Trait futures::future::Future

1.36.0 · source ·
pub trait Future {
    type Output;

    // Required method
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}
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 “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.

Required Associated Types§

1.36.0 · source

type Output

The type of value produced on completion.

Required Methods§

1.36.0 · source

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 a while, 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.

Trait Implementations§

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

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fn into_raw(self) -> *mut dyn Future<Output = T> + 'a

Convert an owned instance into a (conceptually owned) fat pointer. Read more
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unsafe fn drop(_ptr: *mut dyn Future<Output = T> + 'a)

Drops the future represented by the given fat pointer. Read more
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impl<'a, T> UnsafeFutureObj<'a, T> for Box<dyn Future<Output = T> + 'a>
where T: 'a,

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fn into_raw(self) -> *mut dyn Future<Output = T> + 'a

Convert an owned instance into a (conceptually owned) fat pointer. Read more
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unsafe fn drop(ptr: *mut dyn Future<Output = T> + 'a)

Drops the future represented by the given fat pointer. Read more
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impl<'a, T> UnsafeFutureObj<'a, T> for Box<dyn Future<Output = T> + Send + 'a>
where T: 'a,

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fn into_raw(self) -> *mut dyn Future<Output = T> + 'a

Convert an owned instance into a (conceptually owned) fat pointer. Read more
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unsafe fn drop(ptr: *mut dyn Future<Output = T> + 'a)

Drops the future represented by the given fat pointer. Read more
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impl<'a, T> UnsafeFutureObj<'a, T> for Pin<&'a mut dyn Future<Output = T>>

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fn into_raw(self) -> *mut dyn Future<Output = T> + 'a

Convert an owned instance into a (conceptually owned) fat pointer. Read more
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unsafe fn drop(_ptr: *mut dyn Future<Output = T> + 'a)

Drops the future represented by the given fat pointer. Read more

Implementors§

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impl<'a, R> Future for FillBuf<'a, R>
where R: AsyncBufRead + Unpin + ?Sized,

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type Output = Result<&'a [u8], Error>

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impl<'a, St> Future for Peek<'a, St>
where St: Stream,

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type Output = Option<&'a <St as Stream>::Item>

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impl<'a, St> Future for PeekMut<'a, St>
where St: Stream,

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type Output = Option<&'a mut <St as Stream>::Item>

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

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type Output = MutexGuard<'a, T>

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

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

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

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type Output = <A as Future>::Output

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

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

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

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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)>>

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impl<F> Future for &mut F
where F: Future + Unpin + ?Sized,

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type Output = <F as Future>::Output

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impl<F> Future for AssertUnwindSafe<F>
where F: Future,

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type Output = <F as Future>::Output

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impl<F> Future for Flatten<F>
where Flatten<F, <F as Future>::Output>: Future, F: Future,

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type Output = <Flatten<F, <F as Future>::Output> as Future>::Output

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impl<F> Future for JoinAll<F>
where F: Future,

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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<F> Future for TryJoinAll<F>
where F: TryFuture,

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

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impl<F, A> Future for Box<F, A>
where F: Future + Unpin + ?Sized, A: Allocator,

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type Output = <F as Future>::Output

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impl<F, R> Future for Lazy<F>
where F: FnOnce(&mut Context<'_>) -> R,

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type Output = R

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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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impl<Fut1, Fut2> Future for TryFlatten<Fut1, Fut2>
where TryFlatten<Fut1, Fut2>: Future,

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type Output = <TryFlatten<Fut1, Fut2> as Future>::Output

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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 TryFlatten<MapOk<Fut1, F>, Fut2>: Future,

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type Output = <TryFlatten<MapOk<Fut1, F>, Fut2> as Future>::Output

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impl<Fut1, Fut2, F> Future for OrElse<Fut1, Fut2, F>
where TryFlattenErr<MapErr<Fut1, F>, Fut2>: Future,

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type Output = <TryFlattenErr<MapErr<Fut1, F>, Fut2> as Future>::Output

