All of the answers ignore the fact, that the OP wants to sum i32, not u32 from 0 to n. This implies that there may also be negative sums involved (0 + -1 + -2 + ... + n). Hence, I propose:

fn summation(n: i32) -> i32 {
    if n < 0 {
        (n..0).sum()
    } else {
        (0..=n).sum()
    }
}