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<main>
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<h1 id="rust-新版解读--179--内联-const临时变量生命周期延长"><a class="header" href="#rust-新版解读--179--内联-const临时变量生命周期延长">Rust 新版解读 | 1.79 | 内联 const，临时变量生命周期延长</a></h1>
|
|
|
<blockquote>
|
|
|
<p>Rust 1.79 官方 release doc: <a href="https://blog.rust-lang.org/2024/06/13/Rust-1.79.0.html">Announcing Rust 1.79.0 | Rust Blog</a></p>
|
|
|
</blockquote>
|
|
|
<p>通过 <a href="https://www.rust-lang.org/tools/install">rustup</a> 安装的同学可以使用以下命令升级到 1.79 版本:</p>
|
|
|
<pre><code class="language-shell">$ rustup update stable
|
|
|
</code></pre>
|
|
|
<h2 id="内联-const-表达式"><a class="header" href="#内联-const-表达式">内联 <code>const</code> 表达式</a></h2>
|
|
|
<p>如今可以写内联 const 块 <code>const {...}</code> 作为表达式，显式地进入 const 上下文，而不需要额外的声明（例如，定义 <code>const</code> 常量或 Trait 的关联常量）。</p>
|
|
|
<p>与 const 常量 <code>const ITEM: ... = ...</code> 不同，内联 const 里类型可以被推断而不需要显式写出，并且还能使用泛型参数。来看一个很实用的例子：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>const EMPTY: Option<Vec<u8>> = None;
|
|
|
let foo = [EMPTY; 100];
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<p>如今可以写成如下形式，（foo 的类型 <code>Option<T></code> 可以不标注，可以根据上下文推断出来）</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let foo = [const { None }; 100];
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<p>泛型的例子：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn create_none_array<T, const N: usize>() -> [Option<T>; N] {
|
|
|
[const { None }; N]
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<p>更多细节见<a href="https://doc.rust-lang.org/nightly/reference/expressions/block-expr.html#const-blocks">参考文档</a></p>
|
|
|
<h2 id="关联类型约束"><a class="header" href="#关联类型约束">关联类型约束</a></h2>
|
|
|
<p>Rust 1.79 稳定了一些关联类型约束的语法，允许我们在类型约束里写其它类型约束，即 <code>T: Trait<Assoc: Bounds...></code>。这避免了提供额外的显式泛型类型来约束关联类型。</p>
|
|
|
<p>这个新特性允许我们在一些情况下更简单地指定好约束关系，解决了一些之前不可能或者会引入额外不必要约束的场景。</p>
|
|
|
<ul>
|
|
|
<li><strong><code>where</code> 子句</strong> - 在这个位置，这等同于将约束拆分为两个（或更多）<code>where</code> 语句。例如，<code>where T: Trait<Assoc: Bound></code> 等同于 <code>where T: Trait, <T as Trait>::Assoc: Bound</code>。</li>
|
|
|
<li><strong>Supertraits</strong> - 类似于上面，<code>trait CopyIterator: Iterator<Item: Copy> {}</code>。这也等同于将约束拆分为两个（或更多）<code>where</code> 语句；不过当 trait 被使用时，这个对关联类型 Item 的约束是隐含的。</li>
|
|
|
<li><strong>关联类型 Item 约束</strong> - 允许约束与 trait 的关联类型相关的嵌套类型约束。例如 <code>trait Trait { type Assoc: Trait2<Assoc2: Copy>; }</code>。</li>
|
|
|
<li><strong>模糊类型约束</strong>（RPIT: return position <code>impl Trait</code>, TAIT: type alias <code>impl Trait</code>） - 允许约束与模糊类型相关的关联类型。例如 <code>impl Iterator<Item: Copy></code> 定义了 Item 满足 Copy 的迭代器，而不必实际命名该约束。</li>
|
|
|
</ul>
|
|
|
<p>更多细节见 <a href="https://github.com/rust-lang/rust/pull/122055/#issue-2170532454">issue</a></p>
|
|
|
<p>译注：很绕，但是整体上就是一次让 Rust 编译器变得更符合你期望它应该正常工作的样子的更新。</p>
|
|
|
<h2 id="临时变量生命周期延长"><a class="header" href="#临时变量生命周期延长">临时变量生命周期延长</a></h2>
|
|
|
<p>现在，在 <code>match</code> 和 <code>if</code> 结构中构造并立刻被使用的临时变量的生命周期会自动延长。这与代码结构中的临时变量生命周期延长的效果一致。</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let a = if true {
|
|
|
..;
|
|
|
&temp() // used to error, but now gets lifetime extended
|
|
|
} else {
|
|
|
..;
|
|
|
&temp() // used to error, but now gets lifetime extended
|
|
|
};
|
|
|
|
|
|
let a = match () {
|
|
|
_ => {
|
|
|
..;
|
|
|
&temp() // used to error, but now gets lifetime extended
|
|
|
}
|
|
|
};
|
|
|
|
|
|
// 之前已有的代码块临时变量生命周期延长
|
|
|
let a = {
|
|
|
..;
|
|
|
&temp() // lifetime is extended
|
|
|
};
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<h2 id="others"><a class="header" href="#others">Others</a></h2>
|
|
|
<p>其它更新细节，和稳定的 API 列表，参考<a href="https://blog.rust-lang.org/2024/06/13/Rust-1.79.0.html#stabilized-apis">原Blog</a></p>
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