I don't think Ruby 3 itself will provide a typechecker, just a standard for type definition file formats. You have to use a third-party tool, like Steep or Sorbet, to do the type-checking – and Sorbet at least does support inline type information. See more at my comment here: https://news.ycombinator.com/item?id=23991258

You won't need to use the header RBS files at all (types are optional in any case) but you'll likely want to use Sorbet or Steep to generate them if you're sharing your code more widely, since community tooling like YARD will probably use those for code navigation.