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<h1 id="rust-新版解读--159--重点-内联汇编解构式赋值"><a class="header" href="#rust-新版解读--159--重点-内联汇编解构式赋值">Rust 新版解读 | 1.59 | 重点: 内联汇编、解构式赋值</a></h1>
<p>Rust 团队于今天凌晨( 2022-02-25 )发布了最新的 1.59 版本，其中最引人瞩目的特性应该就是支持在代码中内联汇编了，一起来看看。</p>
<h2 id="内联汇编-inline-assembly-"><a class="header" href="#内联汇编-inline-assembly-">内联汇编( inline assembly )</a></h2>
<p>该特性对于需要底层控制的应用非常有用，例如想要控制底层执行、访问特定的机器指令等。</p>
<p>例如，如果目标平台是 <code>x86-64</code> 时，你可以这么写：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::arch::asm;
// 使用 shifts 和 adds 实现 x 乘以 6
let mut x: u64 = 4;
unsafe {``
asm!(
&quot;mov {tmp}, {x}&quot;,
&quot;shl {tmp}, 1&quot;,
&quot;shl {x}, 2&quot;,
&quot;add {x}, {tmp}&quot;,
x = inout(reg) x,
tmp = out(reg) _,
);
}
assert_eq!(x, 4 * 6);
<span class="boring">}</span></code></pre></pre>
<p>大家发现没，这里的格式化字符串的使用方式跟我们平时的 <code>println!</code>、<code>format!</code> 并无区别， 除了 <code>asm!</code> 之外， <code>global_asm!</code> 宏也可以这么使用。</p>
<p>内联汇编中使用的汇编语言和指令取决于相应的机器平台，截至目前，Rust 支持以下平台的内联汇编：</p>
<ul>
<li>x86 和 x86-64</li>
<li>ARM</li>
<li>AArch64</li>
<li>RISC-V</li>
</ul>
<p>如果大家希望深入了解，可以看官方的 <a href="https://doc.rust-lang.org/nightly/reference/inline-assembly.html">Reference</a> 文档，同时在 <a href="https://zh.exercise.rs/unsafe/inline-asm">Rust Exercise</a> 中提供了更多的示例(目前正在翻译中..)。</p>
<h2 id="解构式赋值-destructuring-assignments"><a class="header" href="#解构式赋值-destructuring-assignments">解构式赋值( Destructuring assignments)</a></h2>
<p>现在你可以在赋值语句的左式中使用元组、切片和结构体模式了。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let (a, b, c, d, e);
(a, b) = (1, 2);
[c, .., d, _] = [1, 2, 3, 4, 5];
Struct { e, .. } = Struct { e: 5, f: 3 };
assert_eq!([1, 2, 1, 4, 5], [a, b, c, d, e]);
<span class="boring">}</span></code></pre></pre>
<p>这种使用方式跟 <code>let</code> 保持了一致性，但是需要注意，使用 <code>+=</code> 的赋值语句还不支持解构式赋值。</p>
<h2 id="const-泛型"><a class="header" href="#const-泛型">const 泛型</a></h2>
<h4 id="为参数设置默认值"><a class="header" href="#为参数设置默认值">为参数设置默认值</a></h4>
<p>现在我们可以为 const 泛型参数设置默认值：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>struct ArrayStorage&lt;T, const N: usize = 2&gt; {
arr: [T; N],
}
impl&lt;T&gt; ArrayStorage&lt;T&gt; {
fn new(a: T, b: T) -&gt; ArrayStorage&lt;T&gt; {
ArrayStorage {
arr: [a, b],
}
}
}
<span class="boring">}</span></code></pre></pre>
<h4 id="取消参数顺序的限制"><a class="header" href="#取消参数顺序的限制">取消参数顺序的限制</a></h4>
<p>在之前版本中，类型参数必须要在所有的 const 泛型参数之前，现在，这个限制被放宽了，例如你可以这样交替排列它们：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn cartesian_product&lt;
T, const N: usize,
U, const M: usize,
V, F
&gt;(a: [T; N], b: [U; M], f: F) -&gt; [[V; N]; M]
where
F: FnMut(&amp;T, &amp;U) -&gt; V
{
// ...
}
<span class="boring">}</span></code></pre></pre>
<h2 id="缩小二进制文件体积删除-debug-信息"><a class="header" href="#缩小二进制文件体积删除-debug-信息">缩小二进制文件体积：删除 debug 信息</a></h2>
<p>对于受限的环境来说，缩小编译出的二进制文件体积是非常重要的。</p>
<p>以往我们可以在二进制文件被创建后，手动的来完成。现在 cargo 和 rustc 支持在链接( linked )后就删除 debug 信息，在 <code>Cargo.toml</code> 中新增以下配置：</p>
<pre><code class="language-toml">[profile.release]
strip = &quot;debuginfo&quot;
</code></pre>
<p>以上配置会将 <code>release</code> 二进制文件中的 debug 信息移除。你也可以使用 <code>&quot;symbols&quot;</code> 或 <code>true</code> 选项来移除所有支持的 <code>symbol</code> 信息。</p>
<p>根据 reddit 网友的测试，如果使用了 <code>strip = true</code>，那编译后的体积将大幅减少(50% 左右):</p>
<ul>
<li>先使用 <code>lto = true</code> : 4,397,320 bytes</li>
<li>再使用 <code>strip = true</code> : 2,657,304 bytes</li>
<li>最后 <code>opt-level = &quot;z&quot;</code> : 1,857,680 bytes</li>
</ul>
<p>如果是 WASM，还可以使用以下配置进一步减少体积:</p>
<pre><code class="language-toml">[package.metadata.wasm-pack.profile.release]
wasm-opt = ['-Os']
</code></pre>
<p><a href="https://github.com/rsadsb/adsb_deku/blob/master/CHANGELOG.md#unreleased">github 上一个开源仓库</a>也证明了这一点，总体来看，这个配置的效果是非常显著的！</p>
<h2 id="默认关闭增量编译"><a class="header" href="#默认关闭增量编译">默认关闭增量编译</a></h2>
<p>1.59.0 版本默认关闭了增量编译的功能（你可以通过环境变量显式地启用：<code>RUSTC_FORCE_INCREMENTAL=1</code> ），这会降低已知 Bug <a href="https://github.com/rust-lang/rust/issues/94124">#94124</a> 的影响，该 Bug 会导致增量编译过程中的反序列化错误和 <code>panic</code>。</p>
<p>不过大家也不用担心，这个 Bug 会在 1.60.0 版本修复，也就是 6 周后，增量编译会重新设置为默认开启，如果没有意外的话 :)</p>
<h2 id="稳定化的-api-列表"><a class="header" href="#稳定化的-api-列表">稳定化的 API 列表</a></h2>
<p>一些方法和特征实现现在已经可以 stable 中使用，具体见<a href="https://blog.rust-lang.org/2022/02/24/Rust-1.59.0.html#stabilized-apis">官方发布说明</a></p>
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