Add cop to make sure we don't use ivar in a module

app/controllers/concerns/boards_responses.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module BoardsResponses
   def authorize_read_list
     authorize_action_for!(board.parent, :read_list)

app/controllers/concerns/creates_commit.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module CreatesCommit
   extend ActiveSupport::Concern
 

app/controllers/concerns/cycle_analytics_params.rb

 module CycleAnalyticsParams
   extend ActiveSupport::Concern
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def options(params)
     @options ||= { from: start_date(params), current_user: current_user }
   end

app/controllers/concerns/issuable_actions.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module IssuableActions
   extend ActiveSupport::Concern
 

app/controllers/concerns/issuable_collections.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module IssuableCollections
   extend ActiveSupport::Concern
   include SortingHelper

app/controllers/concerns/issues_action.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module IssuesAction
   extend ActiveSupport::Concern
   include IssuableCollections

app/controllers/concerns/lfs_request.rb

@@ -90,6 +90,7 @@ module LfsRequest
     has_authentication_ability?(:build_download_code) && can?(user, :build_download_code, project)
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def storage_project
     @storage_project ||= begin
       result = project
@@ -103,6 +104,7 @@ module LfsRequest
     end
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def objects
     @objects ||= (params[:objects] || []).to_a
   end

app/controllers/concerns/membership_actions.rb

@@ -76,6 +76,7 @@ module MembershipActions
     end
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def source_type
     @source_type ||= membershipable.class.to_s.humanize(capitalize: false)
   end

app/controllers/concerns/merge_requests_action.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module MergeRequestsAction
   extend ActiveSupport::Concern
   include IssuableCollections

app/controllers/concerns/milestone_actions.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module MilestoneActions
   extend ActiveSupport::Concern
 

app/controllers/concerns/notes_actions.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module NotesActions
   include RendersNotes
   extend ActiveSupport::Concern

app/controllers/concerns/oauth_applications.rb

@@ -13,6 +13,7 @@ module OauthApplications
     end
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def load_scopes
     @scopes = Doorkeeper.configuration.scopes
   end

app/controllers/concerns/renders_commits.rb

 module RendersCommits
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def prepare_commits_for_rendering(commits)
     Banzai::CommitRenderer.render(commits, @project, current_user)
 

app/controllers/concerns/renders_notes.rb

 module RendersNotes
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def prepare_notes_for_rendering(notes, noteable = nil)
     preload_noteable_for_regular_notes(notes)
     preload_max_access_for_authors(notes, @project)

app/controllers/concerns/requires_whitelisted_monitoring_client.rb

@@ -17,6 +17,7 @@ module RequiresWhitelistedMonitoringClient
     ip_whitelist.any? { |e| e.include?(Gitlab::RequestContext.client_ip) }
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def ip_whitelist
     @ip_whitelist ||= Settings.monitoring.ip_whitelist.map(&IPAddr.method(:new))
   end

app/controllers/concerns/service_params.rb

@@ -65,6 +65,7 @@ module ServiceParams
   # Parameters to ignore if no value is specified
   FILTER_BLANK_PARAMS = [:password].freeze
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def service_params
     dynamic_params = @service.event_channel_names + @service.event_names
     service_params = params.permit(:id, service: ALLOWED_PARAMS_CE + dynamic_params)

app/controllers/concerns/snippets_actions.rb

@@ -4,6 +4,7 @@ module SnippetsActions
   def edit
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def raw
     disposition = params[:inline] == 'false' ? 'attachment' : 'inline'
 

app/controllers/concerns/spammable_actions.rb

@@ -17,6 +17,7 @@ module SpammableActions
 
   private
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def ensure_spam_config_loaded!
     return @spam_config_loaded if defined?(@spam_config_loaded)
 

app/controllers/concerns/toggle_subscription_action.rb

@@ -11,6 +11,7 @@ module ToggleSubscriptionAction
 
   private
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def subscribable_project
     @project || raise(NotImplementedError)
   end

