Pydantic is quite popular library. Its author is doing exactly this - rewriting its core [0] in Rust. It's still WIP, but readme mentions that "Pydantic-core is currently around 17x faster than Pydantic Standard."
Pydantic is not just popular (and awesome) on its own but serves as the underpinnings of a lot of the FastAPI functionality - faster Pydantic would make a LOT of apps faster