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Okay, I need to talk about what just happened with the [Ladybird browser](https://ladybird.org) project, because this might be the most impressive real-world demonstration of AI-assisted coding I’ve seen so far.

Andreas Kling and the Ladybird team just [announced](https://ladybird.org/posts/adopting-rust/) they’re adopting Rust as their successor language to C++. That alone would be news. But here’s the part that made my jaw drop: they ported the entire LibJS JavaScript engine from C++ to Rust in roughly two weeks. The lexer, the parser, the abstract syntax tree, the bytecode generator — all of it. About 25,000 lines of safe Rust, translated with the help of Claude Code and Codex.

And before you say “yeah but AI-generated code is always buggy” — the translated Rust code passed all 52,898 ECMAScript conformance tests and 12,461 Ladybird regression tests. Zero regressions. Identical performance. That’s not a toy demo. That’s a production JavaScript engine running correctly after a language swap.

What makes this especially interesting is the approach. This wasn’t some “press a button and let the AI go wild” situation. Kling described it as human-directed translation — he decided what to port, in what order, and what the Rust code should look like, using hundreds of small prompts to steer the agents. Think of it as pair programming where your partner happens to type really, really fast.

The backstory matters too. Ladybird originally tried Swift as their C++ replacement, but the C++ interop was rough and platform support outside Apple’s ecosystem was limited. Rust was actually rejected back in 2024 because it doesn’t play nicely with C++ style object-oriented patterns. But sometimes the pragmatic choice wins out, and with Rust’s growing adoption in Firefox, Chromium, and the Linux kernel, it just made sense to commit.

The [Hacker News thread](https://news.ycombinator.com/item?id=47120899) absolutely exploded — over 1,200 points and 680+ comments. It hit the top of the front page, and the discussion ranged from deep technical dives into C++-to-Rust interop to broader debates about what this means for the future of large codebases. If you want to follow along with the actual code, the [Ladybird GitHub repo](https://github.com/LadybirdBrowser/ladybird) is where all the action is.

Going forward, Rust and C++ will coexist in the Ladybird codebase through well-defined interop boundaries, with more subsystems gradually making the jump. It’s a long road, but this first milestone is hard to argue with. Two weeks, zero regressions, and a clear signal that AI-assisted language migration on real codebases isn’t hypothetical anymore — it’s happening right now.