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<main>
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<h1 id="rust-新版解读--183--const-能力拓展"><a class="header" href="#rust-新版解读--183--const-能力拓展">Rust 新版解读 | 1.83 | Const 能力拓展</a></h1>
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<blockquote>
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<p>Rust 1.83 官方 release doc: <a href="https://blog.rust-lang.org/2024/11/28/Rust-1.83.0.html">Announcing Rust 1.83.0 | Rust Blog</a></p>
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</blockquote>
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<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 1.83 版本:</p>
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<pre><code class="language-shell">$ rustup update stable
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</code></pre>
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<h2 id="新的常量能力"><a class="header" href="#新的常量能力">新的常量能力</a></h2>
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<p>此版本包含了对 <code>const</code> 上下文中运行的代码的几个重大扩展。这指的是编译器必须在编译时求值的代码：包括 <code>const</code> 和 <code>static</code> 项的初始值、数组长度、枚举判别值、<code>const</code> 泛型参数，以及可从这些上下文中调用的函数（<code>const fn</code>）。</p>
|
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<p><strong>对静态值的引用。</strong> 此前，除了 <code>static</code> 项的初始化表达式外，<code>const</code> 上下文中不能引用 <code>static</code> 项。这一限制现已解除：</p>
|
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
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|
</span><span class="boring">fn main() {
|
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|
</span>static S: i32 = 25;
|
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|
const C: &i32 = &S;
|
|
|
<span class="boring">}</span></code></pre>
|
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<p>然而，在 <code>const</code> 上下文中仍然不允许读取可变或内部可变静态项的值。此外，常量的最终值不能引用任何可变或内部可变静态项：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>static mut S: i32 = 0;
|
|
|
|
|
|
const C1: i32 = unsafe { S };
|
|
|
// error: constant accesses mutable global memory
|
|
|
|
|
|
const C2: &i32 = unsafe { &S };
|
|
|
// error: encountered reference to mutable memory in `const`
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这些限制确保了常量仍然是“常量”：它们求值后的值，及其作为模式的意义（可能涉及解引用引用）在整个程序执行过程中将保持不变。</p>
|
|
|
<p>也就是说，常量允许是指向可变或内部可变静态变量的原始指针：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>static mut S: i32 = 64;
|
|
|
const C: *mut i32 = &raw mut S;
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p><strong>可变引用和指针。</strong> 现在可以在 <code>const</code> 上下文中使用可变引用：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>const fn inc(x: &mut i32) {
|
|
|
*x += 1;
|
|
|
}
|
|
|
|
|
|
const C: i32 = {
|
|
|
let mut c = 41;
|
|
|
inc(&mut c);
|
|
|
c
|
|
|
};
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>可变原始指针和内部可变性也得到了支持：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use std::cell::UnsafeCell;
|
|
|
|
|
|
const C: i32 = {
|
|
|
let c = UnsafeCell::new(41);
|
|
|
unsafe { *c.get() += 1 };
|
|
|
c.into_inner()
|
|
|
};
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>然而，可变引用和指针只能在常量的计算过程中使用，不能成为常量最终值的一部分：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>const C: &mut i32 = &mut 4;
|
|
|
// error[E0764]: mutable references are not allowed in the final value of constants
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>此版本还带来了一批新的函数，这些函数现在在 <code>const</code> 上下文中是稳定的（参见原文连接里的“稳定的API”部分的末尾）。</p>
|
|
|
<p>这些新能力和稳定的API解锁了在 <code>const</code> 上下文中执行的全新代码类别，我们很高兴看到Rust生态系统将如何利用这一点！</p>
|
|
|
<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/2024/11/28/Rust-1.83.0.html#stabilized-apis">原Blog</a></p>
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