perf(core): optimize waker capture in AsyncRefCell (denoland#12332)

core/async_cell.rs

@@ -322,9 +322,8 @@ mod internal {
  // is updated, it should be impossible to add it to the current borrow.
  assert!(id > turn || borrow_count.try_add(M::borrow_mode()).is_none());
  // Save or update the waiter's Waker.
- // TODO(piscisaureus): Use will_wake() to make this more efficient.
  let waiter_mut = waiters[id - turn].as_mut().unwrap();
- waiter_mut.set_waker(cx.waker().clone());
+ waiter_mut.set_waker(cx.waker());
  Poll::Pending
  }
  }
@@ -593,8 +592,15 @@ mod internal {
  self.borrow_mode
  }
 
- pub fn set_waker(&mut self, waker: Waker) {
- self.waker.replace(waker);
+ pub fn set_waker(&mut self, new_waker: &Waker) {
+ if self
+ .waker
+ .as_ref()
+ .filter(|waker| waker.will_wake(new_waker))
+ .is_none()
+ {
+ self.waker.replace(new_waker.clone());
+ }
  }
 
  pub fn take_waker(&mut self) -> Option<Waker> {