Rewriting Bun in Rust

Bun, a runtime originally written in Zig, has been rewritten in Rust to address persistent memory management issues. While Zig was instrumental in the project's early success, the team struggled with frequent bugs related to manual memory handling, such as use-after-free and double-free errors. Rust was chosen for its safety features, specifically its compiler-enforced memory management and automatic cleanup capabilities, which eliminate these common failure points.

The rewrite was made possible by Bun’s extensive TypeScript test suite, which served as a conformance guide for an automated agent harness. By using AI models to port the code, the team successfully translated the codebase over 11 days. This process involved sophisticated agentic workflows, including adversarial code reviews and iterative testing, to ensure the new Rust implementation maintained parity with the original Zig version.

This project demonstrates a shift in software engineering where large-scale rewrites are no longer considered impossible. By leveraging AI agents to handle the heavy lifting, the team processed billions of tokens to complete the migration. The new Rust-based Bun has been live in production for nearly a month, resulting in a 10 percent improvement in startup speed on Linux. The transition was seamless, proving that AI-assisted engineering can responsibly manage massive code migrations while maintaining system stability.