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#![unstable(feature = "futures_api",
reason = "futures in libcore are unstable",
issue = "50547")]
use marker::Unpin;
use ops;
use pin::Pin;
use task::{Poll, Waker};
/// 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 `poll`ing again. The wake up is performed using
/// the `waker` argument of the `poll()` method, 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.
#[doc(spotlight)]
#[must_use = "futures do nothing unless polled"]
pub trait Future {
/// The type of value produced on completion.
type 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`] 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 [`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 most recent
/// [`Waker`] passed to `poll` should be scheduled to receive a
/// wakeup.
///
/// # Runtime 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 -- 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.
///
/// An implementation of `poll` may also never cause memory unsafety.
///
/// # Panics
///
/// Once a future has completed (returned `Ready` from `poll`),
/// then any future calls to `poll` may panic, block forever, or otherwise
/// cause any kind of bad behavior except causing memory unsafety.
/// The `Future` trait itself provides no guarantees about the behavior
/// of `poll` after a future has completed.
///
/// [`Poll::Pending`]: ../task/enum.Poll.html#variant.Pending
/// [`Poll::Ready(val)`]: ../task/enum.Poll.html#variant.Ready
/// [`Waker`]: ../task/struct.Waker.html
/// [`Waker::wake`]: ../task/struct.Waker.html#method.wake
fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output>;
}
impl<F: ?Sized + Future + Unpin> Future for &mut F {
type Output = F::Output;
fn poll(mut self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output> {
F::poll(Pin::new(&mut **self), waker)
}
}
impl<P> Future for Pin<P>
where
P: Unpin + ops::DerefMut,
P::Target: Future,
{
type Output = <<P as ops::Deref>::Target as Future>::Output;
fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output> {
Pin::get_mut(self).as_mut().poll(waker)
}
}
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