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Learnings from 100K lines of Rust with AI (2025)

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Re: Learnings from 100K lines of Rust with AI (2025)

#91

Post by Aurornis »

chadd wrote: post #54

We're working on a large Rust codebase, heavily assisted development with Claude and Codex, and one critical workflow is after you have written a spec, have the other LLM critique it thoroughly. This back and forth will take quite a while, but the resulting implementation plan will be 10x better than the original. You can automate this by giving Codex a goal, and a skill to call Claude to review the implementation sp…

I take this idea even further: After the LLMs have critiqued each other, I introduce a third critique and review it myself as a human. This third party review is most effective at highlighting problems that the LLMs miss, in my experience.

Jokes aside, I agree about having LLMs iterate. Bouncing between GPT and Opus is good in my experience, but even having the same LLM review its own output in a new session started fresh without context will surface a lot of problems.

This process takes a lot of tokens and a lot of time, which is find because I’m reviewing and editing everything myself during that time.

Re: Learnings from 100K lines of Rust with AI (2025)

#92

Post by Tval »

embedding-shape wrote: post #19
jdw64 wrote: post #9

I'm also shifting to an vibe coding workflow, but I have a genuine question: whenever I use AI for Rust, it makes an insane amount of lifetime errors. I have no idea how people are churning out so many lines of code so quickly. Honestly, despite all the hype around Rust in the community, the fact that AI can't handle lifetimes reliably makes me reluctant to use it. The AI constantly defaults to spamming .clone() or w…

> I'm also shifting to an vibe coding workflow, but I have a genuine question: whenever I use AI for Rust, it makes an insane amount of lifetime errors. I have no idea how people are churning out so many lines of code so quickly. What harness and model you've been using? For the last few months, essentially since I did the whole "One Human + One Agent = One Browser From Scratch" experiment, I've almost exclusively be…

[flagged]

Re: Learnings from 100K lines of Rust with AI (2025)

#93

Post by CrazyStat »

b40d-48b2-979e wrote: post #88
CrazyStat wrote: post #83

Earlier quoted context omitted.

I didn’t claim that LLMs are people or that they reason. If the behavior of the llm is the same as the behavior of reasonable people then the behavior of the llm is reasonable, regardless of how black of a box they generate tokens out of. Reasonable people will generate divergent specs for the same prompt. Thus it is reasonable for an LLM to generate divergent specs out of the same prompt. Edit: I use “reasonable” he…

[flagged]

Please point to where in my initial comment I indicated that LLMs are human or reason.

If you are unable to do so please withdraw your accusation of gaslighting, a serious form of psychological abuse, and apologize.

Re: Learnings from 100K lines of Rust with AI (2025)

#94

Post by mike_hearn »

onlyrealcuzzo wrote: post #49
mike_hearn wrote: post #48

Earlier quoted context omitted.

It's not nearly the perfect language for LLMs and Rust is dramatically harder to read and reason about than Python with types. Other options work better for nearly all apps. I found Kotlin works well: - Garbage collected so no reasoning tokens or dev cycles are wasted on manual memory management. You say if you're vibe coding you can ignore lifetimes, but in response to a post that says AI can't do a good job and con…

Write a 250k LOC compiler in Python and then get back to me how well LLMs write in Python... Sure if you want to vibe code a TODO app where it's literally just copying and pasting one it's already seen 10,000 times before, it can do it in Python.

I didn't say it's better to use Python!

Re: Learnings from 100K lines of Rust with AI (2025)

#95

Post by mbbutler »

jdw64 wrote: post #9

I'm also shifting to an vibe coding workflow, but I have a genuine question: whenever I use AI for Rust, it makes an insane amount of lifetime errors. I have no idea how people are churning out so many lines of code so quickly. Honestly, despite all the hype around Rust in the community, the fact that AI can't handle lifetimes reliably makes me reluctant to use it. The AI constantly defaults to spamming .clone() or w…

Yeah, LLMs suck at named lifetimes. The number of times I have seen Claude reach for indices and clones instead of just using proper named lifetimes is too many to count at this point. Not great for high-performance code!

