Add comment why we use zero for COMPLETE

Author: orlp

library/std/src/sys/sync/once/futex.rs

@@ -17,6 +17,8 @@ const POISONED: Primitive = 2;
 /// so all future threads need to wait for it to finish.
 const RUNNING: Primitive = 1;
 /// Initialization has completed and all future calls should finish immediately.
+/// By choosing this state as the all-zero state the `is_completed` check can be
+/// a bit faster on some platforms.
 const COMPLETE: Primitive = 0;
 
 // An additional bit indicates whether there are waiting threads:

library/std/src/sys/sync/once/queue.rs

@@ -74,7 +74,8 @@ pub struct OnceState {
 }
 
 // Four states that a Once can be in, encoded into the lower bits of
-// `state_and_queue` in the Once structure.
+// `state_and_queue` in the Once structure. By choosing COMPLETE as the all-zero
+// state the `is_completed` check can be a bit faster on some platforms.
 const INCOMPLETE: usize = 0x3;
 const POISONED: usize = 0x2;
 const RUNNING: usize = 0x1;