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<main>
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<h1 id="rust-新版解读--158--重点-格式化字符串捕获环境中的值"><a class="header" href="#rust-新版解读--158--重点-格式化字符串捕获环境中的值">Rust 新版解读 | 1.58 | 重点: 格式化字符串捕获环境中的值</a></h1>
|
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<p>众所周知，Rust 小版本发布非常频繁，6 周就发布一次，因此通常不会有特别值得普通用户关注的内容，但是这次 1.58 版本不同，新增了(stable 化了)一个非常好用的功能： <strong>在格式化字符串时捕获环境中的值</strong>。</p>
|
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<blockquote>
|
|
|
<p>Rust 1.58 官方 release doc: <a href="https://blog.rust-lang.org/2022/01/13/Rust-1.58.0.html">Announcing Rust 1.58.0 | Rust Blog</a></p>
|
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</blockquote>
|
|
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<h2 id="在格式化字符串时捕获环境中的值"><a class="header" href="#在格式化字符串时捕获环境中的值">在格式化字符串时捕获环境中的值</a></h2>
|
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<p>在以前，想要输出一个函数的返回值，你需要这么做：</p>
|
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<pre><pre class="playground"><code class="language-rust edition2021">fn get_person() -> String {
|
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String::from("sunface")
|
|
|
}
|
|
|
fn main() {
|
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|
let p = get_person();
|
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|
println!("Hello, {}!", p); // implicit position
|
|
|
println!("Hello, {0}!", p); // explicit index
|
|
|
println!("Hello, {person}!", person = p);
|
|
|
}</code></pre></pre>
|
|
|
<p>问题倒也不大，但是一旦格式化字符串长了后，就会非常冗余，而在 1.58 后，我们可以这么写：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn get_person() -> String {
|
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|
String::from("sunface")
|
|
|
}
|
|
|
fn main() {
|
|
|
let person = get_person();
|
|
|
println!("Hello, {person}!");
|
|
|
}</code></pre></pre>
|
|
|
<p>是不是清晰、简洁了很多？甚至还可以将环境中的值用于格式化参数:</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let (width, precision) = get_format();
|
|
|
for (name, score) in get_scores() {
|
|
|
println!("{name}: {score:width$.precision$}");
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<p>但也有局限，它只能捕获普通的变量，对于更复杂的类型（例如表达式），可以先将它赋值给一个变量或使用以前的 <code>name = expression</code> 形式的格式化参数。</p>
|
|
|
<p>目前除了 <code>panic!</code> 外，其它接收格式化参数的宏，都可以使用新的特性。对于 <code>panic!</code> 而言，如果还在使用 <code>2015版本</code> 或 <code>2018版本</code> 版本 ，那 <code>panic!("{ident}")</code> 依然会被当成 正常的字符串来处理，同时编译器会给予 <code>warn</code> 提示。而对于 <code>2021版本</code> ，则可以正常使用:</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn get_person() -> String {
|
|
|
String::from("sunface")
|
|
|
}
|
|
|
fn main() {
|
|
|
let person = get_person();
|
|
|
panic!("Hello, {person}!");
|
|
|
}</code></pre></pre>
|
|
|
<p>输出:</p>
|
|
|
<pre><code class="language-console">thread 'main' panicked at 'Hello, sunface!', src/main.rs:6:5
|
|
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|
|
</code></pre>
|
|
|
<h2 id="比-unwrap-更危险的-unwrap_unchecked"><a class="header" href="#比-unwrap-更危险的-unwrap_unchecked">比 unwrap 更危险的 unwrap_unchecked</a></h2>
|
|
|
<p>在 1.58 中为 <code>Option</code> 和 <code>Result</code> 新增了 <code>unwrap_unchecked</code> 方法，与 <code>unwrap</code> 遇到错误或者空值直接 <code>panic</code> 不同，<code>unwrap_unchecked</code> 遇到错误时处理方式糟糕的多：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn get_num() -> Option<i32> {
|
|
|
None
|
|
|
}
|
|
|
fn main() {
|
|
|
unsafe {
|
|
|
let n = get_num().unwrap_unchecked();
|
|
|
}
|
|
|
}</code></pre></pre>
|
|
|
<p>输出如下：</p>
|
|
|
<pre><code class="language-console">zsh: segmentation fault cargo run
|
|
|
</code></pre>
|
|
|
<p>嗯，段错误了，对比下 <code>panic</code>，有一种泪流满面的冲动：我要这不安全的方法何用？</p>
|
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<p>其实，还真有些用：</p>
|
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<ul>
|
|
|
<li>想要较小的可执行文件时（嵌入式，WASM 等），该方法就可以大显身手。因为 <code>panic</code> 会导致<a href="https://zhuanlan.zhihu.com/p/445465092">二进制可执行文件变大不少</a></li>
|
|
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<li>它还可以提高一点性能， 因为编译器可能无法优化掉 <code>unwrap</code> 的指令分支， 虽然它只会增加区区几条分支预测指令</li>
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