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<main>
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<h1 id="rust-新版解读--166--重点-有字段枚举的显示判别"><a class="header" href="#rust-新版解读--166--重点-有字段枚举的显示判别">Rust 新版解读 | 1.66 | 重点: 有字段枚举的显示判别</a></h1>
|
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<blockquote>
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<p>Rust 1.66 官方 release doc: <a href="https://blog.rust-lang.org/2022/12/15/Rust-1.66.0.html">Announcing Rust 1.66.0 | 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 1.66 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="对有字段枚举的显示判别"><a class="header" href="#对有字段枚举的显示判别">对有字段枚举的显示判别</a></h2>
|
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<p>枚举的显示判别在跨语言传递值时很关键，需要两个语言里每个枚举值的判别是一致的，比如：</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>#[repr(u8)]
|
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|
enum Bar {
|
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A,
|
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|
B,
|
|
|
C = 42,
|
|
|
D,
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}
|
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<span class="boring">}</span></code></pre>
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|
|
<p>这个例子里，枚举 <code>Bar</code> 使用了 <code>u8</code> 作为原语表形(representation)，并且 <code>Bar::C</code> 使用 42 来判别，其它没有显示判别的枚举值会按照源码里地顺序自动地递增赋值，这里的 <code>Bar::A</code> 是0，<code>Bar::B</code> 是1，<code>Bar::D</code> 是43。如果没有显示判别，那就只能在 <code>Bar::B</code> 和 <code>Bar::C</code> 之间加上 40 个无意义的枚举值了。</p>
|
|
|
<p>在1.66之前，枚举的显示判别只能用在无字段枚举上。现在对有字段枚举的显示判别也稳定了：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>#[repr(u8)]
|
|
|
enum Foo {
|
|
|
A(u8),
|
|
|
B(i8),
|
|
|
C(bool) = 42,
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>注意：可以通过 <code>as</code> 转换（比如 <code>Bar::C as u8</code> ）来判断一个无字段枚举的判别值，但是 Rust 还没有给有字段枚举提供语言层面上的获取原始判别值的方法，只能通过 unsafe 的代码来检查有字段枚举的判别值。考虑到这个使用场景往往出现在必须使用 unsafe 代码的跨语言的 FFI 里，希望这没有造成太大的负担。如果你的确需要的话，参考 <code>std::mem::discriminant</code>。</p>
|
|
|
<h2 id="黑盒方法-corehintblack_box"><a class="header" href="#黑盒方法-corehintblack_box">黑盒方法 <code>core::hint::black_box</code></a></h2>
|
|
|
<p>当对编译器产生的代码做基准测试时，常常需要阻止一些优化，比如下面的代码里， <code>push_cap</code> 在一个循环里执行了4次 <code>Vec::push</code> ：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn push_cap(v: &mut Vec<i32>) {
|
|
|
for i in 0..4 {
|
|
|
v.push(i);
|
|
|
}
|
|
|
}
|
|
|
|
|
|
pub fn bench_push() -> Duration {
|
|
|
let mut v = Vec::with_capacity(4);
|
|
|
let now = Instant::now();
|
|
|
push_cap(&mut v);
|
|
|
now.elapsed()
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>如果你检查一下在 x86_64 机器上编译的优化输出结果，你会注意到整个 <code>push_cap</code> 方法都被优化掉了…</p>
|
|
|
<pre><code class="language-text">example::bench_push:
|
|
|
sub rsp, 24
|
|
|
call qword ptr [rip + std::time::Instant::now@GOTPCREL]
|
|
|
lea rdi, [rsp + 8]
|
|
|
mov qword ptr [rsp + 8], rax
|
|
|
mov dword ptr [rsp + 16], edx
|
|
|
call qword ptr [rip + std::time::Instant::elapsed@GOTPCREL]
|
|
|
add rsp, 24
|
|
|
ret
|
|
|
</code></pre>
|
|
|
<p>现在可以通过调用 <code>black_box</code> 来避免类似情况的发送。 虽然实际上 <code>black_box</code> 内部只会取走值并直接返回，但是编译器会认为这个方法可能做任何事情。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use std::hint::black_box;
|
|
|
|
|
|
fn push_cap(v: &mut Vec<i32>) {
|
|
|
for i in 0..4 {
|
|
|
v.push(i);
|
|
|
black_box(v.as_ptr());
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这样就可以得到展开循环的<a href="https://rust.godbolt.org/z/Ws1GGbY6Y">结果</a>：</p>
|
|
|
<pre><code class="language-text"> mov dword ptr [rbx], 0
|
|
|
mov qword ptr [rsp + 8], rbx
|
|
|
mov dword ptr [rbx + 4], 1
|
|
|
mov qword ptr [rsp + 8], rbx
|
|
|
mov dword ptr [rbx + 8], 2
|
|
|
mov qword ptr [rsp + 8], rbx
|
|
|
mov dword ptr [rbx + 12], 3
|
|
|
mov qword ptr [rsp + 8], rbx
|
|
|
</code></pre>
|
|
|
<p>你还能发现结果里有 <code>black_box</code> 带来的副作用，无意义的 <code>mov qword ptr [rsp + 8], rbx</code> 指令在每一次循环后出现，用来获取 <code>v.as_ptr()</code> 作为参数传递给并未真正使用的方法。</p>
|
|
|
<p>注意到上面的例子里，<code>push</code> 指令都不用考虑内存分配的问题，这是因为编译器运行在 <code>Vec::with_capacity(4)</code> 的条件下。你可以尝试改动一下 <code>black_box</code> 的位置或者在多处使用，来看看其对编译的优化输出的影响。</p>
|
|
|
<h2 id="cargo-remove"><a class="header" href="#cargo-remove"><code>cargo remove</code></a></h2>
|
|
|
<p>1.62里我们引入了 <code>cargo add</code> 来通过命令行给你的项目增加依赖项。现在可以使用 <code>cargo remove</code> 来移除依赖了。</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/2022/12/15/Rust-1.66.0.html#stabilized-apis">原Blog</a></p>
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<template id=fa-copy><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M502.6 70.63l-61.25-61.25C435.4 3.371 427.2 0 418.7 0H255.1c-35.35 0-64 28.66-64 64l.0195 256C192 355.4 220.7 384 256 384h192c35.2 0 64-28.8 64-64V93.25C512 84.77 508.6 76.63 502.6 70.63zM464 320c0 8.836-7.164 16-16 16H255.1c-8.838 0-16-7.164-16-16L239.1 64.13c0-8.836 7.164-16 16-16h128L384 96c0 17.67 14.33 32 32 32h47.1V320zM272 448c0 8.836-7.164 16-16 16H63.1c-8.838 0-16-7.164-16-16L47.98 192.1c0-8.836 7.164-16 16-16H160V128H63.99c-35.35 0-64 28.65-64 64l.0098 256C.002 483.3 28.66 512 64 512h192c35.2 0 64-28.8 64-64v-32h-47.1L272 448z"/></svg></span></template>
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