app/models/concerns/ignorable_column.rb

@@ -17,6 +17,7 @@ module IgnorableColumn
       super.reject { |column| ignored_columns.include?(column.name) }
     end
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def ignored_columns
       @ignored_columns ||= Set.new
     end

app/models/concerns/mentionable.rb

@@ -5,6 +5,7 @@
 #
 # Used by Issue, Note, MergeRequest, and Commit.
 #
+# # rubocop:disable Cop/ModuleWithInstanceVariables
 module Mentionable
   extend ActiveSupport::Concern
 

app/models/concerns/milestoneish.rb

@@ -94,6 +94,7 @@ module Milestoneish
     end
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def memoize_per_user(user, method_name)
     @memoized ||= {}
     @memoized[method_name] ||= {}

app/models/concerns/noteable.rb

@@ -12,6 +12,7 @@ module Noteable
   #
   #   noteable.class           # => MergeRequest
   #   noteable.human_class_name # => "merge request"
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def human_class_name
     @human_class_name ||= base_class_name.titleize.downcase
   end
@@ -34,6 +35,7 @@ module Noteable
 
   delegate :find_discussion, to: :discussion_notes
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def discussions
     @discussions ||= discussion_notes
       .inc_relations_for_view
@@ -46,6 +48,7 @@ module Noteable
     notes.inc_relations_for_view.grouped_diff_discussions(*args)
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def resolvable_discussions
     @resolvable_discussions ||=
       if defined?(@discussions)
@@ -67,6 +70,7 @@ module Noteable
     discussions_resolvable? && !discussions_resolved?
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def discussions_to_be_resolved
     @discussions_to_be_resolved ||= resolvable_discussions.select(&:to_be_resolved?)
   end

app/models/concerns/participable.rb

@@ -55,6 +55,7 @@ module Participable
   # This method processes attributes of objects in breadth-first order.
   #
   # Returns an Array of User instances.
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def participants(current_user = nil)
     @participants ||= Hash.new do |hash, user|
       hash[user] = raw_participants(user)

app/models/concerns/protected_ref.rb

@@ -55,6 +55,7 @@ module ProtectedRef
 
   private
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def ref_matcher
     @ref_matcher ||= ProtectedRefMatcher.new(self)
   end

app/models/concerns/relative_positioning.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module RelativePositioning
   extend ActiveSupport::Concern
 

app/models/concerns/resolvable_discussion.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module ResolvableDiscussion
   extend ActiveSupport::Concern
 

app/models/concerns/routable.rb

 # Store object full path in separate table for easy lookup and uniq validation
 # Object must have name and path db fields and respond to parent and parent_changed? methods.
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Routable
   extend ActiveSupport::Concern
 

app/models/concerns/spammable.rb

@@ -35,7 +35,7 @@ module Spammable
   end
 
   def spam?
-    @spam
+    spam
   end
 
   def check_for_spam

app/models/concerns/storage/legacy_namespace.rb

@@ -49,6 +49,7 @@ module Storage
 
     private
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def old_repository_storage_paths
       @old_repository_storage_paths ||= repository_storage_paths
     end

app/models/concerns/strip_attribute.rb

@@ -17,6 +17,7 @@ module StripAttribute
       strip_attrs.concat(attrs)
     end
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def strip_attrs
       @strip_attrs ||= []
     end

app/models/concerns/taskable.rb

@@ -6,6 +6,7 @@ require 'task_list/filter'
 # bugs".
 #
 # Used by MergeRequest and Issue
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Taskable
   COMPLETED    = 'completed'.freeze
   INCOMPLETE   = 'incomplete'.freeze

app/models/concerns/time_trackable.rb

@@ -4,7 +4,7 @@
 #
 # Used by Issue and MergeRequest.
 #
-
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module TimeTrackable
   extend ActiveSupport::Concern
 

