#![allow(dead_code, deprecated, unused_variables, unused_mut)]
fn main() {
    let x = &mut [0, 1, 2];
    if let Some((first, elements)) = x.split_first_mut() {
        *first = 3;
        elements[0] = 4;
        elements[1] = 5;
    }
    assert_eq!(x, & [3, 4, 5]);
}