Rewriting Bun in Rust

9 hours ago

Bun has been rewritten from Zig to Rust using AI-powered coding agents, significantly reducing memory-related bugs.
The rewrite was enabled by a comprehensive TypeScript test suite that served as a conformance benchmark for automated porting.
Key engineering techniques included dynamic workflows, trial runs, adversarial review, and process-based fixes rather than manual code editing.
Jarred Sumner praised Zig for Bun's initial development but chose Rust for its memory safety features like RAII and compiler checks for use-after-free errors.
The rewrite took about 11 days of intensive AI-assisted work, with an estimated cost of $165,000 in API tokens, funded by Anthropic.
The new Rust version has been live in Claude Code for nearly a month, improving startup speed by 10% on Linux with minimal user impact.
The project demonstrates how advanced language models can enable ambitious, large-scale software rewrites previously considered infeasible.

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