> Unsafe does not allow unsound behavior, or rather, it lets you write unsound behavior but that is a bug. You still have to uphold various invariants in unsafe code. It’s there for when you know you can, but the compiler cannot, which is an inherent property of any sound system.

I don't understand what you're saying here.

Rust-without-'unsafe' is a sound language; i.e. any program which types will be unable to violate runtime invariants.

Rust-with-'unsafe' is an unsound language; i.e. there are programs which type but yet violate runtime invariants.

That is a bug in those programs—sure, yes, of course. It is not a bug in rust; it is the language functioning as intended. However the topic at hand is the relationship between rust's type system and c's; hence unsafe is completely uninteresting, because it is a tool for subverting rust's type system.

(It occurs to me now that I might have gotten a less generally defensive response had I made the original comparison between rust and a hypothetical rust without unsafe. Oh well)

> However the topic at hand is the relationship between rust's type system and c's; hence unsafe is completely uninteresting,

Gotcha, I was just trying to clarify the semantics of unsafe, lots of people think it suddenly lets you do whatever you want, and while it will let you, that doesn't mean that you can, which is an important distinction. You're right that this puts it in the same general category of C in a theoretical perspective, I think practice will bear out that it's not a binary proposition, but time will tell :)

If manually managing memory is like wielding a gun, the borrow checker is an automatic safety that prevents you from pulling the trigger when you're roughly pointing it at yourself. But it's coarse-grained and errs on the side of caution; it simulates your foot as as the rectangular box that would contain it, not as a detailed 3D mesh. If you really think you can aim it between your toes and avoid hitting yourself (for example, "the value returned by this function must remain alive for no more than 15 successive invocations of this function"), unsafe will let you try, but the borrow checker's built-in rules isn't granular enough to help you, though it will still stop you if you accidentally put your hand in front without declaring it.

C is like a laser gun in a room full of oddly-shaped mirrors with no safety at all. Good luck.

> it's coarse-grained and errs on the side of caution

That's what type systems in general do. There is no type system for a turing-complete language that is both sound and complete. (Trivial proof: if (undecidable) halt; unsound.)