merge_sort in safe Rust

lib.rs

pub fn merge_sort<T: Ord>(a: &mut [T]) {
    let len = a.len();
    if len < 2 { return }
    
    let mid = len / 2;
    {
        let (left, right) = a.split_at_mut(mid);
        merge_sort(left);
        merge_sort(right);
    }
    
    let mut l = 0;
    let mut r = mid;
    let mut o = 0;
    
    loop {
        match (l < r, r < len) {
            (true, true) => if a[l] <= a[r] {
                a.swap(o, l);
                if l == o { l += 1; }
                else { a[l..r].rotate_left(1); }
                o += 1;
            } else {
                a.swap(o, r);
                if l == o { l = r; }
                o += 1;
                r += 1;
            },
            (true, false) => {
                a[o..r].rotate_left(l - o);
                break;
            }
            (false, true) => break,
            (false, false) => break,
        }
    }
}