void runtime_assert_at(int cond, const &char msg, const &char file, int line)Assert that cond is true at runtime, reporting the given location.
On failure: prints "Assertion failed: <msg> at <file>:<line>" to stderr
and calls abort(). Pass FILE and LINE for the call site:
runtime_assert_at(ptr != 0, "pointer must not be null", __FILE__, __LINE__);