In my own usage of servo_arc, I have come across a case where an object, when dropped, will remove itself from a collection. The collection owns a servo_arc::Arc<Thing>, and the user dropping the object is dropping another servo_arc::Arc<Thing>; so the object needs to remove itself from the collection if its reference count is 2, not if it's 1.

Unfortunately, servo_arc::Arc only provides is_unique(). Would it be possibly to add a reference_count(&self) -> usize method to servo_arc::Arc, analogous to std::sync::Arc's strong_count ?