Cap'n proto is basically a schema language that gets you a well defined in-memory representation that's just as good as if you were writing C structs by hand (laboriously avoiding silent padding, carefully using types with well defined sizes) - without all the silent pitfalls of doing it manually in C.

It's extremely well thought out, it's minimalist in all the right ways; I've found the features and optimizations it has to be things that are borne out of real experience that you would want end up building yourself in any real world system.

E.g. it gives you the ability to add new fields without breaking compatibility. That's the right way to approach forwards/backwards compatibility, and it's what I do in bcachefs and if we'd been able to just use cap'n proto it would've taken out a lot of manual fiddly work.

The only blocker to using it more widely in my own code is that it's not sufficiently ergonomic in Rust - Rust needs lenses, from Swift.