The Rust compiler remains unperturbed by the antics of the LLM.
The software engine underpinning GitHub Copilot and some Microsoft products is now written in Rust, with AI agents doing most of the porting work.
Managers also had to grapple with a few dozen regressions in the resulting code, pointing to AI’s ongoing challenges in understanding Rust.
The effort updated the runtime module-by-module until the job was completed, spanning over 135 releases across a 14.5-week time period. About 1.3 port pull requests were opened per day.
Overall, agents converted 430,000 lines of TypeScript into 800,000 lines of production Rust.
One benchmark measured how quickly the runtime could complete 1,000 one-turn session lifecycles, using a shared client and 100 concurrent pipelines. The original TypeScript implementation completed 7.55 of those lifecycles per second, while Rust running in-process managed 120 per second - representing a 15.9x speedup on that particular workload.
In terms of memory, a 10-client batch of agents consumed 1,383 MB with TypeScript while the Rust rewrite consumed only 126 MB serving the same group.
Rust made those goals possible, wrote Microsoft Distinguished Engineer Stephen Toub in a post that explained the entire process.
The project used Copilot to rewrite Copilot, which in turn used several LLMs – GPT-5.6 Sol and Claude Opus 4.8 were both namechecked – to execute different parts of the job, depending on each LLM’s natural strength.
As of July 30, the experimental Rust port was passing 99.8 percent of Bun's existing tests on Linux x64 glibc, with the stable releases still shipping from the Zig codebase.