Code shared from the Rust Playground

playground.rs

fn main() {
    let val = Some(10);
    //let val = None;
    let z = maybe_add_four(val);
    println!("{:?}", z)
}

fn maybe_add_four(y: Option<i32>) -> Option<i32> {
    let ret = y.map(|x| x + 4);
    assert_eq!(y, Some(10));
    ret
}