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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>
<blockquote>
<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>
</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 1.66 版本:</p>
<pre><code class="language-shell">$ rustup update stable
</code></pre>
<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>
<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>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[repr(u8)]
enum Bar {
A,
B,
C = 42,
D,
}
<span class="boring">}</span></code></pre></pre>
<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><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></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><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn push_cap(v: &amp;mut Vec&lt;i32&gt;) {
for i in 0..4 {
v.push(i);
}
}
pub fn bench_push() -&gt; Duration {
let mut v = Vec::with_capacity(4);
let now = Instant::now();
push_cap(&amp;mut v);
now.elapsed()
}
<span class="boring">}</span></code></pre></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><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: &amp;mut Vec&lt;i32&gt;) {
for i in 0..4 {
v.push(i);
black_box(v.as_ptr());
}
}
<span class="boring">}</span></code></pre></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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