Error code E0562

impl Trait is only allowed as a function return and argument type.

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 appears in a function signature.

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 the reference for more details on impl Trait.