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#![stable(feature = "futures_api", since = "1.36.0")]
use crate::ops;
use crate::pin::Pin;
use crate::task::{Context, Poll};
/// A future represents an asynchronous computation, commonly obtained by use of
/// [`async`].
///
/// A future is a value that might 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.
///
/// [`async`]: ../../std/keyword.async.html
/// [`Waker`]: crate::task::Waker
#[doc(notable_trait)]
#[doc(search_unbox)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[stable(feature = "futures_api", since = "1.36.0")]
#[lang = "future_trait"]
#[diagnostic::on_unimplemented(
label = "`{Self}` is not a future",
message = "`{Self}` is not a future"
)]
pub trait Future {
/// The type of value produced on completion.
#[stable(feature = "futures_api", since = "1.36.0")]
#[lang = "future_output"]
type Output;
/// Attempts 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 for the
/// underlying computation 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.
///
/// Having said that, some Futures may represent a value that is being
/// computed in a different task. In this case, the future's underlying
/// computation is simply acting as a conduit for a value being computed
/// by that other task, which will proceed independently of the Future.
/// Futures of this kind are typically obtained when spawning a new task into an
/// async runtime.
///
/// The `poll` function should not be 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 a while, 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::Ready(val)`]: Poll::Ready
/// [`Waker`]: crate::task::Waker
/// [`Waker::wake`]: crate::task::Waker::wake
#[lang = "poll"]
#[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: 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> {
<P::Target as Future>::poll(self.as_deref_mut(), cx)
}
}
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