I had no idea that .rs was conventionally for Rust projects, but I'm also not too bothered by having to figure it out.
I'm not sure it's convention but it's common just due to matching the file extension.
It's technically for the Republic of Serbia. I only chose it because .io was taken by a python library (which I wish I'd checked before deciding on the name)
It's also a pain in the ass to register because you have to have a Serbian tax ID