rails/rails — pull request #8232 by YasuOza (2012-11-16 02:22:48+00:00)
Title: Write config.threasafe! deprecated waning in config/environments/production.rb

Since Rails 4.0 config.threadsafe! is enabled by default.
I think we have to strongly warn users about it by not only showing deprecated warnings when
user use this option, but also writing deprecated warnings in config file.