Rust 1.50を早めに深掘り

こんにちは、R&Dチームの齋藤(@aznhe21)です。

さて、本日、日本時間2/12(金)、Rust 1.50がリリースされました。 この記事ではRust 1.50での変更点を詳しく紹介します。

2/12はダーウィンの日 1ワードの浪費を何とも思わない言語は、ライフタイムの価値を未だ見出してはいない *1 *2

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。

なお、定数ジェネリクスは1.51に延期されていました。残念!

Rust財団が誕生

Rust 1.50のリリースとは関係ありませんが、直近のビッグニュースなのでここで紹介します。

先日2021/2/8にRust財団が設立されました。Rust財団は、Rust言語とその周辺環境を支援するために設立された独立の非営利組織です。 設立メンバー企業として、AWS、Huawei、Google、Microsoft、そしてMozillaが名を連ねています。

これまでMozillaが管理していた知財(商標・ロゴなど)やインフラ(crates.io含む)は既に財団に移管されており、 より独立した運営がスタートしています。 昨年のMozillaの大規模レイオフによって不安視されていた金銭面への不安もこれで払拭されるでしょう。

設立メンバーの各企業による声明は以下の記事にまとめられています。

ディレクトリ内容を変更した際のbuild.rsの再実行

build.rsにprintln!("cargo:rerun-if-changed:path/to/file")と書くことで、ファイルが更新された際はbuild.rsを再実行する機能があります。 これはコード生成などに使う機能ですが、これまではディレクトリパスを指定しても、ディレクトリ内のファイルの変更ではbuild.rsの再実行はされませんでした。

Rust 1.50からはディレクトリパスを指定するとディレクトリ内のファイルを再帰的にチェックし、 変更されたファイルがあればbuild.rsを再実行されるようになりました。

boolの値でOptionを返す関数

真偽値が真(true)のとき、クロージャーを呼び出してSomeに包む関数bool::thenが使えるようになりました。 無駄なif式が消えてスッキリします。

fn safe_div(a: u32, b: u32) -> Option<u32> {
    // Rust < 1.50
    // if b > 0 {
    //     Some(a / b)
    // } else {
    //     None
    // }

    // Rust >= 1.50
    (b > 0).then(|| a / b)
}

値域を制限する関数

数値の最小値、最大値を指定するとその範囲内に収めてくれる関数Ord::clampf32::clamp及びf64::clampが使えるようになりました。 これもまたコードをスッキリさせる便利関数です。

これはOrd及びf32f64に実装されています。

fn clamp_array(f: &[f32]) -> Vec<f32> {
    // Rust < 1.50
    // f.iter().map(|&v| if v < 0. { 0. } else if v > 1. { 1. } else { v }).collect()

    // Rust >= 1.50
    f.iter().map(|v| v.clamp(0., 1.)).collect()
}

配列を同じ値で埋める関数

配列をループせず、同じ値で埋める関数slice::fillが使えるようになりました。 こちらも嬉しい便利関数ですね。

struct Image {
    pub data: Vec<u8>,
    pub width: u32,
    pub height: u32,
}

impl Image {
    pub fn make_white(&mut self) {
        self.data.fill(0xFF);
    }
}

安定化されたAPIのドキュメント

安定化されたAPIのドキュメントを独自に訳して紹介します。リストだけ見たい方は安定化されたAPIをご覧ください。

bool::then

→ピックアップ

原典

#[lang = "bool"]
impl bool {
    #[stable(feature = "lazy_bool_to_option", since = "1.50.0")]
    #[inline]
    pub fn then<T, F: FnOnce() -> T>(self, f: F) -> Option<T>
}

booltrueであればSome(f())を返し、そうでない場合はNoneを返す。

サンプル
assert_eq!(false.then(|| 0), None);
assert_eq!(true.then(|| 0), Some(0));

btree_map::Entry::or_insert_with_key

原典

impl<'a, K: Ord, V> Entry<'a, K, V> {
    #[inline]
    #[stable(feature = "or_insert_with_key", since = "1.50.0")]
    pub fn or_insert_with_key<F: FnOnce(&K) -> V>(self, default: F) -> &'a mut V
}

エントリが空の場合、default関数の結果を挿入することでエントリに値があることを確定させる。 この関数により、.entry(key)の呼び出しによってムーブされたキーをdefault関数から参照することで、キーに由来した値を挿入することが出来る。

.or_insert_with(|| ... )を使うのとは違ってムーブされたキーを参照できるため、キーのクローンやコピーが不要。

サンプル
use std::collections::BTreeMap;

let mut map: BTreeMap<&str, usize> = BTreeMap::new();

map.entry("poneyland").or_insert_with_key(|key| key.chars().count());

assert_eq!(map["poneyland"], 9);

f32::clamp

→ピックアップ

原典

#[lang = "f32"]
#[cfg(not(test))]
impl f32 {
    // メソッドは元の値を変更せず、新しい値を返す
    #[must_use = "method returns a new number and does not mutate the original value"]
    #[stable(feature = "clamp", since = "1.50.0")]
    #[inline]
    pub fn clamp(self, min: f32, max: f32) -> f32
}

