class_variables.rb

require 'active_support/concern'

module A
  extend ActiveSupport::Concern
  module ClassMethods
    @@a = 2
    def a
      @@a
    end
  end
end

class C
  include A
end

C.a
#=> 2
Hash[C.ancestors.map { |c| [c, c.class_variables] }]
#=>{C=>[], A=>[], Object=>[], Kernel=>[], BasicObject=>[]}