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Rollup merge of rust-lang#119984 - kpreid:waker-noop, r=dtolnay
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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))
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matthiaskrgr authored Jan 16, 2024
2 parents f7c43dd + 7052188 commit 89d4390
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Showing 24 changed files with 46 additions and 60 deletions.
17 changes: 12 additions & 5 deletions library/core/src/task/wake.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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
///
/// ```
Expand All @@ -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,
Expand All @@ -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`].
Expand Down
3 changes: 1 addition & 2 deletions library/core/tests/async_iter/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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)));
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3 changes: 1 addition & 2 deletions src/tools/miri/tests/pass/async-fn.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down
3 changes: 1 addition & 2 deletions src/tools/miri/tests/pass/dyn-star.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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) } {
Expand Down
6 changes: 2 additions & 4 deletions src/tools/miri/tests/pass/future-self-referential.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand All @@ -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;
Expand Down
3 changes: 1 addition & 2 deletions src/tools/miri/tests/pass/issues/issue-miri-2068.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down
3 changes: 1 addition & 2 deletions src/tools/miri/tests/pass/move-data-across-await-point.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down
3 changes: 1 addition & 2 deletions tests/coverage/async.coverage
Original file line number Diff line number Diff line change
Expand Up @@ -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) {
Expand Down
3 changes: 1 addition & 2 deletions tests/coverage/async.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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) {
Expand Down
3 changes: 1 addition & 2 deletions tests/coverage/async2.coverage
Original file line number Diff line number Diff line change
Expand Up @@ -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) {
Expand Down
3 changes: 1 addition & 2 deletions tests/coverage/async2.rs
Original file line number Diff line number Diff line change
Expand Up @@ -39,8 +39,7 @@ 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) {
Expand Down
3 changes: 1 addition & 2 deletions tests/coverage/async_block.coverage
Original file line number Diff line number Diff line change
Expand Up @@ -24,8 +24,7 @@
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) {
Expand Down
3 changes: 1 addition & 2 deletions tests/coverage/async_block.rs
Original file line number Diff line number Diff line change
Expand Up @@ -23,8 +23,7 @@ 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) {
Expand Down
16 changes: 8 additions & 8 deletions tests/coverage/closure_macro_async.cov-map
Original file line number Diff line number Diff line change
@@ -1,28 +1,28 @@
Function name: closure_macro_async::load_configuration_files
Raw bytes (9): 0x[01, 01, 00, 01, 01, 1e, 01, 02, 02]
Raw bytes (9): 0x[01, 01, 00, 01, 01, 1f, 01, 02, 02]
Number of files: 1
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 1
- Code(Counter(0)) at (prev + 30, 1) to (start + 2, 2)
- Code(Counter(0)) at (prev + 31, 1) to (start + 2, 2)

Function name: closure_macro_async::test
Raw bytes (9): 0x[01, 01, 00, 01, 01, 22, 01, 00, 2b]
Raw bytes (9): 0x[01, 01, 00, 01, 01, 23, 01, 00, 2b]
Number of files: 1
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 1
- Code(Counter(0)) at (prev + 34, 1) to (start + 0, 43)
- Code(Counter(0)) at (prev + 35, 1) to (start + 0, 43)

Function name: closure_macro_async::test::{closure#0}
Raw bytes (43): 0x[01, 01, 02, 01, 05, 05, 02, 07, 01, 22, 2b, 01, 21, 02, 02, 09, 00, 0f, 05, 00, 12, 00, 13, 02, 00, 12, 00, 13, 05, 00, 54, 00, 55, 02, 02, 09, 02, 0b, 07, 03, 01, 00, 02]
Raw bytes (43): 0x[01, 01, 02, 01, 05, 05, 02, 07, 01, 23, 2b, 01, 21, 02, 02, 09, 00, 0f, 05, 00, 12, 00, 13, 02, 00, 12, 00, 13, 05, 00, 54, 00, 55, 02, 02, 09, 02, 0b, 07, 03, 01, 00, 02]
Number of files: 1
- file 0 => global file 1
Number of expressions: 2
- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
Number of file 0 mappings: 7
- Code(Counter(0)) at (prev + 34, 43) to (start + 1, 33)
- Code(Counter(0)) at (prev + 35, 43) to (start + 1, 33)
- Code(Expression(0, Sub)) at (prev + 2, 9) to (start + 0, 15)
= (c0 - c1)
- Code(Counter(1)) at (prev + 0, 18) to (start + 0, 19)
Expand All @@ -35,10 +35,10 @@ Number of file 0 mappings: 7
= (c1 + (c0 - c1))

