Using serde, how can I keep a copy of the serialized data while deserializing?

Question

I'm using Rust and Serde. Suppose I have a struct that wants to keep a copy of the JSON data that created it, maybe so that it can re-send that same JSON elsewhere without having to re-serialize. (And it's not as simple as just keeping a copy of the original JSON data, since the struct might be inside more JSON data.) Example:

#[derive(Deserialize)]
struct LargerStruct {
    value: i32,
    inner: InnerStruct,
}

#[derive(Deserialize)]
struct InnerStruct {
    this_json: String,
    inner_value: i32,
}

fn main() {
    let json = r#"
        {
          "value": 42, 
          "inner": { "inner_value": 100 }
        }
    "#;

    let val: LargerStruct = // ???

    assert_eq!(&val.inner.this_json, r#"{ "inner_value": 100 }"#);
}

What might fill the ??? comment?

Answer 1

/*
[dependencies]
serde.version = "*"
serde.features = ["derive"]
serde_json.version = "*"
serde_json.features = ["raw_value"]
*/

use serde::*;
use serde_json::value::RawValue;

#[derive(Deserialize)]
struct LargerStruct {
    value: i32,
    inner: InnerStruct,
}

struct InnerStruct {
    this_json: String,
    inner_value: i32,
}

impl<'de> Deserialize<'de> for InnerStruct {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        let this_json: Box<RawValue> = Deserialize::deserialize(deserializer)?;

        #[derive(Deserialize)]
        struct InnerStruct {
            inner_value: i32,
        }
        let this: InnerStruct =
            serde_json::from_str(this_json.get()).map_err(serde::de::Error::custom)?;

        Ok(Self{
            this_json: this_json.get().to_string(),
            inner_value: this.inner_value,
        })
    }
}

fn main() {
    let json = r#"
        {
          "value": 42, 
          "inner": { "inner_value": 100 }
        }
    "#;

    let val: LargerStruct = serde_json::from_str(json).unwrap();

    assert_eq!(&val.inner.this_json, r#"{ "inner_value": 100 }"#);
}

Playground

You can use the serde_json::value::RawValue type to capture the original JSON in raw form. The code uses a custom Deserialize implementation for InnerStruct to achieve the goal of capturing the JSON and deserialize. It deserializes the struct, by calling serde_json again on the captured JSON inside the Deserialize implementation. This is necessary, since the deserializer is consumed after deserializing the RawValue.