How to return JSON as a response in Rocket 0.5?

Question

I'm trying to create a printing server in Rust using Rocket and face a problem when trying to send JSON as a response. The Rocket documentation shows that is really easy to send JSON as a response using the serde library.

Unfortunately, it isn't working for me:

#[derive(Serialize, Deserialize)]
pub struct Printers {
    pub printers: Vec<Printer>,
}

#[derive(Serialize, Deserialize)]
pub struct Printer {
    pub device_type: String,
    pub uid: String,
    pub provider: String,
    pub name: String,
    pub connection: String,
    pub version: u8,
    pub manufacturer: String,
}

#[get("/printers")]
fn printers(key: ApiKey<'_>) -> Json<Printers> {
    let resp = crate::get_printers::get();
    Json(resp)
}

error[E0277]: the trait bound `rocket_contrib::json::Json<order_route::Printers>: Responder<'_, '_>` is not satisfied
   --> src/order_route.rs:25:33
    |
25  | fn printers(key: ApiKey<'_>) -> Json<Printers> {
    |                                 ^^^^^^^^^^^^^^ the trait `Responder<'_, '_>` is not implemented for `rocket_contrib::json::Json<order_route::Printers>`
    |
note: required by `route::handler::<impl Outcome<rocket::Response<'o>, Status, rocket::Data<'o>>>::from`
   --> /home/*****/.cargo/registry/src/github.com-1ecc6299db9ec823/rocket-0.5.0-rc.1/src/route/handler.rs:188:5
    |
188 |     pub fn from<R: Responder<'r, 'o>>(req: &'r Request<'_>, responder: R) -> Outcome<'r> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
  --> src/order_route.rs:27:10
   |
27 |     Json(resp)
   |          ^^^^ expected struct `order_route::Printers`, found enum `Result`
   |
   = note: expected struct `order_route::Printers`
                found enum `Result<structures::Printers, Box<dyn StdError>>`

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `hello` due to 2 previous errors

It seems I need a trait to do this, but don't know how to create it, If someone could help me to find a solution I will be grateful.

Here is my Cargo.toml:

[package]
name = "test"
version = "0.0.0"
edition = "2018"

[dependencies]
rocket = "0.5.0-rc.1"
reqwest = { version = "0.11", features = ["blocking", "json"] }
openssl = { version = "0.10", features = ["vendored"] }
json = "0.12.4"
serde = "1.0.127"
serde_json = "1.0.66"

[dependencies.rocket_contrib]
version = "0.4.10"
default-features = false
features = ["json"]

[workspace]
members = [
    ""
]

Answer 1

You are using rocket 0.5.0-rc.1 and rocket_contrib 0.4.10. While Json from rocket_contrib does implement Responder, it implements the Responder trait of Rocket v4, not that of Rocket v5.

In Rocket v5, Json is not part of rocket_contib anymore, but included in the rocket crate (note that you need to enable the json feature on the rocket crate):

use rocket::serde::json::Json;
use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
pub struct Printers {
    pub printers: Vec<Printer>,
}

#[derive(Serialize, Deserialize)]
pub struct Printer {
    pub device_type: String,
    pub uid: String,
    pub provider: String,
    pub name: String,
    pub connection: String,
    pub version: u8,
    pub manufacturer: String,
}

#[get("/printers")]
fn printers() -> Json<Printers> {
    todo!();
}

Note that you can now also remove rocket_contrib from your Cargo.toml (as you are not using any features from it).

Answer 2

Anyone who's finding it difficult, here's the solution :

• Enable JSON feature for rocket rust in your Cargo.toml file

  [package]
  #...
  
  [dependencies.rocket]
  version = "0.5.0-rc.1"
  features = ["json"]
  
  [dependencies.serde]
  version = "1.0.136"
  features = ["derive"]
  
  [dependencies]
  #...
  
  # rocket_contrib not required anymore
  

• In your src/main.rs file

  // rocket v0.5.0-rc.1
  use rocket::{
     self,
     serde::{json::Json, Deserialize, Serialize},
  };
  
  #[derive(Deserialize, Serialize)]
  struct User {
     name: String,
     age: u8,
  }
  
  #[rocket::post("/post", format = "json", data = "<user>")]
  fn post_data(user: Json<User>) -> Json<User> {
     let name: String = user.name.clone();
     let age: u8 = user.age.clone();
  
     Json(User { name, age })
  }
  
  #[rocket::main]
  async fn main() {
     if let Err(err) = rocket::build()
        .mount("/", rocket::routes![post_data])
        .launch()
        .await
     {
        println!("Rocket Rust couldn't take off successfully!");
        drop(err); // Drop initiates Rocket-formatted panic
     }
  }
  

• Test API using cURL

  curl -d '{"age": 12,"name":"John Doe"}' -H 'Content-Type: application/json' 
  http://localhost:8000/post
  
  # Response
  # {"name":"John Doe","age":12}