Cap'n Proto has good a good Rust library, but it's overall language support versus alternative formats is somewhat lacking, unfortunately. That said, Cap'n Pro

Cap'n Proto has good a good Rust library, but it's overall language support versus alternative formats is somewhat lacking, unfortunately. That said, Cap'n Proto has a substantially better design (both in theory and implementation) than any of the alternatives IMO, so it's worth using anyway if your language supports it well. And especially so if you're using it "internally", so other language clients aren't as big of a deal.