module T::Private::Mixins

def self.declare_mixes_in_class_methods(mixin, class_methods)
  if mixin.is_a?(Class)
    raise "Classes cannot be used as mixins, and so mixes_in_class_methods cannot be used on a Class."
  end
  if Abstract::Data.key?(mixin, :class_methods_mixin)
    raise "mixes_in_class_methods can only be used once per module."
  end
  mixin.singleton_class.include(MixesInClassMethods)
  Abstract::Data.set(mixin, :class_methods_mixin, class_methods)
end