Yeah, so the idea behind it is to check if rendering is included either in `ActionController::Base` or `ActionMailer::Base`. If *not*, it means that you're messing with it on your own, like discourse or spree guys, and you're using it "outside" of the core of the framework. In this situation, we want to inform you that this behavior is deprecated and you should change it because it won't be supported in 4.2.