rocket_ex1.rs

#![feature(proc_macro_hygiene, decl_macro)]

#[macro_use] extern crate rocket;
#[macro_use] extern crate serde_derive;
extern crate rocket_contrib;

use rocket_contrib::json::{Json};

#[derive(Serialize)]
pub struct User {
    name: Option<String>
}

#[derive(Serialize)]
pub struct JsonError {
    message: String
}

#[get("/")]
fn get() -> Result<Json<User>, Json<JsonError>> {
    Err(Json(JsonError { message: "Unknown".to_string() }))
}

fn main() {
    rocket::ignite()
        .mount("/", routes![get])
        .launch();
}

Fails to compile with "the trait rocket::response::Responder<'_> is not implemented for std::result::Result<rocket_contrib::json::Json<User>, rocket_contrib::json::Json<JsonError>>". Switching out JsonError to a String and returning a Json(String) will compile without issues and I do not understand why.

Problem is that the Err type needs to implement Debug, #[derive(Serialize)] -> #[derive(Serialize, Debug)] fixes the problem.