app/services/concerns/issues/resolve_discussions.rb

@@ -2,11 +2,13 @@ module Issues
   module ResolveDiscussions
     attr_reader :merge_request_to_resolve_discussions_of_iid, :discussion_to_resolve_id
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def filter_resolve_discussion_params
       @merge_request_to_resolve_discussions_of_iid ||= params.delete(:merge_request_to_resolve_discussions_of)
       @discussion_to_resolve_id ||= params.delete(:discussion_to_resolve)
     end
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def merge_request_to_resolve_discussions_of
       return @merge_request_to_resolve_discussions_of if defined?(@merge_request_to_resolve_discussions_of)
 
@@ -15,6 +17,7 @@ module Issues
                                                      .find_by(iid: merge_request_to_resolve_discussions_of_iid)
     end
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def discussions_to_resolve
       return [] unless merge_request_to_resolve_discussions_of
 

app/services/spam_check_service.rb

@@ -6,6 +6,7 @@
 # Dependencies:
 # - params with :request
 #
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module SpamCheckService
   def filter_spam_check_params
     @request            = params.delete(:request)

app/services/system_note_service.rb

@@ -662,6 +662,7 @@ module SystemNoteService
     Rack::Utils.escape_html(text)
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def url_helpers
     @url_helpers ||= Gitlab::Routing.url_helpers
   end

app/workers/concerns/new_issuable.rb

@@ -8,12 +8,14 @@ module NewIssuable
     user && issuable
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def set_user(user_id)
     @user = User.find_by(id: user_id)
 
     log_error(User, user_id) unless @user
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def set_issuable(issuable_id)
     @issuable = issuable_class.find_by(id: issuable_id)
 

config/initializers/fix_local_cache_middleware.rb

@@ -4,6 +4,7 @@ module LocalCacheRegistryCleanupWithEnsure
   LocalStore =
     ActiveSupport::Cache::Strategy::LocalCache::LocalStore
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def call(env)
     LocalCacheRegistry.set_cache_for(local_cache_key, LocalStore.new)
     response = @app.call(env)

config/initializers/rspec_profiling.rb

@@ -16,6 +16,7 @@ module RspecProfilingExt
   end
 
   module Run
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def example_finished(*args)
       super
     rescue => err

config/initializers/rugged_use_gitlab_git_attributes.rb

@@ -11,6 +11,7 @@
 # anyway, and there is no great efficiency gain from just fetching the listed
 # attributes with our implementation, so we ignore the additional arguments.
 #
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Rugged
   class Repository
     module UseGitlabGitAttributes

