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 mem::PinMut;
    use marker::Unpin;
    use task::{self, Poll};

    /// 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`. The future will *also* register the
        /// interest of the current task in the value being produced. For example,
        /// if the future represents the availability of data on a socket, then the
        /// task is recorded so that when data arrives, it is woken up (via
        /// [`cx.waker()`]). 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 if `Pending` is returned it only means that the *current* task
        /// (represented by the argument `cx`) will receive a notification. Tasks
        /// from previous calls to `poll` will *not* receive notifications.
        ///
        /// # 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 for
        /// futures, but only whenever the future itself is ready, as signaled via
        /// the `Waker` inside `task::Context`. 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.
        ///
        /// # 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
        /// [`cx.waker()`]: ../task/struct.Context.html#method.waker
        fn poll(self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output>;
    }

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

        fn poll(mut self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
            F::poll(PinMut::new(&mut **self), cx)
        }
    }

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

        fn poll(mut self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
            F::poll((*self).reborrow(), cx)
        }
    }

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