// Cargo.toml: serde = { version = "1", features = ["derive"] }
//              serde_json = "1"
use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize, Debug)]
struct User { name: String, age: u32 }

fn main() -> serde_json::Result<()> {
    let raw = r#"{"name":"chris","age":31}"#;
    let mut u: User = serde_json::from_str(raw)?;
    u.age += 1;
    println!("{}", serde_json::to_string_pretty(&u)?);
    Ok(())
}