Support both serde_json::Value and cbor_json::Value

Question

If I have a JSON value with unknown layout I can deserialise it with serde_json using serde_json::Value:

#[derive(Deserialize)]
struct Foo {
  unknown: serde_json::Value,
}

Similarly I can do the same with CBOR:

#[derive(Deserialize)]
struct Foo {
  unknown: serde_cbor::Value,
}

But what if I want a single data structure that can be loaded from JSON or CBOR. I effectively want this:

enum UnknownValue {
  Json(serde_json::Value),
  Cbor(serde_cbor::Value),
}

#[derive(Deserialize)]
struct Foo {
  unknown: UnknownValue,
}

Is there any way to do this so that I can deserialise JSON or CBOR into this struct?

Answer 1

Ok, I tried serde(untagged), and it sort of works, except that it always results in UnknownValue::Json, even if you deserialise using serde_cbor. I'm not sure how that works exactly:

use serde::Deserialize;

#[derive(Deserialize, Debug)]
#[serde(untagged)]
enum UnknownValue {
    Json(serde_json::Value),
    Cbor(serde_cbor::Value),
}

#[derive(Deserialize, Debug)]
struct Foo {
    unknown: UnknownValue,
}

fn main() {
    let json_input = br#"{"unknown": 23}"#;
    let cbor_input = [0xA1, 0x67, 0x75, 0x6E, 0x6B, 0x6E, 0x6F, 0x77, 0x6E, 0x17];

    let j: Foo = serde_json::from_slice(json_input).unwrap();
    let c: Foo = serde_cbor::from_slice(&cbor_input).unwrap();
    dbg!(j);
    dbg!(c);
}

Prints

[src/main.rs:21] j = Foo {
    unknown: Number(
        23,
    ),
}
[src/main.rs:22] c = Foo {
    unknown: Number(
        23,
    ),
}

So I wondered if maybe it just works if you use serde_json::Value and forget the enum, and lo, it does! Using this instead gives exactly the same output:

#[derive(Deserialize, Debug)]
struct Foo {
    unknown: serde_json::Value,
}

So this doesn't answer my actual question (e.g. if you want to get CBOR tags), but it does satisfy my use case. Probably also worth mentioning serde-value which seems designed for this sort of thing, though I haven't tried it.