We’re pleased to announce Ruby 3s new language for type signatures, RBS. One of the long-stated goals for Ruby 3 has been to add type checking tooling. After much discussion with Matz and the Ruby committer team, we decided to take the incremental step of adding a foundational type signature language called RBS, which will ship with Ruby 3 along with signatures for the stdlib. RBS command line tooling will also ship with Ruby 3, so you can generate signatures for your own Ruby code.

Source: The State of Ruby 3 Typing - Square Corner Blog