strings.Cut is otherwise known in Rust as str::split_once. It's something I've always missed when writing Go. Nice to see it added to the stdlib
split_once() and similar split features in Rust are interesting because Rust doesn't have overloading, yet you can split on a string, or a character. This relies on a Trait, Pattern, that is Nightly, so you can't Implement it yourself in stable Rust today, but eventually this factors out the commonality which is cleverer than overloading (because it applies everywhere automatically)
The point is you can't implement Pattern, because it's a nightly feature. If you just make your own, that isn't Pattern and provides none of the benefits.
Yes, you can implement your own trait, and provide blanket implementations for it, but unlike Pattern yours is not part of the standard.
The nice thing about Pattern is that if Rust added say split_exactly_six_times() to the standard library that would take separator: Pattern too and so things which implement Pattern qualify, I can't see a way to get that benefit for my own traits.
would be neat to imagine a language where this was an optimizer feature rather than a decision the programmer made. many functions that return an array are ultimately using just one, or a few elements of it. the compiler can make a lot of decisions in the caller about how to reduce allocation or short-circuit array processing.
I think, but am not sure, this is what the experimental parsing language 'wuffs' is about https://github.com/google/wuffs -- the language itself is aware of array lengths as a first-class citizen and can make decisions accordingly