    F: FnOnce(<Fut as TryFuture>::Error) -> <Fut as TryFuture>::Ok,
    Fut: TryFuture
[src]

type Output = <Fut as TryFuture>::Ok

impl<Fut, F, E> Future for MapErr<Fut, F> where
    F: FnOnce(<Fut as TryFuture>::Error) -> E,
    Fut: TryFuture
[src]

type Output = Result<<Fut as TryFuture>::Ok, E>

impl<Fut, F, T> Future for Map<Fut, F> where
    F: FnOnce(<Fut as Future>::Output) -> T,
    Fut: Future
[src]

type Output = T

impl<Fut, F, T> Future for MapOk<Fut, F> where
    F: FnOnce(<Fut as TryFuture>::Ok) -> T,
    Fut: TryFuture
[src]

type Output = Result<T, <Fut as TryFuture>::Error>

impl<Fut, T> Future for NeverError<Fut> where
    Fut: Future<Output = T>, 
[src]

type Output = Result<T, Infallible>

impl<Fut, T> Future for UnitError<Fut> where
    Fut: Future<Output = T>, 
[src]

type Output = Result<T, ()>

impl<Fut1, Fut2> Future for Join<Fut1, Fut2> where
    Fut1: Future,
    Fut2: Future
[src]

type Output = (<Fut1 as Future>::Output, <Fut2 as Future>::Output)

impl<Fut1, Fut2> Future for TryJoin<Fut1, Fut2> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>, 
[src]

type Output = Result<(<Fut1 as TryFuture>::Ok, <Fut2 as TryFuture>::Ok), <Fut1 as TryFuture>::Error>

impl<Fut1, Fut2, F> Future for AndThen<Fut1, Fut2, F> where
    F: FnOnce(<Fut1 as TryFuture>::Ok) -> Fut2,
    Fut1: TryFuture,
    Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>, 
[src]

type Output = Result<<Fut2 as TryFuture>::Ok, <Fut2 as TryFuture>::Error>

impl<Fut1, Fut2, F> Future for OrElse<Fut1, Fut2, F> where
    F: FnOnce(<Fut1 as TryFuture>::Error) -> Fut2,
    Fut1: TryFuture,
    Fut2: TryFuture<Ok = <Fut1 as TryFuture>::Ok>, 
[src]

type Output = Result<<Fut2 as TryFuture>::Ok, <Fut2 as TryFuture>::Error>

impl<Fut1, Fut2, F> Future for Then<Fut1, Fut2, F> where
    F: FnOnce(<Fut1 as Future>::Output) -> Fut2,
    Fut1: Future,
    Fut2: Future
[src]

type Output = <Fut2 as Future>::Output

impl<Fut1, Fut2, Fut3> Future for Join3<Fut1, Fut2, Fut3> where
    Fut1: Future,
    Fut2: Future,
    Fut3: Future
[src]

type Output = (<Fut1 as Future>::Output, <Fut2 as Future>::Output, <Fut3 as Future>::Output)

impl<Fut1, Fut2, Fut3> Future for TryJoin3<Fut1, Fut2, Fut3> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>,
    Fut3: TryFuture<Error = <Fut1 as TryFuture>::Error>, 
[src]

type Output = Result<(<Fut1 as TryFuture>::Ok, <Fut2 as TryFuture>::Ok, <Fut3 as TryFuture>::Ok), <Fut1 as TryFuture>::Error>

impl<Fut1, Fut2, Fut3, Fut4> Future for Join4<Fut1, Fut2, Fut3, Fut4> where
    Fut1: Future,
    Fut2: Future,
    Fut3: Future,
    Fut4: Future
[src]

type Output = (<Fut1 as Future>::Output, <Fut2 as Future>::Output, <Fut3 as Future>::Output, <Fut4 as Future>::Output)

impl<Fut1, Fut2, Fut3, Fut4> Future for TryJoin4<Fut1, Fut2, Fut3, Fut4> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>,
    Fut3: TryFuture<Error = <Fut1 as TryFuture>::Error>,
    Fut4: TryFuture<Error = <Fut1 as TryFuture>::Error>, 
[src]

type Output = Result<(<Fut1 as TryFuture>::Ok, <Fut2 as TryFuture>::Ok, <Fut3 as TryFuture>::Ok, <Fut4 as TryFuture>::Ok), <Fut1 as TryFuture>::Error>

impl<Fut1, Fut2, Fut3, Fut4, Fut5> Future for Join5<Fut1, Fut2, Fut3, Fut4, Fut5> where
    Fut1: Future,
    Fut2: Future,
    Fut3: Future,
    Fut4: Future,
    Fut5: Future
[src]

