It is, by implementing a custom struct, however I don't think it's a problem here. I believe it is more simple for everyone (and since it has been designed this way we're not gonna change it since it works and it would invalidate all previous solution).

I was under impression these should be possible to complete using stdlib. However now I am not quite certain about it...

Is it a problem?

Rust version is using 3rd-party crate "Either" which is not part of the standard library.

There is no real advantage, it just makes the solution merely functional (no need for a named argument). It's a different programming style!

Do you know you can fork the solution and see what happens if you remove this part? Just click on fork below the code. Rust compiler explains things very well :)

What's the advantage over a function with .to_uppercase()?

Good catch on non-ASCII characters!