time: remove Box from Sleep (#3278)

Removes the box from `Sleep`, taking advantage of intrusive wakers. The
`Sleep` future is now `!Unpin`.

Closes #3267

tokio-util/src/time/delay_queue.rs

@@ -138,7 +138,7 @@ pub struct DelayQueue<T> {
     expired: Stack<T>,
 
     /// Delay expiring when the *first* item in the queue expires
-    delay: Option<Sleep>,
+    delay: Option<Pin<Box<Sleep>>>,
 
     /// Wheel polling state
     wheel_now: u64,
@@ -342,9 +342,9 @@ impl<T> DelayQueue<T> {
 
             let delay_time = self.start + Duration::from_millis(when);
             if let Some(ref mut delay) = &mut self.delay {
-                delay.reset(delay_time);
+                delay.as_mut().reset(delay_time);
             } else {
-                self.delay = Some(sleep_until(delay_time));
+                self.delay = Some(Box::pin(sleep_until(delay_time)));
             }
         }
 
@@ -553,7 +553,7 @@ impl<T> DelayQueue<T> {
         let next_deadline = self.next_deadline();
         if let (Some(ref mut delay), Some(deadline)) = (&mut self.delay, next_deadline) {
             // This should awaken us if necessary (ie, if already expired)
-            delay.reset(deadline);
+            delay.as_mut().reset(deadline);
         }
     }
 
@@ -759,7 +759,7 @@ impl<T> DelayQueue<T> {
             // We poll the wheel to get the next value out before finding the next deadline.
             let wheel_idx = self.wheel.poll(self.wheel_now, &mut self.slab);
 
-            self.delay = self.next_deadline().map(sleep_until);
+            self.delay = self.next_deadline().map(|when| Box::pin(sleep_until(when)));
 
             if let Some(idx) = wheel_idx {
                 return Poll::Ready(Some(Ok(idx)));