They target different things. Serde is something that simply serializes data into common text based or binary based format.
But there's been some innovation done to make that even more efficient. It started with protocol buffers[1]. So you can then basically write .proto files which are based on protocol buffers' own schema[2] which look like this[3]. What's special about these schemas is that they can be strongly typed, and then after a schema is written, which is a .proto file (in case of protocol buffers), code for any language can be generated to receive and parse the binary encoded message properly, with proper error checking. This avoids re-writing code in different languages if a RPC protocol is changed. It also offers other advantages and you can look into the docs for that.
Then, the author of protocol buffers left Google and created something called Capnproto[4], which improved on it in many ways. Now, what I linked to is a rust program supporting capnproto's own schema[5]
But there's been some innovation done to make that even more efficient. It started with protocol buffers[1]. So you can then basically write .proto files which are based on protocol buffers' own schema[2] which look like this[3]. What's special about these schemas is that they can be strongly typed, and then after a schema is written, which is a .proto file (in case of protocol buffers), code for any language can be generated to receive and parse the binary encoded message properly, with proper error checking. This avoids re-writing code in different languages if a RPC protocol is changed. It also offers other advantages and you can look into the docs for that.
Then, the author of protocol buffers left Google and created something called Capnproto[4], which improved on it in many ways. Now, what I linked to is a rust program supporting capnproto's own schema[5]
[1]: https://developers.google.com/protocol-buffers/
[2]: https://developers.google.com/protocol-buffers/docs/proto3
[3]: https://github.com/WhisperSystems/libsignal-protocol-c/blob/...
[4]: https://capnproto.org/
[5]: https://capnproto.org/language.html