We’ve been searching for a memory-safe programming language to replace C++ in Ladybird for a while now. We previously explored Swift, but the C++ interop never quite got there, and platform support outside the Apple ecosystem was limited. Rust is a different story. The ecosystem is far more mature for systems programming, and many of our contributors already know the language. Going forward, we are rewriting parts of Ladybird in Rust.
Porting LibJS
I used Claude Code and Codex for the translation. This was human-directed, not autonomous code generation. I decided what to port, in what order, and what the Rust code should look like. It was hundreds of small prompts, steering the agents where things needed to go. After the initial translation, I ran multiple passes of adversarial review, asking different models to analyze the code for mistakes and bad patterns.
The requirement from the start was byte-for-byte identical output from both pipelines. The result was about 25,000 lines of Rust, and the entire port took about two weeks. The same work would have taken me multiple months to do by hand. We’ve verified that every AST produced by the Rust parser is identical to the C++ one, and all bytecode generated by the Rust compiler is identical to the C++ compiler’s output. Zero regressions across the board:
Seems like a non-issue.
Your final hope at this point is announcing you will not use LLM to write any part of your program, including tests. Adopting a language which your developers are less familiar is bad enough, and you’re writing it with LLM? I’d rather you stick with C++.
We need competition in the browser market, ANY competition!
* ladybird appears *
Well… that was on me, I set the bar too low
“We want rust for safety. That’s why we made the code in the most unreliable way to code known to humanity”.
Not surprising from the devs that think that not being a fascist is political I guess
Edit : ah apparently the guy is not trying to be an enlightened centrist anymore and is a full-on nazi. Heh, who could have guessed? /s
Woah wait what? The “let’s vibecode it” angle is fucking stupid, to be clear, but I was unaware of that user drama. Very interesting.
It keeps on coming back and ladybird supporters keep on trying to tone it down because they don’t want the project to die. But yeah, the guy seems like a big piece of shit.
Got a source for that? I tried searching on DDG but couldn’t find anything
https://drewdevault.com/2025/09/24/2025-09-24-Cloudflare-and-fascists.html
Here’s one, you can find more stuff by searching the name of the dev
I will copy my comment here:
Just so you know, Drew DeVault is a pedophile apologist. He cannot even remotely be considered a reliable source of information.
However, I completely agree regarding the use of AI, especially in a project such as a browser engine.
EDIT: I accidentally replied to this comment instead of the one with the link to Drew DeVault’s blog. My bad.
That is appalling, and I don’t mean to diminish the importance of this, but if the claims on fascism stand to scrutiny then it’s not relevant, and if the claims don’t stand to scrutiny they can be easily refuted.
Edit: and to be clear, I know DHH, I’ve read his blog and his social media activity, he’s clearly a fascist.
Most of the information that you get about terrible people comes from terrible people because that is who knows them. Should we discount Trump accusations from Epstein because they came from a known pedo? No, pedos are the people that Trump hangs out with because that is what he is.
Given that Drew Devault has pity for pedophiles, the correct choice is to discount anything he says, much like you should discount anything Trump or Epstein claims without further primary sources of evidence from unaffiliated parties
This is not at all comparable to Trump/Epstein. Drew was only pitying pedophiles, not actual rapists This may come as a surprise to you, but there is a very clear difference (one is a choice, the other is not).
Nobody should pity pedophiles, you should want them removed from society permanently. If that’s controversial, so be it
Then you’ve probably never really thought about what pedophilia actually means. My partner works with pedophiles in a clinical setting and there’s way more people with pedophile tendencies than you’d think (or at least than I thought). Most of them realise it’s a problem early on and seek help, some live in shame for most of their lives and only a tiny percentage actually does something to kids. Obviously, this shouldn’t happen at all (!!) but saying „all pedophiles should be removed from society“ is no solution to anything. Just to be sure: I have no idea what the ladybird-guy said about this topic, nor do I know who he is (or even what Ladybird is. I now know it’s a Browser, but that’s it).
Some people are truly sick and need a lot of help to change how their minds work. I respect your partner and their line of work. Hopefully they all can undergo thorough and intensive rehabilitative care as far away from society as possible until they are fit to be around vulnerable people.
With this line of thinking I’d rather remove you from society
You’d agree of course, that pedophiles should be removed from society? Is the idea of shaming pedophiles and pedo apologists some kind of line in the sand that shouldn’t be crossed in your eyes?
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I am completely disappointed in Ladybird, even though I had some hope for the project until now. I guess all that’s left to do is wait for Servo.
Given that Servo was already written in rust (and not just some of it), it does feel a bit like wasted effort.
(Plus, it was started by Mozilla, so theres the irony factor)
The Lunduke vid about this is going to be lit
Actually not a terrible way to use AI. Hopefully the port goes well
There will be people defending ladybird no matter what huh
I read the comment to only refer to the way AI was used.
I came to say this sounds like a good example of how to use ai. Its going to be used but the big problem is folks who will take the output and say good enough.
Yeah, using AI to help you rewrite code (which is tedious) in a scenario where there are clear test cases (it either works or it doesn’t) is basically the ideal usecase.
…no… this is a bad idea. Especially in a browser, given the myriad safety and security concerns in play. This is an objectively bad way to make changes to the project. I can just about guarantee they’re going to introduce some horrid subtle flaws.
I may be misinformed but the article says
We’ve verified that every AST produced by the Rust parser is identical to the C++ one, and all bytecode generated by the Rust compiler is identical to the C++ compiler’s output.
which I took to mean that the new code behaves 100% identically to the old code, like a byte-perfect decompilation. If that’s not the case and it’s just the same in their suite of tests then I’d agree with you.
It depends on how extensive (and correct) the tests are, which can be pretty difficult. There is also maintainability of generated code.