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impl<Fut1, Fut2, F> Future for Then<Fut1, Fut2, F>
where Flatten<Map<Fut1, F>, Fut2>: Future,

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type Output = <Flatten<Map<Fut1, F>, Fut2> as Future>::Output

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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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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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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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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<Fut> Future for MaybeDone<Fut>
where Fut: Future,

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type Output = ()

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impl<Fut> Future for TryMaybeDone<Fut>
where Fut: TryFuture,

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type Output = Result<(), <Fut as TryFuture>::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 + Send>>

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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 NeverError<Fut>
where Map<Fut, OkFn<Infallible>>: Future,

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type Output = <Map<Fut, OkFn<Infallible>> as Future>::Output

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impl<Fut> Future for Remote<Fut>
where Fut: Future,

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type Output = ()

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impl<Fut> Future for SelectAll<Fut>
where Fut: Future + Unpin,

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

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impl<Fut> Future for SelectOk<Fut>
where Fut: TryFuture + Unpin,

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

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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> Future for UnitError<Fut>
where Map<Fut, OkFn<()>>: Future,

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type Output = <Map<Fut, OkFn<()>> as Future>::Output

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impl<Fut, E> Future for ErrInto<Fut, E>
where MapErr<Fut, IntoFn<E>>: Future,

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type Output = <MapErr<Fut, IntoFn<E>> as Future>::Output

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impl<Fut, E> Future for OkInto<Fut, E>
where MapOk<Fut, IntoFn<E>>: Future,

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type Output = <MapOk<Fut, IntoFn<E>> as Future>::Output

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impl<Fut, F> Future for Inspect<Fut, F>
where Map<Fut, InspectFn<F>>: Future,

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type Output = <Map<Fut, InspectFn<F>> as Future>::Output

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impl<Fut, F> Future for InspectErr<Fut, F>
where Inspect<IntoFuture<Fut>, InspectErrFn<F>>: Future,

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type Output = <Inspect<IntoFuture<Fut>, InspectErrFn<F>> as Future>::Output

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impl<Fut, F> Future for InspectOk<Fut, F>
where Inspect<IntoFuture<Fut>, InspectOkFn<F>>: Future,

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type Output = <Inspect<IntoFuture<Fut>, InspectOkFn<F>> as Future>::Output

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impl<Fut, F> Future for Map<Fut, F>
where Map<Fut, F>: Future,

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type Output = <Map<Fut, F> as Future>::Output

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impl<Fut, F> Future for MapErr<Fut, F>
where Map<IntoFuture<Fut>, MapErrFn<F>>: Future,

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type Output = <Map<IntoFuture<Fut>, MapErrFn<F>> as Future>::Output

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impl<Fut, F> Future for MapOk<Fut, F>
where Map<IntoFuture<Fut>, MapOkFn<F>>: Future,

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type Output = <Map<IntoFuture<Fut>, MapOkFn<F>> as Future>::Output

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impl<Fut, F> Future for UnwrapOrElse<Fut, F>
where Map<IntoFuture<Fut>, UnwrapOrElseFn<F>>: Future,

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type Output = <Map<IntoFuture<Fut>, UnwrapOrElseFn<F>> as Future>::Output

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impl<Fut, F, G> Future for MapOkOrElse<Fut, F, G>
where Map<IntoFuture<Fut>, ChainFn<MapOkFn<F>, ChainFn<MapErrFn<G>, MergeResultFn>>>: Future,

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type Output = <Map<IntoFuture<Fut>, ChainFn<MapOkFn<F>, ChainFn<MapErrFn<G>, MergeResultFn>>> as Future>::Output

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impl<Fut, T> Future for MapInto<Fut, T>
where Map<Fut, IntoFn<T>>: Future,

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type Output = <Map<Fut, IntoFn<T>> as Future>::Output

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impl<P> Future for Pin<P>
where P: DerefMut, <P as Deref>::Target: Future,

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

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

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

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

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

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

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impl<R> Future for SeeKRelative<'_, R>
where R: AsyncRead + AsyncSeek,

