The upside of external files is pure incremental implementation that touches no other tooling and requires no buy-in.
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.
That can be covered by the editor to give the user some hint by referencing the external file but for the user, having have to keep adding it on a separate file seems pretty annoying as you need to keep declarations synched in 2 files.
Also how do you type something in an inline function?
Separating type definitions from code can be considered as contributing to readability of idiomatic Ruby on one hand, and type definitions on the other, taken separately on their own—by not imposing constraints on either syntax.
IDEs will likely be able to seamlessly peek/go to RBS type definition on any Ruby identifier in any case.
