rust struct to json

Cargo.toml:

[dependencies]
serde = {version = "1.0.204", features = ["derive"]}
serde_json = "1.0.120"

main.rs:

use serde::{Deserialize, Serialize};
use serde_json;

#[derive(Serialize, Deserialize)]
struct Person {
    first_name: String,
    last_name: String,
    age: u8,
}

fn main() {
    let first_name = String::from("Glenn");
    let last_name = String::from("Gillen");
    let age = 40;
    let p = Person{ first_name, last_name, age };
    let data = serde_json::to_string(&p).expect("Not a serializable type");
    println!("{}", data)
}