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impl<R, W> Future for Copy<'_, R, W>
where R: AsyncRead, W: AsyncWrite + Unpin + ?Sized,

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impl<R, W> Future for CopyBuf<'_, R, W>
where R: AsyncBufRead, W: AsyncWrite + Unpin + ?Sized,

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impl<R, W> Future for CopyBufAbortable<'_, R, W>
where R: AsyncBufRead, W: AsyncWrite + Unpin,

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

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

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

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impl<Si, Item> Future for Feed<'_, Si, Item>
where Si: Sink<Item> + Unpin + ?Sized,

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

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

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

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

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

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impl<Si, St, Ok, Error> Future for SendAll<'_, Si, St>
where Si: Sink<Ok, Error = Error> + Unpin + ?Sized, St: Stream<Item = Result<Ok, Error>> + Unpin + ?Sized,

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type Output = Result<(), Error>

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

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type Output = <St as Stream>::Item

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impl<St> Future for Count<St>
where St: Stream,

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

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

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

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type Output = <St as Stream>::Item

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impl<St> Future for StreamFuture<St>
where St: Stream + Unpin,

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

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

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

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

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

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impl<St, A, B, FromA, FromB> Future for Unzip<St, FromA, FromB>
where St: Stream<Item = (A, B)>, FromA: Default + Extend<A>, FromB: Default + Extend<B>,

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

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type Output = C

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

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

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impl<St, F> Future for NextIf<'_, St, F>
where St: Stream, F: for<'a> FnOnce1<&'a <St as Stream>::Item, Output = bool>,

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

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

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type Output = bool

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

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type Output = bool

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

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type Output = ()

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

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type Output = ()

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impl<St, Fut, F> Future for TryAll<St, Fut, F>
where St: TryStream, F: FnMut(<St as TryStream>::Ok) -> Fut, Fut: Future<Output = bool>,

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

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impl<St, Fut, F> Future for TryAny<St, Fut, F>
where St: TryStream, F: FnMut(<St as TryStream>::Ok) -> Fut, Fut: Future<Output = bool>,

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

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

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

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

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

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

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type Output = T

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

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

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impl<St, Si> Future for Forward<St, Si>
where Forward<St, Si, <St as TryStream>::Ok>: Future, St: TryStream,

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type Output = <Forward<St, Si, <St as TryStream>::Ok> as Future>::Output

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impl<St, T> Future for NextIfEq<'_, St, T>
where St: Stream, <St as Stream>::Item: PartialEq<T>, T: ?Sized,

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

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impl<T> Future for Cancellation<'_, T>

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type Output = ()

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impl<T> Future for Receiver<T>

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impl<T> Future for OwnedMutexLockFuture<T>
where T: ?Sized,

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impl<T> Future for FutureObj<'_, T>

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type Output = T

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impl<T> Future for LocalFutureObj<'_, T>

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type Output = T

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impl<T> Future for AsyncDropInPlace<T>
where T: ?Sized,

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type Output = ()

1.48.0 · source§

impl<T> Future for core::future::pending::Pending<T>

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type Output = T

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impl<T> Future for core::future::ready::Ready<T>

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type Output = T

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impl<T> Future for Exclusive<T>
where T: Future + ?Sized,

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type Output = <T as Future>::Output

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impl<T> Future for futures::future::Pending<T>

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type Output = T

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impl<T> Future for futures::future::Ready<T>

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type Output = T

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impl<T> Future for RemoteHandle<T>
where T: 'static,

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type Output = T

1.64.0 · source§

impl<T, F> Future for core::future::poll_fn::PollFn<F>
where F: FnMut(&mut Context<'_>) -> Poll<T>,

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type Output = T

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impl<T, F> Future for AlwaysReady<T, F>
where F: Fn() -> T,

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type Output = T

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impl<T, F> Future for futures::future::PollFn<F>
where F: FnMut(&mut Context<'_>) -> Poll<T>,

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type Output = T

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impl<T, F> Future for PollImmediate<F>
where F: Future<Output = T>,

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type Output = Option<T>

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

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

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

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

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