Exactly what? I'm not sure what you're referring to here exactly.

Generally newtypes in Rust are not used excessively and largely for sensible safety assertions. I don't see people making `EmailAddress(String)` newtypes.

That said, I have personally used newtypes a reasonable amount, mostly for making sure I don't mess up different kinds of database ids.

It is not. `NonZeroUsize` is (though in Rust "newtype" is only a pattern or idiom), and its definition (inside of a macro which provides numeric newtypes a bunch of `impl`s) is:

    pub struct NonZeroUsize(usize);

    pub struct String {
        vec: Vec<u8>,
    }

1. Marking the kind of data to prevent mixing up values of different kinds. (e.g. Email)

2. Outlawing some of the values of the underlying type. (e.g. OneToFive)

I do not think it being a tuple-struct or not is important at all when deciding whether something uses the newtype pattern, as long as it has a single field.

(I was going to be cheeky here, but Git history shows you didn't write any of this part.)

There's no real difference between a 1-tuple struct and the regular struct, other than the name. So to me, it feels like either are both newtypes. But, I guess I could see some sort of argument the other way too.

"Newtypes as tokens"

Newtypes do work to assert certain checks have already been done and need not be repeated.