[−][src]Trait futures::future::Future

#[must_use]

pub trait Future {
    type Output;
    fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output>;
}
🔬 This is a nightly-only experimental API. (futures_api)

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 cx.waker(), 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

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futures in libcore are unstable

The result of the Future.

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Required methods

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

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

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 LocalWaker to be woken once the future can make progress. For example, a future waiting for a socket to become readable would call .clone() on the LocalWaker and store it. When a signal arrives elsewhere indicating that the socket is readable, [LocalWaker::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 LocalWaker 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 must never 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.

LocalWaker, Waker and thread-safety

The poll function takes a LocalWaker, an object which knows how to awaken the current task. LocalWaker is not Send nor Sync, so in order to make thread-safe futures the LocalWaker::into_waker method should be used to convert the LocalWaker into a thread-safe version. LocalWaker::wake implementations have the ability to be more efficient, however, so when thread safety is not necessary, LocalWaker should be preferred.

Panics

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

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Implementations on Foreign Types

impl<'a, F> Future for AssertUnwindSafe<F> where
    F: Future, 
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type Output = <F as Future>::Output

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impl<'a, F> Future for &'a mut F where
    F: Unpin + Future + ?Sized, 
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type Output = <F as Future>::Output

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futures in libcore are unstable

impl<P> Future for Pin<P> where
    P: Unpin + DerefMut,
    <P as Deref>::Target: Future, 
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type Output = <<P as Deref>::Target as Future>::Output

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impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, 
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type Output = <F as Future>::Output

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futures in libcore are unstable

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

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impl<'a, St> Future for SelectNextSome<'a, St> where
    St: Unpin + Stream + FusedStream, 
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type Output = <St as Stream>::Item

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impl<'a, T> Future for BiLockAcquire<'a, T>
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type Output = BiLockGuard<'a, T>

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futures in libcore are unstable

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

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

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impl<Fut, T> Future for UnitError<Fut> where
    Fut: Future<Output = T>, 
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type Output = Result<T, ()>

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

futures in libcore are unstable

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Implementors

impl<'_, A> Future for ReadToEnd<'_, A> where
    A: AsyncRead + ?Sized, 
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type Output = Result<(), Error>

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impl<'_, R> Future for Read<'_, R> where
    R: AsyncRead + ?Sized, 
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type Output = Result<usize, Error>

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impl<'_, R> Future for ReadExact<'_, R> where
    R: AsyncRead + ?Sized, 
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type Output = Result<(), Error>

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futures in libcore are unstable

impl<'_, R, W> Future for CopyInto<'_, R, W> where
    R: AsyncRead + ?Sized,
    W: AsyncWrite + ?Sized, 
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type Output = Result<u64, Error>

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impl<'_, Si> Future for futures::sink::Close<'_, Si> where
    Si: Unpin + Sink + ?Sized, 
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type Output = Result<(), <Si as Sink>::SinkError>

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impl<'_, Si> Future for futures::sink::Flush<'_, Si> where
    Si: Unpin + Sink + ?Sized, 
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type Output = Result<(), <Si as Sink>::SinkError>

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

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impl<'_, Si> Future for Send<'_, Si> where
    Si: Unpin + Sink + ?Sized, 
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type Output = Result<(), <Si as Sink>::SinkError>

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

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impl<'_, Si, St> Future for SendAll<'_, Si, St> where
    Si: Unpin + Sink + ?Sized,
    St: Stream<Item = <Si as Sink>::SinkItem> + Unpin + ?Sized, 
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type Output = Result<(), <Si as Sink>::SinkError>

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

futures in libcore are unstable

impl<'_, St> Future for Next<'_, St> where
    St: Unpin + Stream, 
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type Output = Option<<St as Stream>::Item>

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

futures in libcore are unstable

impl<'_, St> Future for TryNext<'_, St> where
    St: Unpin + TryStream, 
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type Output = Result<Option<<St as TryStream>::Ok>, <St as TryStream>::Error>

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

futures in libcore are unstable

impl<'_, W> Future for futures::io::Close<'_, W> where
    W: AsyncWrite + ?Sized, 
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type Output = Result<(), Error>

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

futures in libcore are unstable

impl<'_, W> Future for futures::io::Flush<'_, W> where
    W: AsyncWrite + ?Sized, 
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type Output = Result<(), Error>

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

futures in libcore are unstable

impl<'_, W> Future for WriteAll<'_, W> where
    W: AsyncWrite + ?Sized, 
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type Output = Result<(), Error>

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

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impl<'a, T> Future for FutureObj<'a, T>
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type Output = T

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impl<'a, T> Future for LocalFutureObj<'a, T>
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type Output = T

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

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impl<'a, T> Future for MutexLockFuture<'a, T>
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type Output = MutexGuard<'a, T>

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impl<F> Future for JoinAll<F> where
    F: Future + Unpin,
    <F as Future>::Output: Unpin, 
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type Output = Vec<<F as Future>::Output>

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impl<F> Future for OptionFuture<F> where
    F: Future, 
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type Output = Option<<F as Future>::Output>

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impl<Fut> Future for MaybeDone<Fut> where
    Fut: Future, 
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type Output = ()

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impl<Fut> Future for Compat01As03<Fut> where
    Fut: Future, 
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type Output = Result<<Fut as Future>::Item, <Fut as Future>::Error>

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impl<Fut> Future for Abortable<Fut> where
    Fut: Future, 
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type Output = Result<<Fut as Future>::Output, Aborted>

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impl<Fut> Future for CatchUnwind<Fut> where
    Fut: Future + UnwindSafe, 
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type Output = Result<<Fut as Future>::Output, Box<dyn Any + 'static + Send>>

