If you don't join the threads a panic get stopped at thread boundaries (unless you have taken a lock when the panic happens, in which case the lock is poisoned

If you don't join the threads a panic get stopped at thread boundaries (unless you have taken a lock when the panic happens, in which case the lock is poisoned and attempting to lock it again will return an error. Error that will most likely trigger a further panic since you usually write something like let mutex_guard= mutex.lock().expect("poisoned lock");