Migrating the GitHub Copilot runtime to Rust, using Copilot

The headline is Rust. The real story is how it got written. GitHub ported ~430,000 lines of TypeScript into 832,378 lines of Rust in about 14.5 weeks and 128 PRs, with agents writing most of it across sessions that spawned up to 15 children each. The lead engineer supervised: architecture, design calls, quality gates. My favourite detail — the compiler threw ~8,678 errors, and almost none were about ownership.