値がNaNで無い限り、値を指定された区間に限定させる。

selfmaxより大きい場合はmaxを、selfminより小さい場合はminを、それ以外はselfを返す。

初期値がNaNの場合はNaNを返すことに注意されたい。

パニック

min > maxの場合、あるいはminmaxがNaNの場合はパニックする。

サンプル
assert!((-3.0f32).clamp(-2.0, 1.0) == -2.0);
assert!((0.0f32).clamp(-2.0, 1.0) == 0.0);
assert!((2.0f32).clamp(-2.0, 1.0) == 1.0);
assert!((f32::NAN).clamp(-2.0, 1.0).is_nan());

f64::clamp

→ピックアップ

原典

#[lang = "f64"]
#[cfg(not(test))]
impl f64 {
    // メソッドは元の値を変更せず、新しい値を返す
    #[must_use = "method returns a new number and does not mutate the original value"]
    #[stable(feature = "clamp", since = "1.50.0")]
    #[inline]
    pub fn clamp(self, min: f64, max: f64) -> f64
}

値がNaNで無い限り、値を指定された区間に限定させる。

selfmaxより大きい場合はmaxを、selfminより小さい場合はminを、それ以外はselfを返す。

初期値がNaNの場合はNaNを返すことに注意されたい。

パニック

min > maxの場合、あるいはminmaxがNaNの場合はパニックする。

サンプル
assert!((-3.0f64).clamp(-2.0, 1.0) == -2.0);
assert!((0.0f64).clamp(-2.0, 1.0) == 0.0);
assert!((2.0f64).clamp(-2.0, 1.0) == 1.0);
assert!((f64::NAN).clamp(-2.0, 1.0).is_nan());

hash_map::Entry::or_insert_with_key

原典

impl<'a, K, V> Entry<'a, K, V> {
    #[inline]
    #[stable(feature = "or_insert_with_key", since = "1.50.0")]
    pub fn or_insert_with_key<F: FnOnce(&K) -> V>(self, default: F) -> &'a mut V
}

エントリが空の場合、default関数の結果を挿入することでエントリに値があることを確定させる。 この関数により、.entry(key)の呼び出しによってムーブされたキーをdefault関数から参照することで、キーに由来した値を挿入することが出来る。

.or_insert_with(|| ... )を使うのとは違ってムーブされたキーを参照できるため、キーのクローンやコピーが不要。

サンプル
use std::collections::HashMap;

let mut map: HashMap<&str, usize> = HashMap::new();

map.entry("poneyland").or_insert_with_key(|key| key.chars().count());

assert_eq!(map["poneyland"], 9);

Ord::clamp

→ピックアップ

原典

#[doc(alias = "<")]
#[doc(alias = ">")]
#[doc(alias = "<=")]
#[doc(alias = ">=")]
#[stable(feature = "rust1", since = "1.0.0")]
pub trait Ord: Eq + PartialOrd<Self> {
    #[must_use]
    #[stable(feature = "clamp", since = "1.50.0")]
    fn clamp(self, min: Self, max: Self) -> Self
    where
        Self: Sized,
}

値を指定された区間に限定させる。

selfmaxより大きい場合はmaxを、selfminより小さい場合はminを、それ以外はselfを返す。

パニック

min > maxの場合はパニックする。

サンプル
assert!((-3).clamp(-2, 1) == -2);
assert!(0.clamp(-2, 1) == 0);
assert!(2.clamp(-2, 1) == 1);

RefCell::take

原典

impl<T: Default> RefCell<T> {
    #[stable(feature = "refcell_take", since = "1.50.0")]
    pub fn take(&self) -> T
}

内包する値を取り出し、Default::default()で返る値と入れ替える。

パニック

値が借用されている場合はパニックする。

サンプル
use std::cell::RefCell;

let c = RefCell::new(5);
let five = c.take();

assert_eq!(five, 5);
assert_eq!(c.into_inner(), 0);

slice::fill

→ピックアップ

原典

#[lang = "slice"]
#[cfg(not(test))]
impl<T> [T] {
    #[doc(alias = "memset")]
    #[stable(feature = "slice_fill", since = "1.50.0")]
    pub fn fill(&mut self, value: T)
    where
        T: Clone,
}

selfの各要素を、valueを複製して埋める。

サンプル
let mut buf = vec![0; 10];
buf.fill(1);
assert_eq!(buf, vec![1; 10]);

UnsafeCell::get_mut

原典

impl<T: ?Sized> UnsafeCell<T> {
    #[inline]
    #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
    pub fn get_mut(&mut self) -> &mut T
}

保持しているデータへの可変参照を返す。

この呼び出しによってUnsafeCellは可変で(コンパイル時に)借用されるため、唯一の参照を所有することが保証される。

サンプル
use std::cell::UnsafeCell;

let mut c = UnsafeCell::new(5);
*c.get_mut() += 1;

assert_eq!(*c.get_mut(), 6);

変更点リスト

公式リリースノートをベースに意訳・編集・追記をした変更点リストです。

言語

コンパイラ

※Rustのティアで管理されるプラットフォームの詳細はPlatform Supportのページ(英語)を参照

ライブラリ

安定化されたAPI

※各APIのドキュメントを独自に訳しています。安定化されたAPIのドキュメントもご参照ください。

また、以前から安定化されていたAPIのうち以下のものが定数化された。

Cargo

その他

互換性メモ

関連リンク

さいごに

次のRust 1.51は2021/3/26(金)に予定されています。 今度こそ定数ジェネリクスが使えるようになる他、配列の各要素の所有権を奪ってイテレートするための機能などが入るようです。

オプティムでは進化する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Rust財団によって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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*1:"A man who dares to waste an hour of time has not discovered the value of life." - Charles Darwin https://www.darwinproject.ac.uk/letter/DCP-LETT-306.xml

*2:"A language which dares to waste a word of memory has not discovered the value of lifetime." - Rust 1.50を早めに深堀り