【Rust】jsonの読み書き

### Jsonの読み込み

use serde::{Serialize, Deserialize};
use std::fs;
use std::io::prelude::*;

#[derive(Serialize, Deserialize, Debug)]
#[allow(non_snake_case)]
struct Rotator {
    Pitch:f32,
    Roll:f32,
    Yaw:f32,
}

#[derive(Serialize, Deserialize, Debug)]
#[allow(non_snake_case)]
struct CharaParam {
    Name: String,
    Id: i32,
    ItemIdList: Vec<u32>,
    RotData: Rotator,
}

fn main(){
    let input_fn = fs::read_to_string("test_input.json")
        .expect("JSON Read Failed.");
    let deserialized: Vec<CharaParam> = serde_json::from_str(&input_fn).unwrap();

    for data in &deserialized {
        println!("{:?}", data);
    }
}

Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.67s
Running `target/debug/sample`
CharaParam { Name: “Kiwi”, Id: 3, ItemIdList: [2000, 2001], RotData: Rotator { Pitch: 0.0, Roll: 0.0, Yaw: 0.0 } }
CharaParam { Name: “Orange”, Id: 4, ItemIdList: [2002, 2003], RotData: Rotator { Pitch: 1.0, Roll: 50.2, Yaw: 66.5 } }

### Jsonの書き込み

use serde::{Serialize, Deserialize};
use std::fs::File;
use std::io::prelude::*;

#[derive(Serialize, Deserialize, Debug)]
#[allow(non_snake_case)]
struct Rotator {
    Pitch:f32,
    Roll:f32,
    Yaw:f32,
}

#[derive(Serialize, Deserialize, Debug)]
#[allow(non_snake_case)]
struct CharaParam {
    Name: String,
    Id: i32,
    ItemIdList: Vec<u32>,
    RotData: Rotator,
}

fn main(){
    let chara00 = CharaParam{Name:String::from("Apple"), Id:0x01, ItemIdList:vec![1000, 1001], RotData:Rotator{Pitch:0.0, Roll:0.0, Yaw:32.0} };
    let chara01 = CharaParam{Name:String::from("Banana"), Id:0x02, ItemIdList:vec![1002, 1003], RotData:Rotator{Pitch:0.0, Roll:-70.0, Yaw:66.0}};

    let mut param_list:Vec<CharaParam> = Vec::new();
    param_list.push(chara00);
    param_list.push(chara01);

    let output_fn = String::from("test_output.json");
    let result = output_json(&output_fn, param_list);
    match result {
        Ok(..) => { println!("Json output finished.【{}】", output_fn) }
        Err(err) => { println!("Error: {}", err)}
    }

}

fn output_json(output_fn: &str, charaparam_list: Vec<CharaParam>) -> std::io::Result<()>{

    let serialized: String = serde_json::to_string(&charaparam_list).unwrap();

    let mut file = File::create(output_fn)?;
    file.write_all(serialized.as_bytes())?;
    Ok(())
}

読み込み、書き込みはわかったが、書き込み時に、最終行にappendとしたい。。果たして出来るんか？？
単純にjsonファイルを読み取って、vectorにデータを追加してjsonファイルに保存で良い？