Abstract return types (written impl Trait for some trait Trait) are only allowed as function and inherent impl return types.

Erroneous code example:

fn main() {
    let count_to_ten: impl Iterator<Item=usize> = 0..10;
    // error: `impl Trait` not allowed outside of function and inherent method
    //        return types
    for i in count_to_ten {
        println!("{}", i);
    }
}

Make sure impl Trait only appears in return-type position.

fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
    0..n
}

fn main() {
    for i in count_to_n(10) {  // ok!
        println!("{}", i);
    }
}

See RFC 1522 for more details.