Remove spawning from task::Context

[rust.git] / src / libcore / future / future.rs

diff --git a/src/libcore/future/future.rs b/src/libcore/future/future.rs
index 520b6ebbbaaa59a072eda6529d6f22acd1e4249e..9176e0d32cbf2703c8a7d656614080caf6c8a320 100644 (file)
--- a/src/libcore/future/future.rs
+++ b/src/libcore/future/future.rs
@@ -12,9 +12,10 @@
             reason = "futures in libcore are unstable",
             issue = "50547")]
 
-use pin::PinMut;
 use marker::Unpin;
-use task::{self, Poll};
+use ops;
+use pin::Pin;
+use task::{Poll, LocalWaker};
 
 /// A future represents an asychronous computation.
 ///
@@ -49,18 +50,18 @@ pub trait Future {
     ///
     /// 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.
+    /// 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 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.
+    /// Note that on multiple calls to `poll`, only the most recent
+    /// [`LocalWaker`] passed to `poll` should be scheduled to receive a
+    /// wakeup.
     ///
     /// # Runtime characteristics
     ///
@@ -68,9 +69,9 @@ pub trait Future {
     /// 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
+    /// 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)`.
@@ -82,6 +83,16 @@ pub trait Future {
     /// 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`),
@@ -91,22 +102,29 @@ pub trait Future {
     ///
     /// [`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>;
+    /// [`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: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
-        F::poll(PinMut::new(&mut **self), cx)
+    fn poll(mut self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output> {
+        F::poll(Pin::new(&mut **self), lw)
     }
 }
 
-impl<'a, F: ?Sized + Future> Future for PinMut<'a, F> {
-    type Output = F::Output;
+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(mut self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
-        F::poll((*self).reborrow(), cx)
+    fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output> {
+        Pin::get_mut(self).as_mut().poll(lw)
     }
 }