Rollup merge of #121622 - dtolnay:wake, r=cuviper

Preserve same vtable pointer when cloning raw waker, to fix Waker::will_wake

Fixes #121600.

As `@jkarneges` identified in #121600 (comment), the issue is two different const promotions produce two statics at different addresses, which may or may not later be deduplicated by the linker (in this case not).

Prior to #119863, the content of the statics was compared, and they were equal. After, the address of the statics are compared and they are not equal.

It is documented that `will_wake` _"works on a best-effort basis, and may return false even when the Wakers would awaken the same task"_ so this PR fixes a quality-of-implementation issue, not a correctness issue.

library/alloc/src/task.rs

@@ -136,6 +136,15 @@ impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for RawWaker {
 #[inline(always)]
 fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker {
     // Increment the reference count of the arc to clone it.
+    //
+    // The #[inline(always)] is to ensure that raw_waker and clone_waker are
+    // always generated in the same code generation unit as one another, and
+    // therefore that the structurally identical const-promoted RawWakerVTable
+    // within both functions is deduplicated at LLVM IR code generation time.
+    // This allows optimizing Waker::will_wake to a single pointer comparison of
+    // the vtable pointers, rather than comparing all four function pointers
+    // within the vtables.
+    #[inline(always)]
     unsafe fn clone_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) -> RawWaker {
         unsafe { Arc::increment_strong_count(waker as *const W) };
         RawWaker::new(
@@ -304,6 +313,10 @@ impl<W: LocalWake + 'static> From<Rc<W>> for RawWaker {
 #[inline(always)]
 fn local_raw_waker<W: LocalWake + 'static>(waker: Rc<W>) -> RawWaker {
     // Increment the reference count of the Rc to clone it.
+    //
+    // Refer to the comment on raw_waker's clone_waker regarding why this is
+    // always inline.
+    #[inline(always)]
     unsafe fn clone_waker<W: LocalWake + 'static>(waker: *const ()) -> RawWaker {
         unsafe { Rc::increment_strong_count(waker as *const W) };
         RawWaker::new(

library/alloc/tests/lib.rs

@@ -41,6 +41,7 @@
 #![feature(thin_box)]
 #![feature(strict_provenance)]
 #![feature(drain_keep_rest)]
+#![feature(local_waker)]
 #![allow(internal_features)]
 #![deny(fuzzy_provenance_casts)]
 #![deny(unsafe_op_in_unsafe_fn)]
@@ -62,6 +63,7 @@ mod rc;
 mod slice;
 mod str;
 mod string;
+mod task;
 mod thin_box;
 mod vec;
 mod vec_deque;

library/alloc/tests/task.rs

@@ -0,0 +1,34 @@
+use alloc::rc::Rc;
+use alloc::sync::Arc;
+use alloc::task::{LocalWake, Wake};
+use core::task::{LocalWaker, Waker};
+
+#[test]
+fn test_waker_will_wake_clone() {
+    struct NoopWaker;
+
+    impl Wake for NoopWaker {
+        fn wake(self: Arc<Self>) {}
+    }
+
+    let waker = Waker::from(Arc::new(NoopWaker));
+    let clone = waker.clone();
+
+    assert!(waker.will_wake(&clone));
+    assert!(clone.will_wake(&waker));
+}
+
+#[test]
+fn test_local_waker_will_wake_clone() {
+    struct NoopWaker;
+
+    impl LocalWake for NoopWaker {
+        fn wake(self: Rc<Self>) {}
+    }
+
+    let waker = LocalWaker::from(Rc::new(NoopWaker));
+    let clone = waker.clone();
+
+    assert!(waker.will_wake(&clone));
+    assert!(clone.will_wake(&waker));
+}