time: defer waker clone on spurious poll (#8107)

tokio/src/runtime/time_alt/entry.rs

@@ -228,11 +228,16 @@ impl Handle {
         }
     }
 
-    pub(crate) fn register_waker(&self, waker: Waker) {
+    pub(crate) fn register_waker(&self, waker: &Waker) {
         let mut lock = self.entry.state.lock();
         if !lock.cancelled && !lock.woken_up {
-            let maybe_old_waker = lock.waker.replace(waker);
+            // PANIC: no intermediary state is possible should the user-controllable `Waker`
-            // unlock before calling waker
+            // panic on `Clone` or `Drop`.
+            let maybe_old_waker = match &lock.waker {
+                Some(current_waker) if current_waker.will_wake(waker) => None,
+                _ => lock.waker.replace(waker.clone()),
+            };
+            // unlock before dropping waker
             drop(lock);
             drop(maybe_old_waker);
         }

tokio/src/runtime/time_alt/timer.rs

@@ -97,7 +97,7 @@ impl Timer {
         match self.entry.as_ref() {
             Some(entry) if entry.is_woken_up() => Poll::Ready(()),
             Some(entry) => {
-                entry.register_waker(cx.waker().clone());
+                entry.register_waker(cx.waker());
                 Poll::Pending
             }
             None => self.register(cx),