doc/development/module_with_instance_variables.md

+## Usually modules with instance variables considered harmful
+
+### Background
+
+Rails somehow encourages people using modules and instance variables
+everywhere. For example, using instance variables in the controllers,
+helpers, and views. They're also encouraging the use of
+`ActiveSupport::Concern`, which further strengthens the idea of
+saving everything in a giant, single object, and people could access
+everything in that one giant object.
+
+### The problems
+
+Of course this is convenient to develop, because we just have everything
+within reach. However this has a number of downsides when that chosen object
+is growing, it would later become out of control for the same reason.
+
+There are just too many things in the same context, and we don't know if
+those things are tightly coupled or not, depending on each others or not.
+It's very hard to tell when the complexity grows to a point, and it makes
+tracking the code also extremely hard. For example, a class could be using
+3 different instance variables, and all of them could be initialized and
+manipulated from 3 different modules. It's hard to track when those variables
+start giving us troubles. We don't know which module would suddenly change
+one of the variables. Everything could touch anything.
+
+### Similar concerns
+
+People are saying multiple inheritance is bad. Mixing multiple modules with
+multiple instance variables scattering everywhere suffer from the same issue.
+The same applies to `ActiveSupport::Concern`. See:
+[Consider replacing concerns with dedicated classes & composition](
+https://gitlab.com/gitlab-org/gitlab-ce/issues/23786)
+
+There's also a similar idea:
+[Use decorators and interface segregation to solve overgrowing models problem](
+https://gitlab.com/gitlab-org/gitlab-ce/issues/13484)
+
+Note that `included` doesn't solve the whole issue. They define the
+dependencies, but they still allow each modules to talk implicitly via the
+instance variables in the final giant object, and that's where the problem is.
+
+### Solutions
+
+We should split the giant object into multiple objects, and they communicate
+each other with the API, i.e. public methods. In short, composition over
+inheritance. This way, each smaller objects would have their own respective
+limited states, i.e. instance variables. If one instance variable goes wrong,
+we would be very clear that it's from that single small object, because
+no one else could be touching it.
+
+With clearly defined API, this would make things less coupled and much easier
+to debug and track, and much more extensible for other objects to use, because
+they communicate in a clear way, rather than implicit dependencies.
+
+### Exceptions
+
+However, it's not all that bad when using instance variables in a module,
+as long as it's contained in the same module, that is no other modules or
+objects are touching them. If that's the case, then it would be an acceptable
+use. Unfortunately it's a bit hard to code this principle in the cop, so
+for now we rely on people turning off the cops, if they think that the use
+conform this rule.
+
+Here's an acceptable case:
+
+``` ruby
+# This is ok, as long as `@attributes` is never used anywhere else.
+# Consider adding some prefix or suffix to avoid name conflicts though.
+# rubocop:disable Cop/ModuleWithInstanceVariables
+module Rugged
+  class Repository
+    module UseGitlabGitAttributes
+      def fetch_attributes(name, *)
+        @attributes ||= Gitlab::Git::Attributes.new(path)
+        @attributes.attributes(name)
+      end
+    end
+
+    prepend UseGitlabGitAttributes
+  end
+end
+```
+
+Here's a bad example which we should rewrite:
+
+``` ruby
+module SpamCheckService
+  def filter_spam_check_params
+    @request            = params.delete(:request)
+    @api                = params.delete(:api)
+    @recaptcha_verified = params.delete(:recaptcha_verified)
+    @spam_log_id        = params.delete(:spam_log_id)
+  end
+
+  def spam_check(spammable, user)
+    spam_service = SpamService.new(spammable, @request)
+
+    spam_service.when_recaptcha_verified(@recaptcha_verified, @api) do
+      user.spam_logs.find_by(id: @spam_log_id)&.update!(recaptcha_verified: true)
+    end
+  end
+end
+```
+
+There are several implicit dependencies here. First, `params` should be
+defined before using. Second, `filter_spam_check_params` should be called
+before `spam_check`. These are all implicit and the includer could be using
+those instance variables without awareness.
+
+This should be rewritten like:
+
+``` ruby
+class SpamCheckService
+  def initialize(request:, api:, recaptcha_verified:, spam_log_id:)
+    @request            = request
+    @api                = api
+    @recaptcha_verified = recaptcha_verified
+    @spam_log_id        = spam_log_id
+  end
+
+  def spam_check(spammable, user)
+    spam_service = SpamService.new(spammable, @request)
+
+    spam_service.when_recaptcha_verified(@recaptcha_verified, @api) do
+      user.spam_logs.find_by(id: @spam_log_id)&.update!(recaptcha_verified: true)
+    end
+  end
+end
+```
+
+And use it like:
+
+``` ruby
+class UpdateSnippetService < BaseService
+  def execute
+    # ...
+    spam = SpamCheckService.new(params.slice!(:request, :api, :recaptcha_verified, :spam_log_id))
+
+    spam.check(snippet, current_user)
+    # ...
+  end
+end
+```
+
+This way, all those instance variables are isolated in `SpamCheckService`
+rather than who ever include the module, and those modules which were also
+included, making it much easier to track down the issues if there's any,
+and it also reduce the chance of having name conflicts.
+
+### Things we might need to ignore right now
+
+Since the way how Rails helpers and mailers work, we might not be able to
+avoid the use of instance variables there. For those cases, we could ignore
+them at the moment. At least we're not going to share those modules with
+other random objects, so they're still somehow isolated.
+
+### Instance variables in the views
+
+They're terrible, because they're also shared between different controllers,
+and it's very hard to track where those instance variables were set when we
+saw somewhere is using it, neither do we know where those were used when we
+saw somewhere is setting up them. We hit into a number of 500 errors when we
+tried to remove some instance variables in the controller in the past.
+
+Somewhere, some partials might be using it, and we don't know.
+
+We're trying to use something like this instead:
+
+``` haml
+= render 'projects/commits/commit', commit: commit, ref: ref, project: project
+```
+
+And in the partial:
+
+``` haml
+- ref = local_assigns.fetch(:ref)
+- commit = local_assigns.fetch(:commit)
+- project = local_assigns.fetch(:project)
+```
+
+This way it's very clear where those values were coming from. In the future,
+we should also forbid the use of instance variables in partials.

