Singleton Class Include Cop

singleton_class_include.rb

module RuboCop
  module Cop
    module Security
      # This cop checks for includes/extends/prepends inside of
      # class << self blocks
      #
      # @example
      #
      #   # bad
      #
      #   class User
      #     class << self
      #       include SharedFunctionality
      #     end
      #   end
      #
      # @example
      #
      #   # good
      #
      #   class User
      #     include SharedFunctionality
      #     class << self
      #     end
      #   end
      #
      class SingletonClassInclude < Cop
        MSG = '`%<statement>s` is used inside of `class << self`. Use inside `class` ' \
              'or `module`.'.freeze

        def_node_matcher :include_statement, <<-PATTERN
          (send nil? ${:include :extend :prepend}
            const)
        PATTERN

        def on_send(node)
          include_statement(node) do |statement|
            add_offense(node, message: format(MSG, statement: statement)) if belongs_to_sclass_block?(node)
          end
        end

        private

        def belongs_to_sclass_block?(node)
          if !node.parent
            false
          else
            return true if node.parent.sclass_type?

            belongs_to_sclass_block?(node.parent)
          end
        end
      end
    end
  end
end