custr_append_vprintf
ret = custr_append_vprintf(s->s_custr, fmt, ap);
custr_append_vprintf((s)->s_custr, (fmt), (args))
ret = custr_append_vprintf(cus, fmt, ap);
int custr_append_vprintf(custr_t *, const char *, va_list);
ret = custr_append_vprintf(st->rs_demangled, fmt, ap);
if (custr_append_vprintf(st_custr, fmt, va) == -1) {