Well, as long as you avoid unsafe blocks (which turn off a few safety features in a specific scope so you can do complex or performance critical things in that scope) you're supposed to be safe, but to my knowledge it's not formally proven. In practice it seems to be working quite well though.

Well, you can very easily: write bad code in unsafe blocks.

That said, your badness is contained within unsafe blocks, so hopefully you have much less code to closely review.