features/support/env.rb

@@ -42,11 +42,11 @@ module StdoutReporterWithScenarioLocation
   # Override the standard reporter to show filename and line number next to each
   # scenario for easy, focused re-runs
   def before_scenario_run(scenario, step_definitions = nil)
-    @max_step_name_length = scenario.steps.map(&:name).map(&:length).max if scenario.steps.any?
+    max_step_name_length = scenario.steps.map(&:name).map(&:length).max if scenario.steps.any?
     name = scenario.name
 
     # This number has no significance, it's just to line things up
-    max_length = @max_step_name_length + 19
+    max_length = max_step_name_length + 19
     out.puts "\n  #{'Scenario:'.green} #{name.light_green.ljust(max_length)}" \
       " # #{scenario.feature.filename}:#{scenario.line}"
   end

lib/after_commit_queue.rb

@@ -20,6 +20,7 @@ module AfterCommitQueue
     end
   end
 
+  # rubocop:disable Cop/ModuleWithInstanceVariables
   def _after_commit_queue
     @after_commit_queue ||= []
   end

lib/api/api_guard.rb

@@ -2,6 +2,7 @@
 
 require 'rack/oauth2'
 
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module API
   module APIGuard
     extend ActiveSupport::Concern

lib/api/helpers.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module API
   module Helpers
     include Gitlab::Utils

lib/api/helpers/internal_helpers.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module API
   module Helpers
     module InternalHelpers
@@ -57,6 +58,7 @@ module API
 
       private
 
+      # rubocop:disable Cop/ModuleWithInstanceVariables
       def set_project
         if params[:gl_repository]
           @project, @wiki = Gitlab::GlRepository.parse(params[:gl_repository])

lib/api/helpers/runner.rb

@@ -21,6 +21,7 @@ module API
         forbidden! unless current_runner
       end
 
+      # rubocop:disable Cop/ModuleWithInstanceVariables
       def current_runner
         @runner ||= ::Ci::Runner.find_by_token(params[:token].to_s)
       end

lib/extracts_path.rb

 # Module providing methods for dealing with separating a tree-ish string and a
 # file path string when combined in a request parameter
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module ExtractsPath
   # Raised when given an invalid file path
   InvalidPathError = Class.new(StandardError)

lib/gitlab/cache/request_cache.rb

@@ -45,6 +45,7 @@ module Gitlab
         klass.prepend(extension)
       end
 
+      # rubocop:disable Cop/ModuleWithInstanceVariables
       def request_cache_key(&block)
         if block_given?
           @request_cache_key = block

lib/gitlab/ci/charts.rb

@@ -2,6 +2,7 @@ module Gitlab
   module Ci
     module Charts
       module DailyInterval
+        # rubocop:disable Cop/ModuleWithInstanceVariables
         def grouped_count(query)
           query
             .group("DATE(#{::Ci::Pipeline.table_name}.created_at)")
@@ -9,6 +10,7 @@ module Gitlab
             .transform_keys { |date| date.strftime(@format) }
         end
 
