The Zig approach is "memory safe in practice" vs "memory safe in theory". They don't have any aspirations to total memory safety like Rust, but they want to get most of the way there with a lot less overhead.
Basically they have a lot of runtime checks enabled in debug mode, where you do the majority of your testing, that are then disabled in the release binary.
Additionally the approach they've taken to allocators means that you can use special allocators for testing that can perform even more checks, including leak detection.
I think it's a great idea and a really interesting approach but it's definitely not as rigorous as what Rust provides.
I don't think Zig is going to be memory safe in practice, unless they add a GC or introduce a Rust-like system. All of the mitigations I've seen come from that language--for example, quarantine--are things that we've had for years in hardened memory allocators for C++ like Chromium PartitionAlloc [1] and GrapheneOS hardened_malloc [2]. These have been great mitigations, but have not been effective in achieving memory safety.
Put another way: Anything you could do in the malloc/free model that Zig uses right now is something you could do in C++, or C for that matter. Maybe there's some super-hardened malloc design yet to be found that achieves memory safety in practice for C++. But we've been looking for decades and haven't found such a thing--except for one family of techniques broadly known as garbage collection (which, IMO, should be on the table for systems programming; Chromium did it as part of the Oilpan project and it works well there).
There is always a temptation to think "mitigations will eliminate bugs this time around"! But, frankly, at this point I feel that pushing mitigations as a viable alternative to memory safety for new code is dangerous (as opposed to pushing mitigations for existing code, which is very valuable work). We've been developing mitigations for 40 years and they have not eliminated the vulnerabilities. There's little reason to think that if we just try harder we will succeed.
You understand "memory safe in practice" as soundly eliminating all memory safety issues. This is not how I understand it. Zig can exceed Rust's memory safety in practice without soundly eliminating all issues. The reason is that many codebases rely on unsafe code, and finding problems in Zig can be cheaper than finding problems in Rust w/ unsafe. This is even more pronounced when we look at security overall because while many security issues are memory safety issues, many aren't (and most aren't use-after-free bugs); in other words, it's certainly possible that paying to eliminate all use-after-free harms security more than just catching much of it more cheaply. So there is no doubt that Rust programs that don't use unsafe will have fewer use-after-free bugs than Zig programs, but it is very doubtful that they will, on average, be more secure as a result of this tradeoff.
> Basically they have a lot of runtime checks enabled in debug mode, where you do the majority of your testing, that are then disabled in the release binary.
But there's the problem: Testing can't and won't cover all inputs that a malicious attacker will try [1]. Now you've tested all inputs you can think of with runtime checks enabled, you release your software without runtime checks, and you can be sure that some hacker will find a way to exploit a memory bug in your code.
[1] Except for very thorough fuzzing. Maybe. If you're lucky. But probably not.
It's not “memory safe in practice”. It's “we provide tools to make our memory unsafe language with as little memory issue as possible”. Is it better than what C or C++ offer out of the box: yes. It's totally reasonable to think that it may be as good as C or C++ with state of the art tooling that most programmers aren't using today because they don't want to invest the effort, so this is a big progress over C.
Basically they have a lot of runtime checks enabled in debug mode, where you do the majority of your testing, that are then disabled in the release binary.
Additionally the approach they've taken to allocators means that you can use special allocators for testing that can perform even more checks, including leak detection.
I think it's a great idea and a really interesting approach but it's definitely not as rigorous as what Rust provides.