2 files changed, 74 insertions, 0 deletions
diff --git a/rubocop/cop/module_with_instance_variables.rb b/rubocop/cop/module_with_instance_variables.rb
new file mode 100644
index 00000000000..95e612b58e8
--- /dev/null
+++ b/rubocop/cop/module_with_instance_variables.rb
@@ -0,0 +1,73 @@
+module RuboCop
+  module Cop
+    class ModuleWithInstanceVariables < RuboCop::Cop::Cop
+      MSG = <<~EOL.freeze
+        Do not use instance variables in a module. Please read this
+        for the rationale behind it:
+
+        doc/development/module_with_instance_variables.md
+
+        If you think the use for this is fine, please just add:
+        # rubocop:disable Cop/ModuleWithInstanceVariables
+      EOL
+
+      def on_module(node)
+        return if
+          rails_helper?(node) || rails_mailer?(node) || spec_helper?(node)
+
+        check_method_definition(node)
+
+        # Not sure why some module would have an extra begin wrapping around
+        node.each_child_node(:begin) do |begin_node|
+          check_method_definition(begin_node)
+        end
+      end
+
+      private
+
+      # We ignore Rails helpers right now because it's hard to workaround it
+      def rails_helper?(node)
+        node.source_range.source_buffer.name =~
+          %r{app/helpers/\w+_helper.rb\z}
+      end
+
+      # We ignore Rails mailers right now because it's hard to workaround it
+      def rails_mailer?(node)
+        node.source_range.source_buffer.name =~
+          %r{app/mailers/emails/}
+      end
+
+      # We ignore spec helpers because it usually doesn't matter
+      def spec_helper?(node)
+        node.source_range.source_buffer.name =~
+          %r{spec/support/|features/steps/}
+      end
+
+      def check_method_definition(node)
+        node.each_child_node(:def) do |definition|
+          # We allow this pattern:
+          # def f
+          #   @f ||= true
+          # end
+          if only_ivar_or_assignment?(definition)
+            # We don't allow if any other ivar is used
+            definition.each_descendant(:ivar) do |offense|
+              add_offense(offense, :expression)
+            end
+          else
+            definition.each_descendant(:ivar, :ivasgn) do |offense|
+              add_offense(offense, :expression)
+            end
+          end
+        end
+      end
+
+      def only_ivar_or_assignment?(definition)
+        node = definition.child_nodes.last
+
+        definition.child_nodes.size == 2 &&
+          node.or_asgn_type? && node.child_nodes.first.ivasgn_type?
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 1b6e8991a17..bb0d25f3c48 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -6,6 +6,7 @@ require_relative 'cop/polymorphic_associations'
 require_relative 'cop/project_path_helper'
 require_relative 'cop/active_record_dependent'
 require_relative 'cop/in_batches'
+require_relative 'cop/module_with_instance_variables'
 require_relative 'cop/migration/add_column'
 require_relative 'cop/migration/add_column_with_default_to_large_table'
 require_relative 'cop/migration/add_concurrent_foreign_key'