Only add debugger/byebug if on MRI

railties/lib/rails/generators/rails/plugin/templates/Gemfile

@@ -37,7 +37,7 @@ end
 <% end -%>
 
 <% end -%>
-<% unless defined?(JRUBY_VERSION) -%>
+<% if RUBY_ENGINE == 'ruby' -%>
 # To use a debugger
   <%- if RUBY_VERSION < '2.0.0' -%>
 # gem 'debugger', group: [:development, :test]

railties/test/generators/app_generator_test.rb

@@ -340,7 +340,7 @@ def test_inclusion_of_jbuilder
 
   def test_inclusion_of_a_debugger
     run_generator
-    if defined?(JRUBY_VERSION)
+    if defined?(JRUBY_VERSION) || RUBY_ENGINE == 'rbx'
       assert_file "Gemfile" do |content|
         assert_no_match(/byebug/, content)
         assert_no_match(/debugger/, content)