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> To actually get the safety benefits of Rust in a real way you have to rework those files to not treat eachother as C code. This is the interesting and the difficult part of a rewrite in Rust, and one that an unsupervised LLM rewrite is probably not even going to attempt.

You can use deterministic linting for `unsafe` usage, have agents target `unsafe`, etc. It's pretty easy. You can even run `miri` against the code and give that as a tool for LLM feedback. I've done this all before and it works fine.

> The result of the Rust rewrite in Bun's case is actually less safe than the Zig it is replacing, especially because (I am told) a lot of the new Rust code is unsound (introducing UB).

I'm unconvinced that this is true. How could you tell? You only know about the rust bugs because rust makes them grep'able/ trivial to verify, there's no way of knowing which bugs existed in zig that didn't translate. Regardless, the problem is now trivial to understand in Rust and start to target.

by skeledrew7 hours ago|

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> one that an unsupervised LLM rewrite is probably not even going to attempt.

Why not? A complete test suite exists, so it just boils down to "reimplement this code to reduce the number of 'unsafe' references, while keeping the tests passing" (the last part isn't even needed since Claude loves to run tests and linters anyway).

by TazeTSchnitzel5 hours ago|

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An LLM definitely can do that work, but LLMs have a tendency to follow the path of least resistance unless you force them to do things properly and carefully supervise them.

by skeledrew5 hours ago|

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> LLMs have a tendency to follow the path of least resistance

That's a really strong motivation for the Rust port. It's literally impossible to take a path not leading to an unacceptable outcome because the Rust compiler fails the compile if code no longer marked "unsafe" is still actually unsafe, and the tests fail if the implementation is incorrect. The compiler actively helps to force the LLM to do things properly.

by TazeTSchnitzel1 hours ago|

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…only if you actually force the LLM not to use `unsafe` somehow! And one must remember that `unsafe` can have all sorts of non-local effects. One way to be maliciously compliant (or in the LLM's case, I guess it's more incompetence than malice) is to make a “safe” function that does an `unsafe` operation for you, and then replace every usage of that unsafe operation with your “safe” function. You can remove hundreds of unsafe blocks that way without any improvement in safety.