Replace unstable Waker::noop() with Waker::NOOP.

As discussed in
<https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Associated.20constants.20vs.2E.20functions.20in.20std.20API>,
across `std`, outside of argumentless `new()` constructor functions,
stable constant values are generally provided using `const` items rather
than `const fn`s. Therefore, this change is more consistent API design.

WG-async approves of making this change, per
<#98286 (comment)>.

Author: kpreid

library/alloc/src/task.rs

@@ -247,7 +247,7 @@ fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker {
 ///         // cast the Rc<Task> into a `LocalWaker`
 ///         let local_waker: LocalWaker = task.clone().into();
 ///         // Build the context using `ContextBuilder`
-///         let mut cx = ContextBuilder::from_waker(Waker::noop())
+///         let mut cx = ContextBuilder::from_waker(Waker::NOOP)
 ///             .local_waker(&local_waker)
 ///             .build();
 ///

library/core/src/task/wake.rs

@@ -284,7 +284,7 @@ impl fmt::Debug for Context<'_> {
 /// use std::future::Future;
 ///
 /// let local_waker = LocalWaker::noop();
-/// let waker = Waker::noop();
+/// let waker = Waker::NOOP;
 ///
 /// let mut cx = ContextBuilder::from_waker(&waker)
 ///     .local_waker(&local_waker)
@@ -465,7 +465,7 @@ impl Waker {
         Waker { waker }
     }
 
-    /// Returns a reference to a `Waker` that does nothing when used.
+    /// A reference to a `Waker` that does nothing when used.
     ///
     /// This is mostly useful for writing tests that need a [`Context`] to poll
     /// some futures, but are not expecting those futures to wake the waker or
@@ -481,18 +481,13 @@ impl Waker {
     /// use std::future::Future;
     /// use std::task;
     ///
-    /// let mut cx = task::Context::from_waker(task::Waker::noop());
+    /// let mut cx = task::Context::from_waker(task::Waker::NOOP);
     ///
     /// let mut future = Box::pin(async { 10 });
     /// assert_eq!(future.as_mut().poll(&mut cx), task::Poll::Ready(10));
     /// ```
-    #[inline]
-    #[must_use]
     #[unstable(feature = "noop_waker", issue = "98286")]
-    pub const fn noop() -> &'static Waker {
-        const WAKER: &Waker = &Waker { waker: RawWaker::NOOP };
-        WAKER
-    }
+    pub const NOOP: &'static Waker = &Waker { waker: RawWaker::NOOP };
 
     /// Get a reference to the underlying [`RawWaker`].
     #[inline]
@@ -697,7 +692,7 @@ impl LocalWaker {
     /// use std::future::Future;
     /// use std::task::{ContextBuilder, LocalWaker, Waker, Poll};
     ///
-    /// let mut cx = ContextBuilder::from_waker(Waker::noop())
+    /// let mut cx = ContextBuilder::from_waker(Waker::NOOP)
     ///     .local_waker(LocalWaker::noop())
     ///     .build();
     ///

library/core/tests/async_iter/mod.rs

@@ -7,7 +7,7 @@ fn into_async_iter() {
     let async_iter = async_iter::from_iter(0..3);
     let mut async_iter = pin!(async_iter.into_async_iter());
 
-    let mut cx = &mut core::task::Context::from_waker(core::task::Waker::noop());
+    let mut cx = &mut core::task::Context::from_waker(core::task::Waker::NOOP);
 
     assert_eq!(async_iter.as_mut().poll_next(&mut cx), Poll::Ready(Some(0)));
     assert_eq!(async_iter.as_mut().poll_next(&mut cx), Poll::Ready(Some(1)));