Microsoft migrated Copilot's runtime from TypeScript to Rust, converting 430,000 lines of code into 800,000 lines of Rust. The project consumed $120,000 in AI tokens and approximately three weeks of developer time, with dozens of regressions that required fixing afterward.
The payoff was substantial. Internal testing showed the new runtime achieved a 15.9x performance improvement while reducing memory usage by 91%. The dramatic efficiency gains underscore Rust's appeal for performance-critical infrastructure, though the rewrite's complexity and cost highlight the real-world trade-offs involved in such migrations.