#![allow(dead_code, deprecated, unused_variables, unused_mut)]
fn main() {
    let mut v = [100, 400, 300, 200, 600, 500];
    let mut count = 0;
    for group in v.rsplit_mut(|num| *num % 3 == 0) {
        count += 1;
        group[0] = count;
    }
    assert_eq!(v, [3, 400, 300, 2, 600, 1]);
}