Unrolled build for rust-lang#119984

Rollup merge of rust-lang#119984 - kpreid:waker-noop, r=dtolnay

Change return type of unstable `Waker::noop()` from `Waker` to `&Waker`.

The advantage of this is that it does not need to be assigned to a variable to be used in a `Context` creation, which is the most common thing to want to do with a noop waker. It also avoids unnecessarily executing the dynamically dispatched drop function when the noop waker is dropped.

If an owned noop waker is desired, it can be created by cloning, but the reverse is harder to do since it requires declaring a constant. Alternatively, both versions could be provided, like `futures::task::noop_waker()` and `futures::task::noop_waker_ref()`, but that seems to me to be API clutter for a very small benefit, whereas having the `&'static` reference available is a large reduction in boilerplate.

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

Author: rust-timer

library/core/src/task/wake.rs

@@ -329,12 +329,14 @@ impl Waker {
         Waker { waker }
     }
 
-    /// Creates a new `Waker` that does nothing when `wake` is called.
+    /// Returns 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
     /// do not need to do anything specific if it happens.
     ///
+    /// If an owned `Waker` is needed, `clone()` this one.
+    ///
     /// # Examples
     ///
     /// ```
@@ -343,16 +345,20 @@ 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 fn noop() -> &'static Waker {
+        // Ideally all this data would be explicitly `static` because it is used by reference and
+        // only ever needs one copy. But `const fn`s (and `const` items) cannot refer to statics,
+        // even though their values can be promoted to static. (That might change; see #119618.)
+        // An alternative would be a `pub static NOOP: &Waker`, but associated static items are not
+        // currently allowed either, and making it non-associated would be unergonomic.
         const VTABLE: RawWakerVTable = RawWakerVTable::new(
             // Cloning just returns a new no-op raw waker
             |_| RAW,
@@ -364,8 +370,9 @@ impl Waker {
             |_| {},
         );
         const RAW: RawWaker = RawWaker::new(ptr::null(), &VTABLE);
+        const WAKER_REF: &Waker = &Waker { waker: RAW };
 
-        Waker { waker: RAW }
+        WAKER_REF
     }
 
     /// Get a reference to the underlying [`RawWaker`].

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) {