Rust 1.37を早めに深掘り

こんにちは。業務用のPCがつよつよマシン*1になって毎日が楽しいR&Dチームの齋藤(@aznhe21)です。

さて、本日、日本時間8/16(金)、Rust 1.37がリリースされました。 この記事ではRust 1.37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・追記をしています。

f:id:optim-tech:20190815142210p:plain

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。

プロファイルに基づく最適化ができるようになった

バイナリを実行してプロファイルを取り、それをもとに最適化を実施する、 いわゆるPGO*2が利用できるようになりました。 これにより、例えばエラー処理などの滅多に突入しない条件分岐において分岐予測ができるだけミスしないようになるなど、 通常の最適化では行えない高度な最適化が実施できます。

定数の名前としてアンダースコアが使えるようになった

アンダースコアを定数の名前として使えるようになり、式は評価したいが値は不要であるといった場面でconstが使えるようになりました。 これだけ聞くと何の意味があるか分かりませんが、RFCによれば、 カスタム自動導出*3において、型が要件を満たしているかのチェックに使えるようです。

これまでも_FOOのようにアンダースコアで始まる識別子を使えばすれば未使用の警告を抑制することはできました。 しかし、これでは_FOOを別の場所で使うことができず、マクロによって展開される式では使いづらいものでした。 アンダースコアが使えるようになれば、この問題を回避できるわけです。

実行可能なクレートを公開するときにCargo.lockがデフォルトで含まれるようになった

もともと実行可能クレートのレポジトリにはCargo.lockが含まれていましたが、 cargo packagecargo publishの際には取り除かれていました。 これはビルド再現性の観点からは良くないことであるため、クレートを公開する際にもCargo.lockがデフォルトで含まれるようになりました。

実際にビルドが壊れた例が下記記事で紹介されています。

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

BufReader::buffer

内部バッファのうち、読み込まれている部分へのスライスを返す。 内部バッファが空であれば空のまま返されるので、バッファを埋めたい場合はBufRead::fill_bufを呼び出すと良い。

BufWriter::buffer

内部バッファ全体を返す。 可変参照ではないため、既に書き込まれたデータを読み出すことしかできない。

Cell::from_mut

可変参照から新しいCellのインスタンスを生成する。

Cell<[T]>::as_slice_of_cells

スライスのCellCell<[T]>)からCellのスライス(&[Cell<T>])を生成する。

DoubleEndedIterator::nth_back

イテレータを後ろからn個戻り、その要素を返す。

Option::xor

別のOptionとのXOR演算を行う。 すなわち、a.xor(b)としたとき、abが両方Some(v)ないしNoneであればNoneを、 aSome(v)かつbNoneであればaを、aNoneかつbSome(v)であればbを返す。

Wrapping::reverse_bits

数値のビットパターンを逆順にする。

{i8,i16,i32,i64,i128,isize}::reverse_bits

数値のビットパターンを逆順にする。

slice::copy_within

スライスの内部で値をコピーする。コピーにはmemmoveを使用する。

srcselfにおけるコピー元の範囲。 destsrc内のコピー先範囲の開始インデックスで、コピーする長さはsrcと同じ。 この2つの範囲は重なることができ、それぞれの終了インデックスはself.len()未満である必要がある。

どちらかの範囲がスライスを超えている場合や srcの終了インデックスが開始インデックス以下である場合、この関数はパニックする。

{u8,u16,u32,u64,u128,usize}::reverse_bits

数値のビットパターンを逆順にする。

Cargo

互換性メモ

関連リンク

さいごに

PGOが入ったのは少しでもパフォーマンスを稼ぎたいプロジェクトには朗報なのではないでしょうか。 他はあまりパッとしない変更点ばかりでしたが、async/awaitという嵐の前の静けさなのでしょうか。

次のRust 1.38は9/27(金)に予定されています。

オプティムではこの暑過ぎる夏にも耐えられるエンジニアを募集しています。

また、学生向けの夏季インターンシップも募集しています。 暑い夏も涼しいオフィスでコーディングができます。 なお、ご好評につきR&Dコースは募集を締め切っておりますのでご了承ください。

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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*1:Dell XPS 15 7590 スプレマシー

*2:Profile-guided Optimization

*3:custom derives