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)
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 :)
C is like a laser gun in a room full of oddly-shaped mirrors with no safety at all. Good luck.
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.)
For the record, the “topic at hand” was the memory model not the type-system, nor the theorem proving capabilities of the borrow checker.
Your arguments are not wrong per se, but they are completely missing the topic.
> Of course it can
You cannot share objects using rust's ownership system.
Interior mutability involves an unacceptable performance hit (and doesn't actually allow you to share anything). And if you use shared indices into a centralized array, then you have blinded the ownership system, creating pointers that it cannot make sense of.
>> This function is not locally correct; in a vacuum, it is impossible to say whether this function violates invariants.
> That is also a use-case for unsafe: an unsafe function is one for which the caller is responsible for checking invariants.
That's exactly right, and was my thesis; rust's type system does not allow you to prove the correctness of such a function, because it is neither correct nor incorrect without context. 'Unsafe', as a subversion of the type system which allows unsound behaviour, is completely uninteresting wrt the topic at hand; the comparison is a purely theoretical one between non-'unsafe' rust—that is, rust's ownership model—and c.