Trait futures_util::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.

Loading content...

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:

• Poll::Pending if the future is not ready yet
• Poll::Ready(val) with the result val 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 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.

Loading content...

Trait Implementations

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

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

Convert an owned instance into a (conceptually owned) fat pointer.

unsafe fn drop(_ptr: *mut dyn Future<Output = T> + 'a)

Drops the future represented by the given fat pointer.

Implementations on Foreign Types

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

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

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

Loading content...

Implementors

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

type Output = <F as Future>::Output

fn poll(
    self: Pin<&mut &'_ mut F>,
    cx: &mut Context
) -> Poll<<&'_ mut F as Future>::Output>

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

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

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

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

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

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

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

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

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

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

type Output = Result<(), Error>

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

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

type Output = St::Item

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

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

type Output = Option<St::Item>

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

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

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

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

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

type Output = T

fn poll(self: Pin<&mut FutureObj<'_, T>>, cx: &mut Context) -> Poll<T>

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

type Output = T

fn poll(self: Pin<&mut LocalFutureObj<'_, T>>, cx: &mut Context) -> Poll<T>

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

type Output = A::Output

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

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

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

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

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

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

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

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

type Output = R

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

impl<F: Future> Future for OptionFuture<F>

type Output = Option<F::Output>

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

impl<Fut> Future for Flatten<Fut> where
    Fut: Future,
    Fut::Output: Future, 

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

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

impl<Fut, E> Future for ErrInto<Fut, E> where
    Fut: TryFuture,
    Fut::Error: Into<E>, 

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

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

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

type Output = Fut::Output

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

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

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

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

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

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

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

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

type Output = Fut::Ok

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

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

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

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

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

type Output = T

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

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

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

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

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

type Output = Result<T, Never>

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

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

type Output = Result<T, ()>

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

impl<Fut1, Fut2> Future for TryJoin<Fut1, Fut2> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = Fut1::Error>, 

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

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

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

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

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

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

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

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

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

type Output = Fut2::Output

fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Fut2::Output>

impl<Fut1, Fut2, Fut3> Future for TryJoin3<Fut1, Fut2, Fut3> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = Fut1::Error>,
    Fut3: TryFuture<Error = Fut1::Error>, 

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

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

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

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

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

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

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

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

impl<Fut1: Future, Fut2: Future> Future for Join<Fut1, Fut2>

type Output = (Fut1::Output, Fut2::Output)

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

impl<Fut1: Future, Fut2: Future, Fut3: Future> Future for Join3<Fut1, Fut2, Fut3>

type Output = (Fut1::Output, Fut2::Output, Fut3::Output)

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

impl<Fut1: Future, Fut2: Future, Fut3: Future, Fut4: Future> Future for Join4<Fut1, Fut2, Fut3, Fut4>

type Output = (Fut1::Output, Fut2::Output, Fut3::Output, Fut4::Output)

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

impl<Fut1: Future, Fut2: Future, Fut3: Future, Fut4: Future, Fut5: Future> Future for Join5<Fut1, Fut2, Fut3, Fut4, Fut5>

type Output = (Fut1::Output, Fut2::Output, Fut3::Output, Fut4::Output, Fut5::Output)

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

impl<Fut: Future> Future for MaybeDone<Fut>

type Output = ()

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

impl<Fut: Future> Future for Fuse<Fut>

type Output = Fut::Output

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

impl<Fut: TryFuture> Future for IntoFuture<Fut>

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

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

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

type Output = St::Item

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

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

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

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

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

type Output = C

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

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

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

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

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

type Output = ()

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

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

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

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

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

type Output = T

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

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

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

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

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

type Output = Result<(), E>

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

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

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

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

impl<T> Future for Pending<T>

type Output = T

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

impl<T> Future for Ready<T>

type Output = T

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

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

type Output = T

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

Loading content...