Usage You need to tell RuboCop to load the Performance extension. There are three ways to do this: RuboCop configuration file Put this into your .rubocop.yml. require: rubocop-performance Now you can run rubocop and it will automatically load the RuboCop Performance cops together with the standard cops. Command line $ rubocop --require rubocop-performance Rake task RuboCop::RakeTask.new do |task| task.requires << 'rubocop-performance' end Installation Cops