Function stack_unused_bytes_upto 

pub fn stack_unused_bytes_upto(limit: usize) -> usize

stack_unused_bytes, capped at limit: min(unused, limit), reading at most limit bytes of the calling task’s stack (phase-436 E5).

What the stack-headroom-runtime rule needs: whether at least limit bytes are unused. Below limit the answer is the exact headroom, so a breach is reported without a second, full scan. 0 means “not instrumented”, as for stack_unused_bytes.