Function name: closure_macro_async::test::{closure#0}::{closure#0}
Raw bytes (9): 0x[01, 01, 00, 01, 01, 24, 12, 00, 54]
Raw bytes (9): 0x[01, 01, 00, 01, 01, 25, 12, 00, 54]
Number of files: 1
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 1
- Code(Counter(0)) at (prev + 36, 18) to (start + 0, 84)
- Code(Counter(0)) at (prev + 37, 18) to (start + 0, 84)

3 changes: 1 addition & 2 deletions tests/coverage/closure_macro_async.coverage
Original file line number Diff line number Diff line change
Expand Up @@ -54,8 +54,7 @@
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) {
Expand Down
6 changes: 3 additions & 3 deletions tests/coverage/closure_macro_async.rs
Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,8 @@ macro_rules! bail {

macro_rules! on_error {
($value:expr, $error_message:expr) => {
$value.or_else(|e| { // FIXME(85000): no coverage in closure macros
$value.or_else(|e| {
// FIXME(85000): no coverage in closure macros
let message = format!($error_message, e);
if message.len() > 0 {
println!("{}", message);
Expand Down Expand Up @@ -53,8 +54,7 @@ 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) {
Expand Down
3 changes: 1 addition & 2 deletions tests/ui/async-await/for-await-passthrough.rs
Original file line number Diff line number Diff line change
Expand Up @@ -25,8 +25,7 @@ async fn real_main() {

fn main() {
let future = real_main();
let waker = std::task::Waker::noop();
let mut cx = &mut core::task::Context::from_waker(&waker);
let mut cx = &mut core::task::Context::from_waker(std::task::Waker::noop());
let mut future = core::pin::pin!(future);
while let core::task::Poll::Pending = future.as_mut().poll(&mut cx) {}
}
3 changes: 1 addition & 2 deletions tests/ui/async-await/for-await.rs
Original file line number Diff line number Diff line change
Expand Up @@ -17,8 +17,7 @@ async fn real_main() {

fn main() {
let future = real_main();
let waker = std::task::Waker::noop();
let mut cx = &mut core::task::Context::from_waker(&waker);
let mut cx = &mut core::task::Context::from_waker(std::task::Waker::noop());
let mut future = core::pin::pin!(future);
while let core::task::Poll::Pending = future.as_mut().poll(&mut cx) {}
}
3 changes: 1 addition & 2 deletions tests/ui/async-await/in-trait/async-default-fn-overridden.rs
Original file line number Diff line number Diff line change
Expand Up @@ -40,8 +40,7 @@ fn main() {
let mut fut = pin!(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 fut.as_mut().poll(ctx) {
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -43,8 +43,7 @@ fn main() {
let mut fut = pin!(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 fut.as_mut().poll(ctx) {
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -21,7 +21,7 @@ LL | default async fn foo(_: T) -> &'static str {
= note: specialization behaves in inconsistent and surprising ways with async functions in traits, and for now is disallowed

error[E0599]: no method named `poll` found for struct `Pin<&mut impl Future<Output = ()>>` in the current scope
--> $DIR/dont-project-to-specializable-projection.rs:50:28
--> $DIR/dont-project-to-specializable-projection.rs:49:28
|
LL | match fut.as_mut().poll(ctx) {
| ^^^^ method not found in `Pin<&mut impl Future<Output = ()>>`
Expand Down
3 changes: 1 addition & 2 deletions tests/ui/coroutine/async-gen-yield-ty-is-unit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,6 @@ async gen fn gen_fn() -> &'static str {

pub fn main() {
let async_iterator = pin!(gen_fn());
let waker = Waker::noop();
let ctx = &mut Context::from_waker(&waker);
let ctx = &mut Context::from_waker(Waker::noop());
async_iterator.poll_next(ctx);
}
3 changes: 1 addition & 2 deletions tests/ui/coroutine/async_gen_fn_iter.rs
Original file line number Diff line number Diff line change
Expand Up @@ -74,8 +74,7 @@ fn main() {
let mut fut = pin!(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 fut.as_mut().poll(ctx) {
Expand Down
5 changes: 2 additions & 3 deletions tests/ui/dyn-star/dispatch-on-pin-mut.rs
Original file line number Diff line number Diff line change
Expand Up @@ -19,15 +19,14 @@ async fn async_main() {
// ------------------------------------------------------------------------- //
// Implementation Details Below...

use std::task::*;
use std::pin::pin;
use std::task::*;

fn main() {
let mut fut = pin!(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 fut.as_mut().poll(ctx) {
Expand Down

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