Turn unstable Waker::noop() into Waker::NOOP.

The advantage of this is that `&Waker::NOOP` can live for '`static`,
so it does not need to be assigned to a variable to be used in a
`Context` creation, which is, as shown in the changes to the tests in
this commit, is the most common thing to want to do with a noop waker.

An alternative would be to make it be of type `&'static Waker`,
which would make that common case even shorter, but that would mean that
if an owned `Waker` is desired it must be `.clone()`d from the constant.
Or, both versions could be provided, like `futures::task::noop_waker()`
and `futures::task::noop_waker_ref()`. But either option seems less
elegant to me.

Previous discussion on the tracking issue starting here:
rust-lang#98286 (comment)

Author: kpreid

library/core/src/task/wake.rs

@@ -329,7 +329,7 @@ impl Waker {
         Waker { waker }
     }
 
-    /// Creates a new `Waker` that does nothing when `wake` is called.
+    /// A `Waker` that does nothing when its `wake()` is called.
     ///
     /// 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
@@ -343,16 +343,13 @@ impl Waker {
     /// use std::future::Future;
     /// use std::task;
     ///
-    /// let waker = task::Waker::noop();
-    /// let mut cx = task::Context::from_waker(&waker);
+    /// 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() -> Waker {
+    pub const NOOP: Waker = {
         const VTABLE: RawWakerVTable = RawWakerVTable::new(
             // Cloning just returns a new no-op raw waker
             |_| RAW,
@@ -366,7 +363,7 @@ impl Waker {
         const RAW: RawWaker = RawWaker::new(ptr::null(), &VTABLE);
 
         Waker { waker: RAW }
-    }
+    };
 
     /// Get a reference to the underlying [`RawWaker`].
     #[inline]

library/core/tests/async_iter/mod.rs

@@ -7,8 +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 waker = core::task::Waker::noop();
-    let mut cx = &mut core::task::Context::from_waker(&waker);
+    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)));

src/tools/miri/tests/pass/async-fn.rs

@@ -76,8 +76,7 @@ async fn uninhabited_variant() {
 fn run_fut<T>(fut: impl Future<Output = T>) -> T {
     use std::task::{Context, Poll, Waker};
 
-    let waker = Waker::noop();
-    let mut context = Context::from_waker(&waker);
+    let mut context = Context::from_waker(&Waker::NOOP);
 
     let mut pinned = Box::pin(fut);
     loop {

src/tools/miri/tests/pass/dyn-star.rs

@@ -93,8 +93,7 @@ fn dispatch_on_pin_mut() {
     let mut fut = async_main();
 
     // Poll loop, just to test the future...
-    let waker = Waker::noop();
-    let ctx = &mut Context::from_waker(&waker);
+    let ctx = &mut Context::from_waker(&Waker::NOOP);
 
     loop {
         match unsafe { Pin::new_unchecked(&mut fut).poll(ctx) } {

src/tools/miri/tests/pass/future-self-referential.rs

@@ -77,8 +77,7 @@ impl Future for DoStuff {
 }
 
 fn run_fut<T>(fut: impl Future<Output = T>) -> T {
-    let waker = Waker::noop();
-    let mut context = Context::from_waker(&waker);
+    let mut context = Context::from_waker(&Waker::NOOP);
 
     let mut pinned = pin!(fut);
     loop {
@@ -90,8 +89,7 @@ fn run_fut<T>(fut: impl Future<Output = T>) -> T {
 }
 
 fn self_referential_box() {
-    let waker = Waker::noop();
-    let cx = &mut Context::from_waker(&waker);
+    let cx = &mut Context::from_waker(&Waker::NOOP);
 
     async fn my_fut() -> i32 {
         let val = 10;

src/tools/miri/tests/pass/issues/issue-miri-2068.rs

@@ -6,8 +6,7 @@ use std::task::{Context, Poll, Waker};
 
 pub fn fuzzing_block_on<O, F: Future<Output = O>>(fut: F) -> O {
     let mut fut = std::pin::pin!(fut);
-    let waker = Waker::noop();
-    let mut context = Context::from_waker(&waker);
+    let mut context = Context::from_waker(&Waker::NOOP);
     loop {
         match fut.as_mut().poll(&mut context) {
             Poll::Ready(v) => return v,

src/tools/miri/tests/pass/move-data-across-await-point.rs

@@ -56,8 +56,7 @@ fn data_moved() {
 fn run_fut<T>(fut: impl Future<Output = T>) -> T {
     use std::task::{Context, Poll, Waker};
 
-    let waker = Waker::noop();
-    let mut context = Context::from_waker(&waker);
+    let mut context = Context::from_waker(&Waker::NOOP);
 
     let mut pinned = Box::pin(fut);
     loop {

tests/coverage/async.coverage

@@ -117,8 +117,7 @@
    LL|       |    #[coverage(off)]
    LL|       |    pub fn block_on<F: Future>(mut future: F) -> F::Output {
    LL|       |        let mut future = pin!(future);
-   LL|       |        let waker = Waker::noop();
-   LL|       |        let mut context = Context::from_waker(&waker);
+   LL|       |        let mut context = Context::from_waker(&Waker::NOOP);
    LL|       |
    LL|       |        loop {
    LL|       |            if let Poll::Ready(val) = future.as_mut().poll(&mut context) {

tests/coverage/async.rs

@@ -110,8 +110,7 @@ mod executor {
     #[coverage(off)]
     pub fn block_on<F: Future>(mut future: F) -> F::Output {
         let mut future = pin!(future);
-        let waker = Waker::noop();
-        let mut context = Context::from_waker(&waker);
+        let mut context = Context::from_waker(&Waker::NOOP);
 
         loop {
             if let Poll::Ready(val) = future.as_mut().poll(&mut context) {

tests/coverage/async2.coverage

@@ -41,8 +41,7 @@
    LL|       |    #[coverage(off)]
    LL|       |    pub fn block_on<F: Future>(mut future: F) -> F::Output {
    LL|       |        let mut future = pin!(future);
-   LL|       |        let waker = Waker::noop();
-   LL|       |        let mut context = Context::from_waker(&waker);
+   LL|       |        let mut context = Context::from_waker(&Waker::NOOP);
    LL|       |
    LL|       |        loop {
    LL|       |            if let Poll::Ready(val) = future.as_mut().poll(&mut context) {