While not a direct competitor to protobufs, if you are working in the video game space where struct versioning is not needed, there is an alternative language called "schema" that supports C, C++, C#, Golang, Rust and JavaScript.
> video game space where struct versioning is not needed
Save files? Looser than exact version multiplayer?
Of all names, they pick schema? That's like calling a programming language "language."