Function within 

pub fn within(
    func: &Func,
    order: &Order,
    live: &Live,
    env: &Env,
    budget: u64,
) -> Option<Assignment>

The same, with the budget said, or None for a function that went over it.

Panics

Panics on a value in a class the environment hands out no registers from, as the linear scan does.