Both. The borrow checker encodes a lot of rules and habits which experienced C and C++ developers tend to develop anyway, because breaking those rules tends to

Both. The borrow checker encodes a lot of rules and habits which experienced C and C++ developers tend to develop anyway, because breaking those rules tends to result in bugs (and people who learn rust often report that it makes them better C/C++ programmers). But it is possible to break those rules and get away with it in ways which the borrow checker is unable to verify, and unsafe blocks exist in part to allow you to do so if you're really sure you're right (though frequently beginners are not).