> This is simply not true. Perhaps you mean that you don't have a guarantee that your code is memory-safe, but that's not the same thing.

“But people can write correct C code”. Correct Zig != memory safety. It's the opposite: MEMORY SAFETY IS THE GUARANTEE that your code won't have memory error no matter how broken it is!

> Another is to write it in a language that guarantees no such error can happen in development, do some testing, and then remove the guarantees.

That's what the same kind of design as C is with ASan, TSan, MemSan etc. Yes Zig is less broken than C, leading to fewer sources of memory issues, but for what matters most (Double Free, Use After Free[1], Data Races) Zig and C offers the same level of safety guarantees: none.

> As someone who works with formal methods, we do these tradeoffs in formal verification all the time. It is simple false that sound guarantees are always the best way to correctness -- it would be if they were free, but they're not.

This is a straw man: comparing compile-time enforced ownership (Rust borrowck) to formal method doesn't make any more sense than comparing static typing to formal methods. It adds a lot of learning friction, but that's it. I just grepped my current 90kLoc rust project. You know how many lifetime annotation ('x) there is in it? Fifty-four! Which is one every 1666 lines. Please tell me again how much it cripples productivity and the ability to write correct code!

> Because Zig is a simple language, the chances of such paths existing without you noticing are lower;

If you ever try to use shared-memory parallelism, this kind of bugs will be everywhere! That's simple: every call to allocator.free is a minefield.

> ultimately, we don't care what bug breaks our program or leaves it open to security vulnerabilities

Memory safety issues aren't just security vulnerabilities, more than anything they are horrible bugs to track down, and it costs tons of money.

> Which is exactly what I mean by soundness coming at a cost. It guarantees the lack of certain bugs, but because it complicates the language, it can make other bugs more costly to detect.

This is BS. It's not because a language has few symbols or a simple syntax that it is easier to debug. Otherwise brainfuck would be the ultimate productivity tool. Semantic is what matters, and because it has UBs, Zig semantic is more complex than most languages out there. That's why C is one of the most complex language ever in practice, even if it's really “simple” and easy to “learn”.

Again, don't get me wrong, I have nothing against Zig and I find it refreshing because it has tons of cool ergonomic tricks (and having a built-in sanitizer which “just works” out of the box in debug mode without any other programmer intervention is cool!). It's a nice programming language experiment that will probably inspire a lot of others, and it's probably a really cool language for C programmers who like to manage their memory by themselves and don't want the “nany compiler” Rust has and still have a language with a modern look and feel: that's totally legit.

But memory safe, it isn't.

[1]: which cause more than 30% of Google and Microsoft security issues by itself! (https://www.zdnet.com/article/microsoft-70-percent-of-all-se... https://www.chromium.org/Home/chromium-security/memory-safet...)