Function borrowing_sub_mask_u64 

pub const fn borrowing_sub_mask_u64(
    lhs: u64,
    rhs: u64,
    borrow: u64,
) -> (u64, u64)

Subtract rhs and an incoming borrow from lhs, using word-sized borrow masks.

The incoming borrow is the most significant bit of borrow; all other bits are ignored. Returns the wrapping difference and an all-ones mask if the subtraction underflows, or zero otherwise.

The outgoing mask can be passed directly to the next limb’s subtraction, avoiding conversions to and from Choice at each step of a multi-limb subtraction.