Ladybird Browser Embraces Rust to Boost Performance and Security

In a bold move that signals a major shift in browser development, the Ladybird project has officially begun rewriting portions of its codebase in Rust, marking a pivotal moment in the evolution of this ambitious new browser. The decision comes after careful consideration of alternative languages, including Swift, and reflects a pragmatic approach to balancing performance, safety, and ecosystem maturity.

The Swift Experiment and Why It Didn’t Stick

When the Ladybird team first explored alternatives to C++, Swift emerged as a strong contender. Known for its modern syntax and safety features, Swift seemed like a natural fit for a project aiming to build a high-performance, secure browser. However, the team quickly ran into significant roadblocks. Swift’s interoperability with C++ proved to be cumbersome, and its ecosystem support outside of Apple’s platforms was limited. These limitations made Swift a less-than-ideal choice for a cross-platform browser project like Ladybird.

Why Rust Won the Day

After a year of treading water, the Ladybird team decided to revisit Rust—a language they had previously dismissed due to its perceived shortcomings in object-oriented programming (OOP). This time, however, the team recognized Rust’s growing ecosystem, its robust safety guarantees, and its increasing adoption by major players in the browser world, including Mozilla’s Firefox and Google’s Chromium. As Andreas Kling, the founder of Ladybird, put it: “Rust has the ecosystem and the safety guarantees we need. Both Firefox and Chromium have already begun introducing Rust into their codebases, and we think it’s the right choice for Ladybird too.”

The First Step: Rewriting LibJS

The team’s first major undertaking in Rust is the LibJS component, which includes the lexer, parser, abstract syntax tree, and bytecode generator. These modules were chosen because they are largely self-contained and have comprehensive test coverage, including Ladybird’s own regression tests. The transition to Rust for LibJS generated approximately 25,000 lines of code and took about two weeks to complete. While the process was led by human developers, AI tools like Claude Code and Codex played a supportive role, handling hundreds of small prompts and streamlining the workflow.

Testing: A Resounding Success

The results of the transition speak for themselves. The Rust version of LibJS passed an impressive 52,898 tests and 12,461 Ladybird regression tests without introducing any new bugs. Performance metrics also remained consistent with the C++ version across all tracked JavaScript tasks. This achievement underscores Rust’s ability to deliver on its promises of safety and efficiency without sacrificing performance.

A Pragmatic Approach to Transition

For now, the Rust code is designed to mirror C++ patterns, even in areas like register allocation, to ensure that the compiler output remains identical. This approach allows the team to maintain stability while gradually building confidence in the new codebase. The plan is to refactor and simplify the Rust code over time, once the team is more comfortable and can safely remove some of the C++ components.

A Gradual Shift, Not a Revolution

The Ladybird team is taking a measured approach to this transition. C++ will remain the primary language for the foreseeable future, with Rust being introduced as a long-term project. New Rust components will be integrated into the existing C++ codebase, with clear rules governing their interaction. This gradual shift ensures that the browser remains stable and functional throughout the transition process.

The Bigger Picture

Ladybird’s decision to embrace Rust is part of a broader trend in the tech industry. As browsers become increasingly complex and security-critical, developers are turning to languages that offer both performance and safety. Rust’s growing adoption by major browser projects like Firefox and Chromium is a testament to its potential to address these challenges.

For those interested in following Ladybird’s journey, the team has published a detailed announcement on their website, providing further insights into their decision-making process and future plans.

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