Function safe_arch::bit_lowest_set_mask_u64

#[must_use]pub fn bit_lowest_set_mask_u64(a: u64) -> u64

This is supported with target feature bmi1 only.

Gets the mask of all bits up to and including the lowest set bit in a u64.

If the input is 0, you get u64::MAX

• Formula: (a - 1) ^ a

assert_eq!(bit_lowest_set_mask_u64(0b0), u64::MAX);
assert_eq!(bit_lowest_set_mask_u64(0b1), 0b1);
assert_eq!(bit_lowest_set_mask_u64(0b10), 0b11);
assert_eq!(bit_lowest_set_mask_u64(0b100), 0b111);
assert_eq!(bit_lowest_set_mask_u64(0b111100), 0b111);