Why did Stephen Toub choose Rust over C#, Go or Java for the Copilot runtime?

Stephen Toub had agents build small prototypes of five to ten thousand lines each in TypeScript, Python, Go, C#, Java and Rust, then measured performance, working set, throughput and embeddability. TypeScript and Python dropped out quickly, Go was fast but not designed for interop, and Java was only okay. C# with native AOT and Rust were roughly tied, so softer factors decided it: GitHub has more Rust projects and more engineers who know Rust.

• Toub said that if the runtime had already been written in C#, there would have been no reason to rewrite it.
• He contrasted the choice with the TypeScript compiler's port to Go, which has no requirement to be embedded in other apps.

Source: our own transcription of the episode. The full conversation is on Ryan Peterman (2026-11-02). Read the whole episode summary →

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