Review recommendations:

* Added missing ';' after unsafe blocks
* Improved documentation in `State::transition_to_join_handle_dropped`
  on the scenarios in which `JOIN_WAKER` gets unset

tokio/src/runtime/task/harness.rs

@@ -317,7 +317,7 @@ where
             // 2. The task is not completed so the `JoinHandle` was able to unset
             //    `JOIN_WAKER` bit itself to get mutable access to the waker.
             //    The runtime will not access the waker when this flag is unset.
-            unsafe { self.trailer().set_waker(None) }
+            unsafe { self.trailer().set_waker(None) };
         }
 
         // Drop the `JoinHandle` reference, possibly deallocating the task
@@ -358,7 +358,7 @@ where
                 {
                     // SAFETY: We have COMPLETE=1 and JOIN_INTEREST=0, so
                     // we have exclusive access to the waker.
-                    unsafe { self.trailer().set_waker(None) }
+                    unsafe { self.trailer().set_waker(None) };
                 }
             }
         }));

tokio/src/runtime/task/state.rs

@@ -406,7 +406,12 @@ impl State {
 
             if !snapshot.is_join_waker_set() {
                 // If the `JOIN_WAKER` bit is unset and the `JOIN_HANDLE` has exclusive access to
-                // the the join waker and should drop it following this transition.
+                // the join waker and should drop it following this transition.
+                // This might happen in two situations:
+                //  1. The task is not completed and we just unset the `JOIN_WAKer` above in this
+                //     function.
+                //  2. The task is completed. In that case the `JOIN_WAKER` bit was already unset
+                //     by the runtime during completion.
                 transition.drop_waker = true;
             }