size_of and align_of

fn make_small_bigint(value: i64) -> BigNum{
// Do whatever simple logic you need here.
}

// Can also have e.g. make_small_rational(numerator, denominator), etc.

// Global scope:
static OnceLock<BigNum> FOUR = OnceLock::new();

// Callsite:
let four = FOUR.get_or_init(|| make_small_bigint(4));

LazyLock just means you can write let four = *FOUR; instead of let four = FOUR.get_or_init(|| make_small_bigint(4));. You still have to write the function make_small_bigint() either way, because you will need to pass exactly the same lambda to the LazyLock at construction time.