Added platform dependent gems

See #12475

If people are on rbx they will get the desired gem

railties/lib/rails/generators/app_base.rb

@@ -110,6 +110,7 @@ def gemfile_entries
           javascript_gemfile_entry,
           jbuilder_gemfile_entry,
           sdoc_gemfile_entry,
+          platform_dependent_gemfile_entry,
           @extra_entries].flatten.find_all(&@gem_filter)
       end
 
@@ -316,6 +317,14 @@ def assets_gemfile_entry
         gems
       end
 
+      def platform_dependent_gemfile_entry
+        gems = []
+        if RUBY_ENGINE == 'rbx'
+          gems << GemfileEntry.version('rubysl', nil)
+        end
+        gems
+      end
+
       def jbuilder_gemfile_entry
         comment = 'Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder'
         GemfileEntry.version('jbuilder', '~> 1.2', comment)

railties/test/generators/app_generator_test.rb

@@ -305,6 +305,13 @@ def test_inclusion_of_javascript_runtime
     end
   end
 
+  def test_inclusion_of_plateform_dependent_gems
+    run_generator([destination_root])
+    if RUBY_ENGINE == 'rbx'
+      assert_gem 'rubysl'
+    end
+  end
+
   def test_creation_of_a_test_directory
     run_generator
     assert_file 'test'