问如何知道迭代器的当前位置？

EN

fn one_indexed<T>((n, x): (usize, T)) -> (usize, T) {
    (n+1, x)
}
fn main() {
    let s = "abc def\nghi jkl";
    for (line_n, line) in s.lines().enumerate().map(one_indexed) {
        for (char_n, char) in line.chars().enumerate().map(one_indexed) {
            println!("character {} at {}:{}", char, line_n, char_n);
        }
    }
}

character a at 1:1
character b at 1:2
character c at 1:3
character   at 1:4
character d at 1:5
character e at 1:6
character f at 1:7
character g at 2:1
character h at 2:2
character i at 2:3
character   at 2:4
character j at 2:5
character k at 2:6
character l at 2:7