Find elements in container within given bounds

main.rs

fn main() {
    println!(
        "{:?}",
        elements_in_container(String::from("|**|*|*|"), 0, 5)
    );
    println!(
        "{:?}",
        elements_in_container(String::from("|**|*|*|"), 0, 6)
    );
    println!(
        "{:?}",
        elements_in_container(String::from("|**|*|*|"), 1, 7)
    );
    println!(
        "{:?}",
        elements_in_container(String::from("**||**|*|***"), 1, 7)
    );
    println!(
        "{:?}",
        elements_in_container(String::from("************"), 1, 7)
    );
    println!("{:?}", elements_in_container(String::from("**|***"), 1, 5));
    println!("{:?}", elements_in_container(String::from("|*****"), 1, 5));
    println!("{:?}", elements_in_container(String::from("||****"), 1, 5));
    println!("{:?}", elements_in_container(String::from("||||||"), 1, 5));
}

fn elements_in_container(s: String, from: usize, to: usize) -> Option<usize> {
    if to < from {
        return None;
    }

    if s.len() <= 1 {
        return None;
    }

    let sub = &s[from..to];

    let mut fwd = sub.char_indices();
    let mut rev = sub.char_indices().rev();

    let mut left = fwd.next().unwrap();
    let mut right = rev.next().unwrap();
    while left.1 != '|' || right.1 != '|' {
        if left.1 != '|' {
            match fwd.next() {
                Some(n) => left = n,
                None => return None,
            }
        }
        if right.1 != '|' {
            match rev.next() {
                Some(n) => right = n,
                None => return None,
            }
        }

        if left.0 > right.0 {
            return None;
        }
    }

    let n = sub[left.0..right.0].chars().filter(|x| *x == '*').count();

    if n == 0 {
        return None;
    }
    Some(n)
}