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<h1 id="rust-新版解读--161--重点-自定义-main-函数-exitcodeconst-fn-增强为锁定的-stdio-提供静态句柄"><a class="header" href="#rust-新版解读--161--重点-自定义-main-函数-exitcodeconst-fn-增强为锁定的-stdio-提供静态句柄">Rust 新版解读 | 1.61 | 重点: 自定义 main 函数 ExitCode、const fn 增强、为锁定的 stdio 提供静态句柄</a></h1>
<blockquote>
<p>原文链接: https://blog.rust-lang.org/2022/05/19/Rust-1.61.0.html
翻译 by ：<a href="https://github.com/AllanDowney">AllanDowney</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 1.61 版本:</p>
<pre><code class="language-shell">$ rustup update stable
</code></pre>
<h2 id="支持自定义-main-函数-exitcode"><a class="header" href="#支持自定义-main-函数-exitcode">支持自定义 main 函数 ExitCode</a></h2>
<p>一开始， Rust <code>main</code> 函数只能返回单元类型 <code>()</code>（隐式或显式），总是指示成功的退出状态，如果您要你想要其它的，必须调用 <code>process::exit(code)</code>。从 Rust 1.26 开始， <code>main</code> 允许返回一个 <code>Result</code> ，其中 <code>Ok</code> 转换为 <code>C EXIT_SUCCESS</code>，<code>Err</code> 转换为 <code>EXIT_FAILURE</code>（也调试打印错误）。在底层，这些返回类型统一使用不稳定的 <code>Termination</code> 特征。</p>
<p>在此版本中，最终稳定了 <code>Termination</code> 特征，以及一个更通用的 <code>ExitCode</code> 类型，它封装了特定于平台的返回类型。它具有 <code>SUCCESS</code> 和 <code>FAILURE</code> 常量，并为更多任意值实现 <code>From&lt;u8&gt;</code>。也可以为您自己的类型实现 <code>Termination</code> 特征，允许您在转换为 <code>ExitCode</code> 之前定制任何类型的报告。</p>
<p>例如，下面是一种类型安全的方式来编写 <code>git bisect</code> 运行脚本的退出代码：</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::process::{ExitCode, Termination};
#[repr(u8)]
pub enum GitBisectResult {
Good = 0,
Bad = 1,
Skip = 125,
Abort = 255,
}
impl Termination for GitBisectResult {
fn report(self) -&gt; ExitCode {
// Maybe print a message here
ExitCode::from(self as u8)
}
}
fn main() -&gt; GitBisectResult {
std::panic::catch_unwind(|| {
todo!(&quot;test the commit&quot;)
}).unwrap_or(GitBisectResult::Abort)
}</code></pre></pre>
<h2 id="const-fn-增强"><a class="header" href="#const-fn-增强">const fn 增强</a></h2>
<p>这个版本稳定了几个增量特性，以支持 const 函数的更多功能：</p>
<ul>
<li><code>fn</code> 指针的基本处理：现在可以在 <code>const fn</code> 中创建、传递和强制转换函数指针。例如，在为解释器构建编译时函数表时，这可能很有用。但是，仍然不允许调用 <code>fn</code> 指针。</li>
<li>特征约束：现在可以将特征约束写在 <code>const fn</code> 的泛型参数上，如 <code>T: Copy</code>，以前只允许 <code>Sized</code>。</li>
<li><code>dyn Trait</code> 类型：类似地，<code>const fn</code> 现在可以处理特征对象 <code>dyn Trait</code>。</li>
<li><code>impl Trait</code> 类型：<code>const fn</code> 的参数和返回值现在可以是不透明的 <code>impl Trait</code> 类型。</li>
</ul>
<p>注意，特征特性还不支持在 <code>const fn</code> 中调用这些特征的方法。</p>
<h2 id="为锁定的-stdio-提供静态句柄"><a class="header" href="#为锁定的-stdio-提供静态句柄">为锁定的 stdio 提供静态句柄</a></h2>
<p>三种标准 I/O 流 —— <code>Stdin</code> 、<code>Stdout</code> 和 <code>Stderr</code> —— 都有一个 <code>锁(&amp;self)</code>，允许对同步读写进行更多控制。但是，它们返回的锁守卫具有从 <code>&amp;self</code> 借来的生命周期，因此它们被限制在原始句柄的范围内。这被认为是一个不必要的限制，因为底层锁实际上是在静态存储中，所以现在守卫返回一个 <code>'static</code> 生命期，与句柄断开连接。</p>
<p>例如，一个常见的错误来自于试图获取一个句柄并将其锁定在一个语句中:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// error[E0716]: temporary value dropped while borrowed
let out = std::io::stdout().lock();
// ^^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
// |
// creates a temporary which is freed while still in use
<span class="boring">}</span></code></pre></pre>
<p>现在锁守卫是 <code>'static</code>，而不是借用那个临时的，所以这个可以正常工作！</p>
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