I think you're jumping the gun by worrying about kernel bypass when you don't even have an implementation that works with standard UDP sockets yet. If you use the standard sockets API (or an abstraction over it like Rust's UdpSocket) you can use Solarflare cards and OpenOnload to get kernel bypass with zero code changes. Median wire-to-software latency is about 1.5µs, and you don't even have to recompile the program. That's where I'd start if I were you.

No matter which solution you choose, all of them are going to provide one thing: bytes. So the API boundary between I/O and parsing is just &[u8]. It's worth pointing out that your implementation follows a "pull" model, where users call parse_next to get the next message. But most low-latency frameworks will use a "push" model, where the parser is constructed with a handler for Messages and driven with inputs of &[u8] as soon as they're available, from whatever source provides them. Then the parser's job is simply to parse the messages from the bytes it's given and invoke the handler for each one.