Ladybird Starts Rewriting Its Browser Engine in Rust with Help from AI

Ladybird, the new browser currently under active development, has begun rewriting parts of its codebase in Rust.

Founder Andreas Kling explained that the team had previously evaluated Swift as a potential successor to C++. However, Swift proved impractical due to weak interoperability with C++ and limited cross-platform support beyond Apple ecosystems.

As Kling noted, the team had also assessed Rust back in 2024 but initially dismissed it because it doesn’t naturally align with traditional C++-style object-oriented programming. After another year of limited progress, they reconsidered. Rust’s mature ecosystem and strong safety guarantees ultimately made it the pragmatic choice. With both Firefox and Chromium already integrating Rust into their codebases, the Ladybird team concluded it was the right direction for their project as well.

The migration effort begins with LibJS. Its lexer, parser, abstract syntax tree (AST), and bytecode generator are largely self-contained components and benefit from extensive automated test coverage, including Ladybird’s own regression suite.

The initial port produced roughly 25,000 lines of Rust code and was completed in about two weeks. While developers led the effort, AI tools such as Claude Code and Codex assisted by handling hundreds of smaller prompts throughout the process.

Testing showed the transition met expectations. The Rust implementation successfully passed 52,898 tests along with 12,461 Ladybird regression tests, without introducing new bugs. Performance also remained consistent across all monitored JavaScript workloads.

For now, the Rust implementation closely mirrors existing C++ design patterns, even down to register allocation strategies, ensuring identical compiler output. The team plans to refactor and streamline the code later, once confidence grows and certain C++ components can be phased out.

The shift to Rust will be gradual. C++ continues to drive primary development, while Rust components are being introduced incrementally as part of a long-term strategy. Clear integration boundaries are being established to allow Rust and C++ code to coexist safely.

More details are available in the official announcement.