Your last point is the relevant here in this context. If the defauklt "push_all" has no access to member variables (how could it - it's defined on the trait) an

Your last point is the relevant here in this context. If the defauklt "push_all" has no access to member variables (how could it - it's defined on the trait) and only calls public methods (such as push) then it is no more powerful than a free function or extension function. Hence it is okay. However, if you where to have another private function "private_push_all" and "push_all" could decide whether to call "push" or "private_push_all" then we would have the same situation again.