Use Waker::will_wake() to avoid a cloning op (#2723)

Author: daxpedda

futures-core/src/task/__internal/atomic_waker.rs

@@ -271,7 +271,12 @@ impl AtomicWaker {
             WAITING => {
                 unsafe {
                     // Locked acquired, update the waker cell
-                    *self.waker.get() = Some(waker.clone());
+
+                    // Avoid cloning the waker if the old waker will awaken the same task.
+                    match &*self.waker.get() {
+                        Some(old_waker) if old_waker.will_wake(waker) => (),
+                        _ => *self.waker.get() = Some(waker.clone()),
+                    }
 
                     // Release the lock. If the state transitioned to include
                     // the `WAKING` bit, this means that at least one wake has