+        # rubocop:disable Cop/ModuleWithInstanceVariables
         def interval_step
           @interval_step ||= 1.day
         end
@@ -28,6 +30,7 @@ module Gitlab
           end
         end
 
+        # rubocop:disable Cop/ModuleWithInstanceVariables
         def interval_step
           @interval_step ||= 1.month
         end

lib/gitlab/ci/config/entry/configurable.rb

@@ -13,6 +13,7 @@ module Gitlab
         #   script: ...
         #   artifacts: ...
         #
+        # rubocop:disable Cop/ModuleWithInstanceVariables
         module Configurable
           extend ActiveSupport::Concern
 

lib/gitlab/ci/config/entry/validatable.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   module Ci
     class Config

lib/gitlab/ci/model.rb

@@ -5,6 +5,7 @@ module Gitlab
         "ci_"
       end
 
+      # rubocop:disable Cop/ModuleWithInstanceVariables
       def model_name
         @model_name ||= ActiveModel::Name.new(self, nil, self.name.split("::").last)
       end

lib/gitlab/current_settings.rb

@@ -52,6 +52,7 @@ module Gitlab
       ::ApplicationSetting.create_from_defaults || in_memory_application_settings
     end
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def in_memory_application_settings
       @in_memory_application_settings ||= ::ApplicationSetting.new(::ApplicationSetting.defaults)
     rescue ActiveRecord::StatementInvalid, ActiveRecord::UnknownAttributeError

lib/gitlab/cycle_analytics/base_event_fetcher.rb

@@ -59,6 +59,12 @@ module Gitlab
         nil
       end
 
+      def load_allowed_ids
+        allowed_ids_finder_class
+          .new(@options[:current_user], project_id: @project.id)
+          .execute.where(id: event_result_ids).pluck(:id)
+      end
+
       def event_result_ids
         event_result.map { |event| event['id'] }
       end

lib/gitlab/cycle_analytics/base_query.rb

@@ -7,6 +7,7 @@ module Gitlab
 
       private
 
+      # rubocop:disable Cop/ModuleWithInstanceVariables
       def base_query
         @base_query ||= stage_query
       end

lib/gitlab/cycle_analytics/code_event_fetcher.rb

 module Gitlab
   module CycleAnalytics
     class CodeEventFetcher < BaseEventFetcher
-      include MergeRequestAllowed
-
       def initialize(*args)
         @projections = [mr_table[:title],
                         mr_table[:iid],
@@ -20,6 +18,14 @@ module Gitlab
       def serialize(event)
         AnalyticsMergeRequestSerializer.new(project: @project).represent(event)
       end
+
+      def allowed_ids
+        load_allowed_ids
+      end
+
+      def allowed_ids_finder_class
+        MergeRequestsFinder
+      end
     end
   end
 end

lib/gitlab/cycle_analytics/issue_allowed.rb

-module Gitlab
-  module CycleAnalytics
-    module IssueAllowed
-      def allowed_ids
-        @allowed_ids ||= IssuesFinder.new(@options[:current_user], project_id: @project.id).execute.where(id: event_result_ids).pluck(:id)
-      end
-    end
-  end
-end

lib/gitlab/cycle_analytics/issue_event_fetcher.rb

 module Gitlab
   module CycleAnalytics
     class IssueEventFetcher < BaseEventFetcher
-      include IssueAllowed
-
       def initialize(*args)
         @projections = [issue_table[:title],
                         issue_table[:iid],
@@ -18,6 +16,14 @@ module Gitlab
       def serialize(event)
         AnalyticsIssueSerializer.new(project: @project).represent(event)
       end
+
+      def allowed_ids
+        load_allowed_ids
+      end
+
+      def allowed_ids_finder_class
+        IssuesFinder
+      end
     end
   end
 end

