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

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    #![stable(feature = "futures_api", since = "1.36.0")]

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

    /// 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 `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.
    ///
    /// [`Waker`]: ../task/struct.Waker.html
    #[doc(spotlight)]
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    #[stable(feature = "futures_api", since = "1.36.0")]
    #[lang = "future_trait"]
    pub trait Future {
        /// The type of value produced on completion.
        #[stable(feature = "futures_api", since = "1.36.0")]
        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`] 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* `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.
        ///
        /// # 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.
        ///
        /// [`Poll::Pending`]: ../task/enum.Poll.html#variant.Pending
        /// [`Poll::Ready(val)`]: ../task/enum.Poll.html#variant.Ready
        /// [`Context`]: ../task/struct.Context.html
        /// [`Waker`]: ../task/struct.Waker.html
        /// [`Waker::wake`]: ../task/struct.Waker.html#method.wake
        #[stable(feature = "futures_api", since = "1.36.0")]
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
    }

    #[stable(feature = "futures_api", since = "1.36.0")]
    impl<F: ?Sized + Future + Unpin> Future for &mut F {
        type Output = F::Output;

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

    #[stable(feature = "futures_api", since = "1.36.0")]
    impl<P> Future for Pin<P>
    where
        P: Unpin + ops::DerefMut<Target: Future>,
    {
        type Output = <<P as ops::Deref>::Target as Future>::Output;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            Pin::get_mut(self).as_mut().poll(cx)
        }
    }