Spawn short lived thread

fn main() {
  let arr = &[1, 25, -4, 10];
  let max = find_max(arr);
  assert_eq!(max, Some(25));
}
fn find_max(arr: &[i32]) -> Option<i32> {
  const THRESHOLD: usize = 2;
  if arr.len() <= THRESHOLD {
    return arr.iter().cloned().max();
  }
  let mid = arr.len() / 2;
  let (left, right) = arr.split_at(mid);
  crossbeam::scope(|s| {
    let thread_l = s.spawn(|_| find_max(left));
    let thread_r = s.spawn(|_| find_max(right));
    let max_l = thread_l.join().unwrap()?;
    let max_r = thread_r.join().unwrap()?;
    Some(max_l.max(max_r))
  }).unwrap()
}