lib/gitlab/cycle_analytics/merge_request_allowed.rb

-module Gitlab
-  module CycleAnalytics
-    module MergeRequestAllowed
-      def allowed_ids
-        @allowed_ids ||= MergeRequestsFinder.new(@options[:current_user], project_id: @project.id).execute.where(id: event_result_ids).pluck(:id)
-      end
-    end
-  end
-end

lib/gitlab/cycle_analytics/production_helper.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   module CycleAnalytics
     module ProductionHelper

lib/gitlab/cycle_analytics/review_event_fetcher.rb

 module Gitlab
   module CycleAnalytics
     class ReviewEventFetcher < BaseEventFetcher
-      include MergeRequestAllowed
-
       def initialize(*args)
         @projections = [mr_table[:title],
                         mr_table[:iid],
@@ -14,9 +12,19 @@ module Gitlab
         super(*args)
       end
 
+      private
+
       def serialize(event)
         AnalyticsMergeRequestSerializer.new(project: @project).represent(event)
       end
+
+      def allowed_ids
+        load_allowed_ids
+      end
+
+      def allowed_ids_finder_class
+        MergeRequestsFinder
+      end
     end
   end
 end

lib/gitlab/database/rename_reserved_paths_migration/v1/migration_classes.rb

@@ -3,6 +3,7 @@ module Gitlab
     module RenameReservedPathsMigration
       module V1
         module MigrationClasses
+          # rubocop:disable Cop/ModuleWithInstanceVariables
           module Routable
             def full_path
               if route && route.path.present?

lib/gitlab/email/handler/reply_processing.rb

@@ -12,6 +12,7 @@ module Gitlab
           raise NotImplementedError
         end
 
+        # rubocop:disable Cop/ModuleWithInstanceVariables
         def message
           @message ||= process_message
         end

lib/gitlab/emoji.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   module Emoji
     extend self

lib/gitlab/git/popen.rb

@@ -13,15 +13,15 @@ module Gitlab
         vars = { "PWD" => path }
         options = { chdir: path }
 
-        @cmd_output = ""
-        @cmd_status = 0
+        cmd_output = ""
+        cmd_status = 0
         Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
-          @cmd_output << stdout.read
-          @cmd_output << stderr.read
-          @cmd_status = wait_thr.value.exitstatus
+          cmd_output << stdout.read
+          cmd_output << stderr.read
+          cmd_status = wait_thr.value.exitstatus
         end
 
-        [@cmd_output, @cmd_status]
+        [cmd_output, cmd_status]
       end
     end
   end

lib/gitlab/identifier.rb

@@ -52,6 +52,7 @@ module Gitlab
       end
     end
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def identification_cache
       @identification_cache ||= {
         email: {},

lib/gitlab/import_export/command_line_util.rb

@@ -30,6 +30,7 @@ module Gitlab
         execute(%W(tar -#{options} #{archive} -C #{dir}))
       end
 
+      # rubocop:disable Cop/ModuleWithInstanceVariables
       def execute(cmd)
         output, status = Gitlab::Popen.popen(cmd)
         @shared.error(Gitlab::ImportExport::Error.new(output.to_s)) unless status.zero?

lib/gitlab/metrics/influx_db.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   module Metrics
     module InfluxDb

lib/gitlab/metrics/prometheus.rb

 require 'prometheus/client'
 
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   module Metrics
     module Prometheus

lib/gitlab/path_regex.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   module PathRegex
     extend self

lib/gitlab/prometheus/additional_metrics_parser.rb

@@ -26,6 +26,7 @@ module Gitlab
         load_yaml_file&.map(&:deep_symbolize_keys).freeze
       end
 
