WIP: add CheckPluginIsEnabled cop

config/default.yml

@@ -90,3 +90,6 @@ Discourse/Plugins/UseRequireRelative:
 
 Discourse/Plugins/NoMonkeyPatching:
   Enabled: true
+
+Discourse/Plugins/CheckPluginIsEnabled:
+  Enabled: true

lib/rubocop/cop/discourse/plugins/check_plugin_is_enabled.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Discourse
+      module Plugins
+        class CheckPluginIsEnabled < Base
+          MSG = "Check the plugin is enabled."
+
+          def_node_matcher :existing_class?, <<~MATCHER
+            (class (const (cbase) _) ...)
+          MATCHER
+
+          def_node_matcher :plugin_enabled_check?, <<~MATCHER
+            (def _ _ (begin (if (send (const nil? :SiteSetting) /_enabled\\?$/) nil? (return)) ...))
+          MATCHER
+
+          def on_def(node)
+            return unless in_plugin_rb_file?
+            return unless in_existing_class?(node)
+            return if plugin_enabled_check?(node)
+            add_offense(node, message: MSG)
+          end
+
+          private
+
+          def in_plugin_rb_file?
+            processed_source.path.split("/").last == "plugin.rb"
+          end
+
+          def in_existing_class?(node)
+            node.each_ancestor(:class).detect(&method(:existing_class?))
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/rubocop/cop/plugins/check_plugin_is_enabled_spec.rb

@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe RuboCop::Cop::Discourse::Plugins::CheckPluginIsEnabled,
+               :config do
+  subject(:cop) { described_class.new(config) }
+
+  let(:config) { RuboCop::Config.new }
+
+  context "when outside `plugin.rb`" do
+    it "does not register an offense" do
+      expect_no_offenses(<<~RUBY, "my_class.rb")
+        class ::MyClass
+          def my_method
+            "my_value"
+          end
+        end
+
+        class AnotherClass
+          def my_method
+            "my_value"
+          end
+        end
+      RUBY
+    end
+  end
+
+  context "when inside `plugin.rb`" do
+    context "when opening an existing class" do
+      it "registers an offense" do
+        expect_offense(<<~RUBY, "plugin.rb")
+          after_initialize do
+            class ::Post
+              after_commit :generate_zendesk_ticket, on: [:create]
+
+              private
+
+              def generate_zendesk_ticket
+              ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Discourse/Plugins/CheckPluginIsEnabled: Check the plugin is enabled.
+                return unless DiscourseZendeskPlugin::Helper.autogeneration_category?(topic.category_id)
+                Jobs.enqueue_in(5.seconds, :zendesk_job, post_id: id)
+              end
+            end
+          end
+        RUBY
+      end
+
+      it "does not register an offense" do
+        expect_no_offenses(<<~RUBY, "plugin.rb")
+          after_initialize do
+            class ::Post
+              after_commit :generate_zendesk_ticket, on: [:create]
+
+              private
+
+              def generate_zendesk_ticket
+                return unless SiteSetting.zendesk_enabled?
+                return unless DiscourseZendeskPlugin::Helper.autogeneration_category?(topic.category_id)
+                Jobs.enqueue_in(5.seconds, :zendesk_job, post_id: id)
+              end
+            end
+          end
+        RUBY
+      end
+    end
+  end
+end