Checked add signed to usize.

usize_add.rs

fn add(u: usize, i: i32) -> Option<usize> {
    if i.is_negative() {
        u.checked_sub(i.wrapping_abs() as u32 as usize)
    } else {
        u.checked_add(i as usize)
    }
}

fn main() {
    let u = 7;
    let i1 = -1;
    let i2 = 1;

    assert_eq!(add(u, i1), Some(6));
    assert_eq!(add(u, i2), Some(8));
    assert_eq!(add(0, -1), None);
}