Referencing the type alias outside of the module

Since it is declared to be public, the type alias OddIntegers can be referenced from outside the module odd. This allows callers to give a name to the return type of odd_integers:

mod odd {
    pub type OddIntegers = impl Iterator<Item = u32>;
    pub fn odd_integers(mut start: u32, stop: u32) -> OddIntegers { /* as above */ }
}

fn main() {
    let odds: odd::OddIntegers = odd::odd_integers(1, 10);
    for i in odds {
        println!("{}", i);
    }
}

Because that code is outside of the defining scope, however, it is not allowed to influence or observe the hidden type. It can only rely on the that were declared¹ (e.g., Iterator<Item = u32>). For example, the following code would not type check:

mod odd {
    pub type OddIntegers = impl Iterator<Item = u32>;
    pub fn odd_integers(mut start: u32, stop: u32) -> OddIntegers { /* as above */ }
}

fn main() {
    // Error!
    let odds: std::iter::StepBy<std::iter::Range<u32>> =
        odd::odd_integers(1, 10);
    for i in odds {
        println!("{}", i);
    }
}

This code fails because odds is not known to have that exact type -- main can only see that odds returns "some kind of iterator".