Ok, I agree. But this is just as well a problem for any language that specifies API in some way. So it is really a decades old debate about static typing disallowing to change things easily. I mean, there might always be certain case when the library author has to change the signature because they made it too restrictive. It just happens that Rust allows you to constrain things that might not be constrainable in other languages, so I guess the chances for that happening accidentally are somewhat higher. But generally I like going from more concrete / restricted to more generic when needed rather than the other way round.

> Off topic, but you could do this in both Java and Go via generics.

I don't think so you can achieve same level of flexibility and efficiency at the same time.

Go doesn't have function level generics, so you'd need interfaces and runtime polymorphism as usual (and runtime penalty).

And in Java you cannot add a new interface implementation to a builtin type like String, so the idea of switching from String to any other type won't work. There is much more upfront ceremony needed to add such flexibility (defining custom interface + implementations and using them from the beginning).