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impl<Fut> Future for Flatten<Fut> where
    Fut: Future,
    <Fut as Future>::Output: Future, 
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type Output = <<Fut as Future>::Output as Future>::Output

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impl<Fut> Future for Fuse<Fut> where
    Fut: Future, 
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type Output = <Fut as Future>::Output

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impl<Fut> Future for IntoFuture<Fut> where
    Fut: TryFuture, 
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type Output = Result<<Fut as TryFuture>::Ok, <Fut as TryFuture>::Error>

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impl<Fut> Future for Remote<Fut> where
    Fut: Future, 
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type Output = ()

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

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impl<Fut> Future for Shared<Fut> where
    Fut: Future,
    <Fut as Future>::Output: Clone, 
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type Output = <Fut as Future>::Output

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impl<Fut, E> Future for ErrInto<Fut, E> where
    Fut: TryFuture,
    <Fut as TryFuture>::Error: Into<E>, 
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type Output = Result<<Fut as TryFuture>::Ok, E>

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impl<Fut, F> Future for Inspect<Fut, F> where
    F: FnOnce(&<Fut as Future>::Output),
    Fut: Future, 
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type Output = <Fut as Future>::Output

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impl<Fut, F> Future for UnwrapOrElse<Fut, F> where
    F: FnOnce(<Fut as TryFuture>::Error) -> <Fut as TryFuture>::Ok,
    Fut: TryFuture, 
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type Output = <Fut as TryFuture>::Ok

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impl<Fut, F, E> Future for MapErr<Fut, F> where
    F: FnOnce(<Fut as TryFuture>::Error) -> E,
    Fut: TryFuture, 
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type Output = Result<<Fut as TryFuture>::Ok, E>

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futures in libcore are unstable

impl<Fut, F, T> Future for Map<Fut, F> where
    F: FnOnce(<Fut as Future>::Output) -> T,
    Fut: Future, 
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type Output = T

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impl<Fut, F, T> Future for MapOk<Fut, F> where
    F: FnOnce(<Fut as TryFuture>::Ok) -> T,
    Fut: TryFuture, 
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type Output = Result<T, <Fut as TryFuture>::Error>

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impl<Fut1, Fut2> Future for Join<Fut1, Fut2> where
    Fut1: Future,
    Fut2: Future, 
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type Output = (<Fut1 as Future>::Output, <Fut2 as Future>::Output)

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futures in libcore are unstable

impl<Fut1, Fut2> Future for TryJoin<Fut1, Fut2> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>, 
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type Output = Result<(<Fut1 as TryFuture>::Ok, <Fut2 as TryFuture>::Ok), <Fut1 as TryFuture>::Error>

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

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futures in libcore are unstable

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

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

futures in libcore are unstable

impl<Fut1, Fut2, F> Future for Then<Fut1, Fut2, F> where
    F: FnOnce(<Fut1 as Future>::Output) -> Fut2,
    Fut1: Future,
    Fut2: Future, 
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type Output = <Fut2 as Future>::Output

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

futures in libcore are unstable

impl<Fut1, Fut2, Fut3> Future for Join3<Fut1, Fut2, Fut3> where
    Fut1: Future,
    Fut2: Future,
    Fut3: Future, 
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type Output = (<Fut1 as Future>::Output, <Fut2 as Future>::Output, <Fut3 as Future>::Output)

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

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impl<Fut1, Fut2, Fut3, Fut4> Future for Join4<Fut1, Fut2, Fut3, Fut4> where
    Fut1: Future,
    Fut2: Future,
    Fut3: Future,
    Fut4: Future, 
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type Output = (<Fut1 as Future>::Output, <Fut2 as Future>::Output, <Fut3 as Future>::Output, <Fut4 as Future>::Output)

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futures in libcore are unstable

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

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futures in libcore are unstable

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

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futures in libcore are unstable

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>, 
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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>

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impl<R, F> Future for Lazy<F> where
    F: FnOnce(&LocalWaker) -> R, 
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type Output = R

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futures in libcore are unstable

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, 
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type Output = <St as Stream>::Item

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impl<St> Future for StreamFuture<St> where
    St: Unpin + Stream, 
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type Output = (Option<<St as Stream>::Item>, St)

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impl<St, C> Future for Collect<St, C> where
    C: Default + Extend<<St as Stream>::Item>,
    St: Stream, 
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type Output = C

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futures in libcore are unstable

impl<St, C> Future for TryCollect<St, C> where
    C: Default + Extend<<St as TryStream>::Ok>,
    St: TryStream, 
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type Output = Result<C, <St as TryStream>::Error>

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futures in libcore are unstable

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

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futures in libcore are unstable

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

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futures in libcore are unstable

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, 
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type Output = T

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futures in libcore are unstable

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

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futures in libcore are unstable

impl<St, Si> Future for Forward<St, Si> where
    Si: Sink + Unpin,
    St: Stream<Item = Result<<Si as Sink>::SinkItem, <Si as Sink>::SinkError>>, 
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type Output = Result<Si, <Si as Sink>::SinkError>

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impl<T> Future for Receiver<T>
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type Output = Result<T, Canceled>

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futures in libcore are unstable

impl<T> Future for Empty<T>
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type Output = T

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

futures in libcore are unstable

impl<T> Future for Ready<T>
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type Output = T

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futures in libcore are unstable

impl<T> Future for RemoteHandle<T> where
    T: 'static + Send, 
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type Output = T

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

futures in libcore are unstable

impl<T, F> Future for PollFn<F> where
    F: FnMut(&LocalWaker) -> Poll<T>, 
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type Output = T

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

futures in libcore are unstable

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