[roca]

Not a nightmare at all. For example, if you're doing JSON processing in Rust, serde_json gives you great tools for this. The serde_json::Value type can represent any JSON value. You parse JSON to specific typed structures when you want to parse-and-validate and detect errors (using #[derive(Serialize, Deserialize)] to autogenerate the code), but those structures can include islands of arbitrary serde_json::Values. You can also parse JSON objects into structs that contain a set of typed fields with all other "unknown" fields collected into a dynamic structure, so those extra fields are reserialized when necessary --- see https://serde.rs/attr-flatten.html for an example.