Rust serde parse Vec of substruct

Question

Is it possible to just extract the Message struct of this json items array?
My goal is to get a Vec of Message(s) if there are multiple entries in items.

{
  "items": [
    {
      "Message": {
        "data": "data",
        "header": "header"
      },
      "structx": {
        "val": 1
      },
      "val1": "test"
    }
  ]
}

As of now i have those structs to deserialize the above.

#[derive(Debug, Deserialize, Serialize)]
struct Items {
    items: Vec<Message>,
}

#[derive(Debug, Deserialize, Serialize)]
struct Message {
    #[serde(rename = "Message")]
    msg: Payload,
}

#[derive(Debug, Deserialize, Serialize)]
struct Payload {
    header: String,
    data: String,
}

I deserialize like that:

let x = json_string; // the above json
let msg: Items = serde_json::from_str(&x)?;

This works but maybe it could be made cleaner. I thought of something like this(which does not work) so that i could remove the unnecessary structs(Items, Message):

let msg: Vec<Payload> = serde_json::from_str(&x)?;

Answer 1

There is 3 solutions:

1. Deal with it
2. Implement custom Deserialize
3. Use unsafe and #[repr(transparent)]:

use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize, Serialize)]
#[repr(transparent)]
struct Items {
    items: Vec<Message>,
}

#[derive(Debug, Deserialize, Serialize)]
#[repr(transparent)]
struct Message {
    #[serde(rename = "Message")]
    msg: Payload,
}

#[derive(Debug, PartialEq, Deserialize, Serialize)]
struct Payload {
    header: String,
    data: String,
}

fn main() {
    let raw = r#"{
    "items": [{
        "Message": {
            "data": "data",
            "header": "header"
        },
        "structx": {
            "val": 1
        },
        "val1": "test"
    }]
}"#;

    let msg: Items = serde_json::from_str(&raw).unwrap();
    let payloads: Vec<Payload> = unsafe { std::mem::transmute(msg) };

    assert_eq!(
        payloads,
        vec![Payload {
            data: String::from("data"),
            header: String::from("header")
        }]
    );
}