Rust 1.48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。 先日の健康診断でついに身長が180cmに到達しました。

さて、本日、日本時間11/20(金)、Rust 1.48がリリースされました。 この記事ではRust 1.48での変更点を詳しく紹介します。

11/20は任天堂設立の日 C++の倒し方、知ってますよ

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。

これまではhtmlのパスとハッシュを使い、[`serialize_struct`](#method.serialize_struct)や[`Deserialize`](trait.Deserialize.html)のようにリンクしていました。 これはリンク先が同じ型なのか、別の型で種類（構造体やトレイトなど）は何なのかを意識しなければなりませんでした。 また、モジュールが違えば[`Value`](../enum.Value.html)のようにパスを辿る必要があり、これもまた面倒です。

Rust 1.48からは、単純に[`serialize_struct`](`Self::serialize_struct`)や[`Deserialize`](`crate::Deserialize`)のように、 コードの中で書くのと同じ形式でアイテムを書けばリンクができるようになりました。

ソースコード:

//! このモジュールには[`random`][`crate::random`]という関数がある。

/// この関数は真にランダムな[`u32`]型の値を返す。
pub fn random() -> u32 {
    42
}

変換されたHTML:

<p>このモジュールには<a href="../test_doc/fn.random.html" title="crate::random"><code>random</code></a>という関数がある。</p>

<p>この関数は真にランダムな<a href="https://doc.rust-lang.org/nightly/std/primitive.u32.html" title="u32"><code>u32</code></a>型の値を返す。</p>

Vec<T>から[T; N]に変換できるようになった

Vec<T>型の変数から特定の要素数の配列に変換できるようになりました。 これにより、Vec<T>の所有権を奪いつつ固定長の配列に変換できるようになります。 なお、要素数が異なる場合はエラーが返されます。

use std::convert::TryInto;

fn main() {
    let v: Vec<u32> = vec![1; 10];
    let array: [u32; 10] = v.try_into().unwrap();
}

futuresクレートの一部メソッドがlibcoreでも実装された

futuresクレートに実装されていたpending及びreadyという関数がlibcore / libstdにも実装されました。 わざわざfuturesクレートに依存しなくてもこれらの関数が使えるようになります。

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。

slice::as_ptr_range

原典

impl<T> [T] {
    #[stable(feature = "slice_ptr_range", since = "1.48.0")]
    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
    #[inline]
    pub const fn as_ptr_range(&self) -> Range<*const T>
}

スライスを架ける2つの生ポインタを返す。

返される範囲は半開区間であるため、終了側のポインタはスライスの最後の要素より1つ後ろである。 このため、空のスライスは2つの同じポインタによって表される他、2つのポインタの差がスライスのサイズとなる。

ポインタを扱う際の注意点はas_ptrを参照されたい。後側のポインタは特に注意が必要で、スライス中での有効な要素を指しているわけではない。

この関数は、C++で一般的な、メモリ間の要素の範囲を2つのポインタで表す外部インターフェースとのやり取りに有用。

また、あるポインタがこのスライス内を指しているかを確認するのにも有用である。

let a = [1, 2, 3];
let x = &a[1] as *const _;
let y = &5 as *const _;

assert!(a.as_ptr_range().contains(&x));
assert!(!a.as_ptr_range().contains(&y));

slice::as_mut_ptr_range

原典

impl<T> [T] {
    #[stable(feature = "slice_ptr_range", since = "1.48.0")]
    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
    #[inline]
    pub const fn as_mut_ptr_range(&mut self) -> Range<*mut T>
}

スライスを架ける、2つの不安全で可変な生ポインタを返す。

返される範囲は半開区間であるため、終了側のポインタはスライスの最後の要素より1つ後ろである。 このため、空のスライスは2つの同じポインタによって表される他、2つのポインタの差がスライスのサイズとなる。

ポインタを扱う際の注意点はas_mut_ptrを参照されたい。後側のポインタは特に注意が必要で、スライス中での有効な要素を指しているわけではない。

この関数は、C++で一般的なメモリ間の要素の範囲を2つのポインタで表す外部インターフェースとのやり取りに有用。

VecDeque::make_contiguous

原典

impl<T> From<VecDeque<T>> for Vec<T> {
    #[stable(feature = "deque_make_contiguous", since = "1.48.0")]
    pub fn make_contiguous(&mut self) -> &mut [T]
}

一塊のスライスとなるよう内部領域を再配置し、スライスを返す。

このメソッドは確保は行わず、挿入された要素の順番も入れ替えない。そして可変スライスを返すため、Dequeのソートや二分探索に使うことが出来る。

内部領域が一塊となれば、as_slicesやas_mut_slicesはVecDeque内の内容を1つのスライスとして返すようになる。

サンプル
use std::collections::VecDeque;

let mut buf = VecDeque::with_capacity(15);

buf.push_back(2);
buf.push_back(1);
buf.push_front(3);

// Dequeをソートする
buf.make_contiguous().sort();
assert_eq!(buf.as_slices(), (&[1, 2, 3] as &[_], &[] as &[_]));

// 逆順でソートする
buf.make_contiguous().sort_by(|a, b| b.cmp(a));
assert_eq!(buf.as_slices(), (&[3, 2, 1] as &[_], &[] as &[_]));

一塊のスライスに不変アクセスする。

use std::collections::VecDeque;

let mut buf = VecDeque::new();

buf.push_back(2);
buf.push_back(1);
buf.push_front(3);

buf.make_contiguous();
if let (slice, &[]) = buf.as_slices() {
    // `buf`への不変参照を保持している間、`slice`はDeque内の全要素を保持していると言える
    assert_eq!(buf.len(), slice.len());
    assert_eq!(slice, &[3, 2, 1] as &[_]);
}

future::pending

原典

#[stable(feature = "future_readiness_fns", since = "1.48.0")]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct Pending<T> {
    _data: marker::PhantomData<T>,
}

計算が永遠に終わらないことを表す、永遠に解決されないfutureを生成する。

サンプル
use core::future;

let future = future::pending();
let () = future.await;
unreachable!();

future::ready

原典

#[stable(feature = "future_readiness_fns", since = "1.48.0")]
pub fn ready<T>(t: T) -> Ready<T> {
    Ready(Some(t))
}

指定された値で即座に解決されるfutureを生成する。

この関数によって作られたfutureは、async {}で作られるものとほぼ同じである。 主に、この関数で作られたfutureは名前があり、そしてUnpinを実装しているという相違点がある。

サンプル
use core::future;

let a = future::ready(1);
assert_eq!(a.await, 1);

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Tierによるプラットフォームサポートの詳細はプラットフォームサポートのページを参照 ※訳注：英語ページ

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APIが定数化された。

Rustdoc

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をもたらすわけではないが、rustc及び関連ツール内部での意味あるパフォーマンス改善を意味する。

関連リンク

さいごに

次のRust 1.49は2020年大晦日〜2021年元日に掛けてのリリースが予定されています。 1つのパターンの中でムーブと参照を同時に使えるようになりそうです。 （記事が書けない気がする・・・）

オプティムではゲーミング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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