RBS, Ruby’s new type signature language

I likely am missing some context, but the comparison to TypeScript's ".d.ts" files seems misplaced. This is a type signature language, but it does not seem to be a type checker.

The comparison to .d.ts files then seems bizarre because that is helpful for language servers¹ to consume types, but there's no proof that say, the implementation matches the type specification.

TypeScript declaration files declare what the types of module exports are. For the most part, a .d.ts file informs the typechecker "the type of module Foo export bar is the interface named Quux". This is not checked, this is simply an assertion. The language server for TypeScript will pick these definitions up, assume they are correct, and provide code completions for those types as if they were correct.

On the other hand, a .ts file, combining types and codes, enables type checking. If the type declarations are incompatible with the code, an error is thrown by TypeScript. While .d.ts files declare types, .ts files verify that the code and the types declare are compatible.

Since .rbs files simply describe the external interface of types and modules, and cannot describe internal variables, I'm not sure how it's doing any type checking.

For example, if I have this code:

    module Foo
      class Bar
        def trivial: () -> String
      end
    end

What prevents me from writing this:

    class Bar
      def trivial
        return 42
      end
    end

Or alternatively, this:

    class Bar
      def trivial
        x = some_function_that_might_not_be_declared_in_an_rbs_file()
        return x
      end
    end

Does x have a type? Can I gradually type "class Bar" via type inference, or do I have to declare and keep in sync all my rbs files with my rb files? What happens when the rbs file is out of sync with the rb file?

¹ Language servers are implementations of an IDE protocol for code completion. The trend in programming language tooling is to use Microsoft's Language Server Protocol (https://microsoft.github.io/language-server-protocol/) to provide code completion, semantic navigation, refactoring, etc.