type Output = (<Fut1 as Future>::Output, <Fut2 as Future>::Output, <Fut3 as Future>::Output, <Fut4 as Future>::Output, <Fut5 as Future>::Output)

impl<Fut1, Fut2, Fut3, Fut4, Fut5> Future for TryJoin5<Fut1, Fut2, Fut3, Fut4, Fut5> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>,
    Fut3: TryFuture<Error = <Fut1 as TryFuture>::Error>,
    Fut4: TryFuture<Error = <Fut1 as TryFuture>::Error>,
    Fut5: TryFuture<Error = <Fut1 as TryFuture>::Error>, 
[src]

type Output = Result<(<Fut1 as TryFuture>::Ok, <Fut2 as TryFuture>::Ok, <Fut3 as TryFuture>::Ok, <Fut4 as TryFuture>::Ok, <Fut5 as TryFuture>::Ok), <Fut1 as TryFuture>::Error>

impl<St> Future for Concat<St> where
    St: Stream,
    <St as Stream>::Item: Extend<<<St as Stream>::Item as IntoIterator>::Item>,
    <St as Stream>::Item: IntoIterator,
    <St as Stream>::Item: Default
[src]

type Output = <St as Stream>::Item

impl<St> Future for StreamFuture<St> where
    St: Unpin + Stream
[src]

type Output = (Option<<St as Stream>::Item>, St)

impl<St> Future for TryConcat<St> where
    St: TryStream,
    <St as TryStream>::Ok: Extend<<<St as TryStream>::Ok as IntoIterator>::Item>,
    <St as TryStream>::Ok: IntoIterator,
    <St as TryStream>::Ok: Default
[src]

type Output = Result<<St as TryStream>::Ok, <St as TryStream>::Error>

impl<St, C> Future for Collect<St, C> where
    C: Default + Extend<<St as Stream>::Item>,
    St: Stream
[src]

type Output = C

impl<St, C> Future for TryCollect<St, C> where
    C: Default + Extend<<St as TryStream>::Ok>,
    St: TryStream
[src]

type Output = Result<C, <St as TryStream>::Error>

impl<St, Fut, F> Future for ForEach<St, Fut, F> where
    F: FnMut(<St as Stream>::Item) -> Fut,
    Fut: Future<Output = ()>,
    St: Stream
[src]

type Output = ()

impl<St, Fut, F> Future for ForEachConcurrent<St, Fut, F> where
    F: FnMut(<St as Stream>::Item) -> Fut,
    Fut: Future<Output = ()>,
    St: Stream
[src]

type Output = ()

impl<St, Fut, F> Future for TryForEach<St, Fut, F> where
    F: FnMut(<St as TryStream>::Ok) -> Fut,
    Fut: TryFuture<Ok = (), Error = <St as TryStream>::Error>,
    St: TryStream
[src]

type Output = Result<(), <St as TryStream>::Error>

impl<St, Fut, F> Future for TryForEachConcurrent<St, Fut, F> where
    F: FnMut(<St as TryStream>::Ok) -> Fut,
    Fut: Future<Output = Result<(), <St as TryStream>::Error>>,
    St: TryStream
[src]

type Output = Result<(), <St as TryStream>::Error>

impl<St, Fut, T, F> Future for Fold<St, Fut, T, F> where
    F: FnMut(T, <St as Stream>::Item) -> Fut,
    Fut: Future<Output = T>,
    St: Stream
[src]

type Output = T

impl<St, Fut, T, F> Future for TryFold<St, Fut, T, F> where
    F: FnMut(T, <St as TryStream>::Ok) -> Fut,
    Fut: TryFuture<Ok = T, Error = <St as TryStream>::Error>,
    St: TryStream
[src]

type Output = Result<T, <St as TryStream>::Error>

impl<St, Si, Item, E> Future for Forward<St, Si> where
    Si: Sink<Item, Error = E>,
    St: Stream<Item = Result<Item, E>>, 
[src]

type Output = Result<(), E>

impl<T> Future for Receiver<T>[src]

type Output = Result<T, Canceled>

impl<T> Future for Pending<T>[src]

type Output = T

impl<T> Future for Ready<T>[src]

type Output = T

impl<T> Future for RemoteHandle<T> where
    T: 'static + Send
[src]

type Output = T

impl<T, F> Future for PollFn<F> where
    F: FnMut(&mut Context) -> Poll<T>, 
[src]

type Output = T

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