Bun's Zig-to-Rust Rewrite Was Not a Prompt

Anatomy and bounded reproduction of an LLM-driven migration system

Abstract. The interesting unit in Bun’s rewrite is not the model call. It is the closed-loop migration system: a porting specification, an externalized lifetime analysis, task sharding, compiler feedback, a language-independent test oracle, adversarial review, and continuous human supervision.

Planned (weeks 6–10). Every statement will be tagged source / observation / inference / speculation; the reported figures (~535K lines of Zig, 1,448 source files, ~50 dynamic workflows, an 11-day path to a passing test suite) are facts to audit, not results to casually reproduce.