Function no_host_bits 

fn no_host_bits(addr: u128, prefix: u8, width: u8) -> bool

Returns true when the low width - prefix host bits of addr are all zero.

Preconditions

Callers must ensure prefix <= width. The host_bits == 0 branch is not merely an optimisation: it is load-bearing for panic-safety. When prefix == width, host_bits is 0 and we return early, avoiding the expression u128::MAX >> 128, which would panic due to Rust’s overflow checks on shift amounts.