future.rs.html -- source
Description: Source to the Rust file `libcore/future/future.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2018 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    #![unstable(feature = "futures_api",
                reason = "futures in libcore are unstable",
                issue = "50547")]

    use marker::Unpin;
    use ops;
    use pin::Pin;
    use task::{Poll, LocalWaker};

    /// A future represents an asychronous 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
    /// `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.
    pub trait Future {
        /// The result of the `Future`.
        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 [`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* `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 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.
        ///
        /// [`Poll::Pending`]: ../task/enum.Poll.html#variant.Pending
        /// [`Poll::Ready(val)`]: ../task/enum.Poll.html#variant.Ready
        /// [`LocalWaker`]: ../task/struct.LocalWaker.html
        /// [`LocalWaker::into_waker`]: ../task/struct.LocalWaker.html#method.into_waker
        /// [`LocalWaker::wake`]: ../task/struct.LocalWaker.html#method.wake
        /// [`Waker`]: ../task/struct.Waker.html
        fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output>;
    }

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

        fn poll(mut self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output> {
            F::poll(Pin::new(&mut **self), lw)
        }
    }

    impl<P> Future for Pin<P>
    where
        P: ops::DerefMut,
        P::Target: Future,
    {
        type Output = <<P as ops::Deref>::Target as Future>::Output;

        fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output> {
            Pin::get_mut(self).as_mut().poll(lw)
        }
    }

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