Rust 1.46を早めに深掘り

Rust
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こんにちは、R&Dチームの齋藤（@aznhe21）です。

さて、本日、日本時間8/28(金)、Rust 1.46がリリースされました。 この記事ではRust 1.46での変更点を詳しく紹介します。 なお、この記事は公式リリースノートをベースに、意訳・編集・追記をしています。

8/28は日本テレビが民法で初めてテレビ放映をした日 若者のNULL離れ

ピックアップ

個人的に注目する変更点を「ピックアップ」としてまとめました。 全ての変更点を網羅したリストは変更点リストをご覧ください。

定数文脈で一部の制御構文が使えるようになった

constによる定数の宣言やstatic、static mutによるグローバル変数の宣言、あるいは定数関数など、 定数の文脈でもifやmatch、loopが使えるようになりました。

以下のコードは、Rust 1.46からはいずれも合法です。

const X: bool = match option_env!("X") {
    None => false,
    Some(x) if x.is_empty() => false,
    _ => true,
};

static Y: u8 = loop {
    break 10;
};

const fn fizzbuzz(x: usize) -> Option<&'static str> {
    if x % 15 == 0 {
        Some("FizzBuzz")
    } else if x % 3 == 0 {
        Some("Fizz")
    } else if x % 5 == 0 {
        Some("Buzz")
    } else {
        None
    }
}

パニックメッセージに関数呼び出し元の情報を出せるようになった

関数に属性#[track_caller]を付けることで、パニックメッセージに含まれるソースコードの情報を、その関数を呼び出した箇所とすることが出来ます （Rust 1.42からパニック時に表示される行番号がlibcoreではなくなったのはこの仕組みを利用しています）。

use std::thread;

fn without_track_caller() {
    panic!();
}

#[track_caller]
fn with_track_caller() {
    panic!();
}

fn main() {
    // without_track_caller内の行番号が表示される
    // thread '<unnamed>' panicked at 'explicit panic', src/main.rs:4:5
    let _ = thread::spawn(|| without_track_caller()).join();

    // thread::spawnの行番号が表示される
    // thread '<unnamed>' panicked at 'explicit panic', src/main.rs:19:30
    let _ = thread::spawn(|| with_track_caller()).join();
}

この仕組みを使うことで、ラッパー関数がパニックした際の行番号に悩まされることが無くなります。

#[track_caller] // Rust 1.45まではこの属性は付けられない
fn read_file<P: AsRef<std::path::Path>>(path: P) -> String {
    // Rust 1.45まではファイルが読めなかった際はこの行の情報が表示されており、デバッグが難しかった
    // Rust 1.46からはread_fileを呼び出した部分の情報が表示される
    std::fs::read_to_string(path).unwrap()
}

fn main() {
    // Rust 1.45まではread_fileの情報、Rust 1.46からはこの行の情報
    // Rust 1.45: thread 'main' panicked at '...', src/main.rs:5:5
    // Rust 1.46: thread 'main' panicked at '...', src/main.rs:10:20
    println!("{}", read_file("./hoge.txt"));
}

併せて、Location::callerによって、（#[track_caller]を付けた関数内では）関数の呼び出し元が分かるようにもなりました。 この時、#[track_caller]の付いた関数から更に#[track_caller]の付いた関数を呼び出していた場合、 #[track_caller]が付いていない関数からの呼び出し位置が使われます。

use std::panic::Location;

#[track_caller]
fn print_location() {
    println!("{}", Location::caller());
}

fn foo() {
    print_location();
}

// #[track_caller]の関数から#[track_caller]の関数を呼び出している
#[track_caller]
fn bar() {
    print_location();
}

fn main() {
    foo(); // src/main.rs:9:3（foo()内の行）
    bar(); // src/main.rs:20:3（main()のこの行）
}

なお、#[track_caller]は内部的には隠し引数として実装されており、乱用するとパフォーマンスの悪化・バイナリサイズの増大に繋がるため注意が必要です。

変更点リスト

言語

コンパイラ

ライブラリ

安定化されたAPI

Option::zip

impl<T> Option<T> {
    #[stable(feature = "option_zip_option", since = "1.46.0")]
    pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)>
}

selfともう一つのOptionをがっちゃんこする。

selfがSome(s)でotherがSome(o)である場合、Some((s, o))を返す。それ以外ではNoneを返す。

サンプル
let x = Some(1);
let y = Some("hi");
let z = None::<u8>;

assert_eq!(x.zip(y), Some((1, "hi")));
assert_eq!(x.zip(z), None);

vec::Drain::as_slice

impl<'a, T> Drain<'a, T> {
    #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
    pub fn as_slice(&self) -> &[T]
}
サンプル
イテレーターの残りのアイテムをスライスにして返す。 
let mut vec = vec!['a', 'b', 'c'];
let mut drain = vec.drain(..);
assert_eq!(drain.as_slice(), &['a', 'b', 'c']);
let _ = drain.next().unwrap();
assert_eq!(drain.as_slice(), &['b', 'c']);

Cargo

コンパイル時に使用できる環境変数がいくつか追加された。

互換性メモ

関連リンク

さいごに

次のRust 1.47は2020/10/9(金)に予定されています。 32要素まででしか使えなかった一部配列のメソッドがどんな長さでも使えるようになったり、TypeIdが定数として取得できるようになったりするようです。

オプティムでは若者に限らずエンジニアを募集しています。

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