let proxy-lockable unlock pass the state by const&

Summary: Avoid questions of what may the mutex unlock method do to the passed state: nothing.

Reviewed By: aary

Differential Revision: D28895552

fbshipit-source-id: 8b73e76711bb553a344d958c80dcf14d56e74ffd

folly/synchronization/detail/ProxyLockable-inl.h

@@ -83,7 +83,7 @@ ProxyLockableUniqueLock<Mutex>& ProxyLockableUniqueLock<Mutex>::operator=(
 
 template <typename Mutex>
 ProxyLockableUniqueLock<Mutex>::ProxyLockableUniqueLock(
-    mutex_type& mutex, std::adopt_lock_t, state_type state)
+    mutex_type& mutex, std::adopt_lock_t, const state_type& state)
     : mutex_{std::addressof(mutex)}, state_{state} {
   proxylockable_detail::throwIfNotLocked(state_);
 }
@@ -123,7 +123,9 @@ void ProxyLockableUniqueLock<Mutex>::unlock() {
   proxylockable_detail::throwIfNoMutex(mutex_);
   proxylockable_detail::throwIfNotLocked(state_);
 
-  mutex_->unlock(std::exchange(state_, state_type{}));
+  const auto& state = state_;
+  mutex_->unlock(state);
+  state_ = state_type{};
 }
 
 template <typename Mutex>

folly/synchronization/detail/ProxyLockable.h

@@ -78,7 +78,7 @@ class ProxyLockableUniqueLock {
    *                    converts to true if the lock was successful
    */
   ProxyLockableUniqueLock(
-      mutex_type& mutex, std::adopt_lock_t, state_type state);
+      mutex_type& mutex, std::adopt_lock_t, const state_type& state);
   ProxyLockableUniqueLock(mutex_type& mutex, std::defer_lock_t) noexcept;
   ProxyLockableUniqueLock(mutex_type& mutex, std::try_to_lock_t);