Ok so this “what uses rust” website lists “Valve Proton” as one of the projects using rust. Curious. It links to a reddit post mentioning that proton 5.13-1 used Rust for media_converter. The only problem: In Proton 9, media_converter was rewritten in C, integrated into winegstreamer, and the rust version was removed.

https://github.com/ValveSoftware/Proton/commit/da36c977f919e4744769b55384cec23000c83419#diff-ed8d88df6b3495f923c623fd4c4c6705b4550a7dc6e5556fd383942c741aee06