Default form helper

Rails
1

On line 1152 of form_helper.rb, there is this line:

    def default_form_builder
      builder = ActionView::Base.default_form_builder
      builder.respond_to?(:constantize) ? builder.constantize :

builder
end

I tried searching for default_form_builder on ActiveView::Base but
couldnt find the definition. Can anyone guide to where it’s defined,
specifically file and line number?