The reason why it's possible to use helper method like current_user, authenticate_user!, and so on without including any module

The title is my question.

devise provide us many useful methods like current_user, authenticate_user!, and so on. I want to know why is it possible to use them without including any module like below.

class ApplicationController < ActionController::Base
  before_action :authenticate_user!
end

Those method's definition is here

Somebody please help me！

The answer is devise included its helper methods into ActionController on behalf of you when Rails on_load

# devise/rails.rb
module Devise
  class Engine < ::Rails::Engine
    # ...
    initializer "devise.url_helpers" do
      Devise.include_helpers(Devise::Controllers)
    end
    # ...
end

# devise.rb
  # ...
  def self.include_helpers(scope)
    ActiveSupport.on_load(:action_controller) do
      include scope::Helpers if defined?(scope::Helpers)
      include scope::UrlHelpers
    end

    ActiveSupport.on_load(:action_view) do
      include scope::UrlHelpers
    end
  end
  # ...   

I saw many 3rd gems using on_load to include their methods (or themselves) into Rails core, maybe it's a typical way to do that (allows Rails to lazily load a lot of components and thus making the app boot faster). If you install some gems and you could use their methods on your model/controller/view then those gems did the same thing devise did above.

About those methods current_user, authenticate_user! ... they are dynamic methods devise will generate when it did include scope::Helpers into Rails (see code above and the link).