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Rewriting Bun in Rust

L4 · DeveloperTools & ProductsSimon Willison's Blog· 7/8/2026

Shows how AI agents are enabling previously impossible large-scale rewrites, changing fundamental assumptions about software engineering best practices.

AI Summary

Bun's creator Jarred Sumner shares how AI agents helped rewrite the entire JavaScript runtime from Zig to Rust in just 11 days, using million-line test suites as conformance benchmarks and adversarial review processes. The post details how dynamic workflows and frontier models made previously impossible ground-up rewrites feasible by automating code porting while maintaining safety through Rust's memory management. This represents a major shift in how large-scale software engineering can approach language migrations using AI-powered toolchains.

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