動機

設定ファイルをsetting.tomlのような形にして外部に吐き出しておいて，あとで読み込みたい．
RustでTOMLを扱うにはtoml-rsを使う．本稿では併せてファイルの入出力を行う．

環境

環境

$ rustc --version
rustc 1.40.0 (73528e339 2019-12-16)
$ sw_vers
ProductName:    Mac OS X
ProductVersion: 10.15
BuildVersion:   19A603

最終結果

ファイル構成

$ tree
.
├── Cargo.lock
├── Cargo.toml
└── src
   └── main.rs

Cargo.toml

[package]
name = "toml-handle"
version = "0.1.0"
authors = ["Scstechr <scstechr@email.com>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
serde = { version = "1.0.104", features = ["derive"] }
toml = "0.5.5"

src/main.rs

extern crate serde;
extern crate toml;

use serde::{Deserialize, Serialize};
use std::fs::{self, File};
use std::io::Write;

# [derive(Debug, Serialize, Deserialize)]
struct Setting {
    os: String,
    version: f32,
    users: Vec<String>,
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Definition of struct instant
    let setting = Setting {
        os: "Mac OS X".into(),
        version: 10.15,
        users: vec!["John Doe".into(), "Alice Carter".into()],
    };
    println!("\n#Original:\n{:#?}", setting);

    // Write TOML to file
    let mut file = File::create("Setting.toml")?;
    let toml = toml::to_string(&setting).unwrap();
    write!(file, "{}", toml)?;
    file.flush()?;
    println!("\n#TOML:\n{}", toml);

    // Read file and parse to Setting
    let foo: String = fs::read_to_string("Setting.toml")?;
    let setting: Result<Setting, toml::de::Error> = toml::from_str(&foo);
    match setting {
        Ok(p) => println!("#Parsed TOML:\n{:#?}", p),
        Err(e) => panic!("Filed to parse TOML: {}", e),
    }
    Ok(())
}

出力

$ cargo run
   Compiling toml-handle v0.1.0 (/Users/Scstechr/Works/projects/toml-handle)
    Finished dev [unoptimized + debuginfo] target(s) in 1.54s
     Running `target/debug/toml-handle`

# Original:
Setting {
    os: "Mac OS X",
    version: 10.14,
    users: [
        "John Doe",
        "Alice Carter",
    ],
}

# TOML:
os = "Mac OS X"
version = 10.14
users = ["John Doe", "Alice Carter"]

# Parsed TOML:
Setting {
    os: "Mac OS X",
    version: 10.14,
    users: [
        "John Doe",
        "Alice Carter",
    ],
}

おわり

TODO

解説を追記する．

参考