2 files changed, 74 insertions, 0 deletions
diff --git a/rubocop/cop/prefer_class_methods_over_module.rb b/rubocop/cop/prefer_class_methods_over_module.rb
new file mode 100644
index 00000000000..0dfa80ccfab
--- /dev/null
+++ b/rubocop/cop/prefer_class_methods_over_module.rb
@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    # Enforces the use of 'class_methods' instead of 'module ClassMethods' for activesupport concerns.
+    # For more information see: https://gitlab.com/gitlab-org/gitlab-ce/issues/50414
+    #
+    # @example
+    #   # bad
+    #   module Foo
+    #     extend ActiveSupport::Concern
+    #
+    #     module ClassMethods
+    #       def a_class_method
+    #       end
+    #     end
+    #   end
+    #
+    #   # good
+    #   module Foo
+    #     extend ActiveSupport::Concern
+    #
+    #     class_methods do
+    #       def a_class_method
+    #       end
+    #     end
+    #   end
+    #
+    class PreferClassMethodsOverModule < RuboCop::Cop::Cop
+      include RangeHelp
+
+      MSG = 'Do not use module ClassMethods, use class_methods block instead.'
+
+      def_node_matcher :extend_activesupport_concern?, <<~PATTERN
+        (:send nil? :extend (:const (:const nil? :ActiveSupport) :Concern))
+      PATTERN
+
+      def on_module(node)
+        add_offense(node) if node.defined_module_name == 'ClassMethods' && module_extends_activesupport_concern?(node)
+      end
+
+      def autocorrect(node)
+        lambda do |corrector|
+          corrector.replace(module_range(node), 'class_methods do')
+        end
+      end
+
+      private
+
+      def module_extends_activesupport_concern?(node)
+        container_module = container_module_of(node)
+        return false unless container_module
+
+        container_module.descendants.any? do |descendant|
+          extend_activesupport_concern?(descendant)
+        end
+      end
+
+      def container_module_of(node)
+        while node = node.parent
+          break if node.type == :module
+        end
+
+        node
+      end
+
+      def module_range(node)
+        module_node, _ = *node
+        range_between(node.loc.keyword.begin_pos, module_node.source_range.end_pos)
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index eaf421a7235..d823fa4edb1 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -7,6 +7,7 @@ require_relative 'cop/include_sidekiq_worker'
 require_relative 'cop/avoid_return_from_blocks'
 require_relative 'cop/avoid_break_from_strong_memoize'
 require_relative 'cop/line_break_around_conditional_block'
+require_relative 'cop/prefer_class_methods_over_module'
 require_relative 'cop/migration/add_column'
 require_relative 'cop/migration/add_concurrent_foreign_key'
 require_relative 'cop/migration/add_concurrent_index'