Use global namespace Rails

railties/lib/rails/generators/rails/plugin_new/templates/lib/%name%/engine.rb

 module <%= camelized %>
-  class Engine < Rails::Engine
+  class Engine < ::Rails::Engine
 <% if mountable? -%>
     isolate_namespace <%= camelized %>
 <% end -%>

railties/test/generators/plugin_new_generator_test.rb

@@ -148,7 +148,7 @@ def test_creating_engine_in_full_mode
     assert_file "app/views"
     assert_file "app/helpers"
     assert_file "config/routes.rb", /Rails.application.routes.draw do/
-    assert_file "lib/bukkits/engine.rb", /module Bukkits\n  class Engine < Rails::Engine\n  end\nend/
+    assert_file "lib/bukkits/engine.rb", /module Bukkits\n  class Engine < ::Rails::Engine\n  end\nend/
     assert_file "lib/bukkits.rb", /require "bukkits\/engine"/
   end