Rust 1.43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。 最近はHalf-Life: Alyxをプレイしすぎて、リアルでも遠くのものを近くに引き寄せようとしてしまいます。

さて、本日、日本時間4/24(金)、Rust 1.43がリリースされました。 この記事ではRust 1.43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。

4/23はYouTubeに初めて動画が投稿された日 4/23はYouTubeに初めて動画が投稿された日

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。

数値型の関連定数を使う時に、モジュールをインポートする必要が無くなった

u32::MAXなど、数値型に関連した定数を使用する際、モジュールをインポートせずにそのまま使うことができるようになりました。 これまではuse std::u32;のように明示的なインポートが必要でした。

// 1.42まではインポートが必要
// use std::u32;

fn main() {
    println!("32ビットの最大値：{}", u32::MAX);
    println!("64ビットの最大値：{}", u64::MAX);
}

primitiveモジュールが追加され、組み込み型を確実に参照できるようになった

Rustでは、変数と同じように型も隠蔽（shadowing）出来ます。 この機能を使ってu32などの組み込み型も隠蔽できるわけですが、一度隠蔽してしまうと元の型を参照できなくなっていました。 これはマクロの中でそれらの型を使っている場合に困ります。

primitiveモジュールは組み込み型を再エクスポートするモジュールであり、このモジュールを使えば確実に組み込み型を参照できるようになりました。

#![allow(non_camel_case_types)]

macro_rules! hoge {
    (u32) => {
        // 型の隠蔽により、u32の最大値が出力されるとは限らない
        println!("32ビットの最大値・・・？：{}", u32::MAX);
    };
    (u64) => {
        // 型が隠蔽されていても、確実に型を参照できる
        // println!も確実に参照できるように絶対パスで指定している
        ::std::println!("64ビットの最大値：{}", ::std::primitive::u64::MAX);
    };
}

fn main() {
    // u32とu64を隠蔽してみる
    type u32 = u16;
    type u64 = u16;

    hoge!(u32);
    hoge!(u64);
}

変更点リスト

言語

構文のみの変更

例えばこう書ける。

macro_rules! mac_trait {
    ($i:item) => {
        trait T { $i }
    }
}
mac_trait! {
    fn foo() {}
}

これらは依然として意味上は弾かれるためエラーになる可能性が高いが、マクロや条件コンパイルといった意味解析しない部分では使うことが出来る。

コンパイラ

ライブラリ

安定化されたAPI

Once::is_completed

call_onceの呼び出しが成功した場合にtrueを返す。 逆に、以下のような場合にはfalseを返す。

  • call_onceが呼び出されていない場合
  • call_onceが呼び出されたが、その呼び出しが完了していない場合
  • Onceのインスタンスが毒された（poison状態になった）

戻り値がfalseだからといってOnceが実行されていない、というわけではない。 例えばis_completedが呼び出されてから戻るまでにOnceの実行が完了した場合など。その場合、戻り値はfalseだが実行されたということになる。

f32::LOG10_2 / f64::LOG10_2

log10(2)

f32::LOG2_10 / f64::LOG2_10

log2(10)

iter::once_with

クロージャーによって、値を一度だけ生成して返すイテレーターを作る。

これは、1つの値を生成するメソッドをIterator::chainなどと組み合わせる場合に使われる。 イテレーターとしても適用できる関数を、1回だけ処理したい場合に有用。

Iterator::onceとは異なり、この関数は、値を要求された時に遅延してクロージャーを呼び出し、値を生成する。

Cargo

その他

互換性メモ

内部の変更

これらの変更は直接ユーザーの利益になるわけではないが、rustcや関連ツールの内部と全体的なパフォーマンスの改善を意味する。 （訳注：コアな部分ということもあって翻訳が難しくルー語になってしまうものがあったため、一律未翻訳とします）

関連リンク

さいごに

次のRust 1.44は2020/6/5(金)に予定されています。 配列をVecにInto出来るようになったり、浮動小数点数を近似して整数に変換するための機能などの追加が予定されています。

オプティムでは好きなことで生きていきたいエンジニアを募集しています。

ライセンス表記

  • この記事はApache 2/MITのデュアルライセンスで公開されている公式リリースノートを翻訳・追記をしています
  • 冒頭の画像中にはRust公式サイトで配布されているロゴを使用しており、 このロゴはMozillaによってCC-BYの下で配布されています
  • 冒頭の画像はいらすとやさんの画像を使っています。いつもありがとうございます

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