Re: Learnings from 100K lines of Rust with AI (2025)

#96

Post by dist-epoch »

olafmol wrote: post #73
CrazyStat wrote: post #69

Earlier quoted context omitted.

If you ask 10 different humans to produce the spec with the same information (prompt and context) they will also produce 10 unique answers that will contradict each other and (depending on who you asked) may be just as confident. There are real decisions to be made when going from a vague prompt to a spec. It's not surprising that an LLM would produce different specs for the same work on different runs. If the prompt…

An LLM should not "generate specs", a human should. The LLM can work from the specs. It can never infer meaning from a vague prompt. If so, it will start guessing. Every human that ever did functional specification or information analysis at some point knows this. Or has learned the hard way, something with assumptions and asses ;)

The guessing of a LLM for a vague prompt is better than the one of your average developer.

A prompt like "write these two files on disk" will very likely make the LLM do some sort of an atomic write/swap operation, unlike the average developer which will just write the two files and maybe later encounter a race condition bug. You can argue the LLM output is overkill, but it will also be more robust on average.

Re: Learnings from 100K lines of Rust with AI (2025)

#97

Post by ndr »

kikimora wrote: post #59
ndr wrote: post #3

Earlier quoted context omitted.

It just an anchor. If it were 50k would you say the same down to 25k? And if so how many more times would it apply? The interesting thing is that it was manageable solo (in many ways it's _more_ manageable solo+AIs than with coworkers+(their)AIs), and in such a short amount of time.

Original RSL library is 36k LoC. And this is C++. Rust should be like 50% smaller, that is, 18k LoC. This library is so big that I bet the author has no idea if it works or not. 1300 test generated by AI say nothing about actual quality. In the end it is just a lot of unmaintainable code quickly generated by AI.

This is uncharitable, but makes a prediction. I imagine you'd bet the author won't be successfully using this, at MS/Uber or wherever they are, in a year time?

Rust makes no promise of being terser than C++, and RSL does less than this considering the optimization.

Also it's only 45/50k LOC so not so very from the 36k LOC.

Re: Learnings from 100K lines of Rust with AI (2025)

#99

Post by stymaar »

boitiga wrote: post #30
jdw64 wrote: post #9

I'm also shifting to an vibe coding workflow, but I have a genuine question: whenever I use AI for Rust, it makes an insane amount of lifetime errors. I have no idea how people are churning out so many lines of code so quickly. Honestly, despite all the hype around Rust in the community, the fact that AI can't handle lifetimes reliably makes me reluctant to use it. The AI constantly defaults to spamming .clone() or w…

Honestly Rust is an UGLY language. For whatever powers it possesses in memory safety, its cryptic symbology is reminiscent of assembly. This is a problem when language designers are mathematicians and don’t understand typographical nuance and visual weights.

I really don't get the complain about Rust's syntax, it's almost identical to TypeScript's and nobody complains about TypeScript Synthax being ugly …

(Yes, I know the 'a lifetimes are a bit weird, and that's not something that exist in typescript, but that's also not something you use everyday in Rust either.

Re: Learnings from 100K lines of Rust with AI (2025)

#100

Post by zahlman »

kikimora wrote: post #59
ndr wrote: post #3

Earlier quoted context omitted.

It just an anchor. If it were 50k would you say the same down to 25k? And if so how many more times would it apply? The interesting thing is that it was manageable solo (in many ways it's _more_ manageable solo+AIs than with coworkers+(their)AIs), and in such a short amount of time.

Original RSL library is 36k LoC. And this is C++. Rust should be like 50% smaller, that is, 18k LoC. This library is so big that I bet the author has no idea if it works or not. 1300 test generated by AI say nothing about actual quality. In the end it is just a lot of unmaintainable code quickly generated by AI.

Has Rust code generally been found shorter than C++ in practice? I don't see an obvious reason for it.
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