Need dokumentation for Redmine client for Rust

I'm developing at the moment an async Rust client for the Redmine REST-API I found already the documentation for the endpoints. But what is missing is a list of which fields are optional because it's important to know in a typed language like Rust. If I don't mark a field as optional it will cause a runtime error while parsing the JSON. I think marking all fields as optional is not a good option.