[kbenson]

I'm pretty sure the answer actually is that certain things are provably safe, at least in a way that can easily be accomplished by the compiler, and some aren't (at least simply by a compiler at our current understanding). We may be able to reason about a situation and prove to our understanding that something is safe, but that doesn't always mean we can encode those rules in a deterministic and terminating way.

I imagine rust would happily extend what it considers safe if it can be determined in an effective way and if it requires additional decoration, doesn't conflict with current syntax.