I much prefer separate files for type declarations. Or at least the ability to define them separately. Type annotation takes away from readability. I like keeping the types and code separate.
I don't see how having to switch files to know that `input` is a `User` increases readability, though. It seems like straight-forward impl-simplicity trade-off, not one of user ergonomics.
IDEs will likely be able to seamlessly peek/go to RBS type definition on any Ruby identifier in any case.
Also how do you type something in an inline function?
Do you mean for Ruby specifically or in general? I've found that it's much easier to (safely, accurately) read, use, and extend e.g. a TypeScript file than its JavaScript counterpart, even when provided with a .d.ts file.
I don't disagree, but I think it's a very minor issue given that it's trivial to use color to highlight code these days. By comparison having to switch between two files (and keep them in sync!) when making changes is a far bigger usability concern.