Code shared from the Rust Playground

playground.rs

use serde; // 1.0.152
use serde_json; // 1.0.91

/// Foo struct
#[derive(Debug, serde::Serialize, serde::Deserialize)]
struct FooV1 {
    msg: String,
}

#[derive(Debug, serde::Serialize, serde::Deserialize)]
struct FooV2 {
    msg: String,
    #[serde(default = "FooV2::default_version")]
    version: u64,
}

impl FooV2 {
    pub fn default_version() -> u64 {
        4
    }
}


fn main() {
    // Assert that v2 encoded message can be read by v1 serde deserializer.
    let foo_v2 = FooV2 {
        msg: "message".to_string(),
        version: 5,
    };
    let serialized_foo_v2 = serde_json::to_string(&foo_v2).unwrap();

    println!("FooV2 debug: {:?}", foo_v2);
    println!("Serialized FooV2: {}", serialized_foo_v2);

    let foo_v1: FooV1 = serde_json::from_str(&serialized_foo_v2).unwrap();
    println!("FooV1 debug: {:?}", foo_v1);

    // Assert that serde::default makes v1 message readble by v2 deserializer
    let serialized_foo_v1 = serde_json::to_string(&foo_v1).unwrap();
    println!("Serialized FooV1: {}", serialized_foo_v1);

    let foo_v2_from_v1: FooV2 = serde_json::from_str(&serialized_foo_v1).unwrap();
    println!("FooV2 from v1 debug: {:?}", &foo_v2_from_v1);
}