What is "frozen_string_literal" in Ruby?

Freezing Strings feature improves apps performance by freezing Strings. So, Matz, Ruby's creator, decided to make all String literals frozen (immutable) by default in Ruby 3.0. Update: the Ruby team changed their mind and this didn't happen when Ruby 3.0 was released.

In order to have a transition path to this coming big change, it was decided to have a magic comment at the beginning of files, so you can use in Ruby 2.x. To do so, just add this comment in the first line of your files:

# frozen_string_literal: true
class YourClass
  # ...
end

Try adding it to your spec_helper or rails_helper file and let me know in the comments if your performance gets any better.

More info: Ruby issue #8976.