Function rustc_apfloat::ieee::sig::widening_mul

pub(super) fn widening_mul(a: u128, b: u128) -> [u128; 2]

[low, high] = a * b.

This cannot overflow, because

(n - 1) * (n - 1) + 2 * (n - 1) == (n - 1) * (n + 1)

which is less than n².