+      # rubocop:disable Cop/ModuleWithInstanceVariables
       def load_yaml_file
         @loaded_yaml_file ||= YAML.load_file(Rails.root.join('config/prometheus/additional_metrics.yml'))
       end

lib/gitlab/prometheus/queries/query_additional_metrics.rb

@@ -56,6 +56,7 @@ module Gitlab
           query
         end
 
+        # rubocop:disable Cop/ModuleWithInstanceVariables
         def available_metrics
           @available_metrics ||= client_label_values || []
         end

lib/gitlab/regex.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   module Regex
     extend self

lib/gitlab/slash_commands/presenters/issue_base.rb

+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   module SlashCommands
     module Presenters

lib/gitlab/themes.rb

@@ -72,6 +72,7 @@ module Gitlab
 
     private
 
+    # rubocop:disable Cop/ModuleWithInstanceVariables
     def default_id
       @default_id ||= begin
         id = Gitlab.config.gitlab.default_theme.to_i

lib/tasks/gitlab/task_helpers.rb

 require 'rainbow/ext/string'
 
+# rubocop:disable Cop/ModuleWithInstanceVariables
 module Gitlab
   TaskFailedError = Class.new(StandardError)
   TaskAbortedByUserError = Class.new(StandardError)

qa/qa/runtime/namespace.rb

@@ -3,6 +3,7 @@ module QA
     module Namespace
       extend self
 
+      # rubocop:disable Cop/ModuleWithInstanceVariables
       def time
         @time ||= Time.now
       end

rubocop/cop/module_with_instance_variables.rb

+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|
+          definition.each_descendant(:ivar, :ivasgn) do |offense|
+            add_offense(offense, :expression)
+          end
+        end
+      end
+    end
+  end
+end

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'

spec/rubocop/cop/module_with_instance_variables_spec.rb

+require 'spec_helper'
+require 'rubocop'
+require 'rubocop/rspec/support'
+require_relative '../../../rubocop/cop/module_with_instance_variables'
+
+describe RuboCop::Cop::ModuleWithInstanceVariables do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  shared_examples('registering offense') do
+    it 'registers an offense when instance variable is used in a module' do
+      inspect_source(cop, source)
+
+      aggregate_failures do
+        expect(cop.offenses.size).to eq(offending_lines.size)
+        expect(cop.offenses.map(&:line)).to eq(offending_lines)
+      end
+    end
+  end
+
+  context 'when source is a regular module' do
+    let(:source) do
+      <<~RUBY
+        module M
+          def f
+            @f ||= true
+          end
+        end
+      RUBY
+    end
+
+    let(:offending_lines) { [3] }
+
+    it_behaves_like 'registering offense'
+  end
+
+  context 'when source is a nested module' do
+    let(:source) do
+      <<~RUBY
+        module N
+          module M
+            def f
+              @f = true
+            end
+          end
+        end
+      RUBY
+    end
+
+    let(:offending_lines) { [4] }
+
+    it_behaves_like 'registering offense'
+  end
+
+  context 'when source is a nested module with multiple offenses' do
+    let(:source) do
+      <<~RUBY
+        module N
+          module M
+            def f
+              @f ||= true
+            end
+
+            def g
+              true
+            end
+
+            def h
+              @h = true
+            end
+          end
+        end
+      RUBY
+    end
+
+    let(:offending_lines) { [4, 12] }
+
+    it_behaves_like 'registering offense'
+  end
+
+  context 'when source is offending but it is a rails helper' do
+    before do
+      allow(cop).to receive(:rails_helper?).and_return(true)
+    end
+
+    it 'does not register offenses' do
+      inspect_source(cop, <<~RUBY)
+        module M
+          def f
+            @f ||= true
+          end
+        end
+      RUBY
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+
+  context 'when source is offending but it is a rails mailer' do
+    before do
+      allow(cop).to receive(:rails_mailer?).and_return(true)
+    end
+
+    it 'does not register offenses' do
+      inspect_source(cop, <<~RUBY)
+        module M
+          def f
+            @f = true
+          end
+        end
+      RUBY
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+end