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<ol class="chapter"><li class="chapter-item expanded affix "><a href="title-page.html">Rust 程序设计语言</a></li><li class="chapter-item expanded affix "><a href="foreword.html">前言</a></li><li class="chapter-item expanded affix "><a href="ch00-00-introduction.html">简介</a></li><li class="chapter-item expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> 入门指南</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> 安装</a></li><li class="chapter-item expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="chapter-item expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> 写个猜数字游戏</a></li><li class="chapter-item expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> 常见编程概念</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> 变量与可变性</a></li><li class="chapter-item expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> 数据类型</a></li><li class="chapter-item expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> 函数</a></li><li class="chapter-item expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> 注释</a></li><li class="chapter-item expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> 控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> 认识所有权</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> 什么是所有权？</a></li><li class="chapter-item expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> 引用与借用</a></li><li class="chapter-item expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slice 类型</a></li></ol></li><li class="chapter-item expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> 使用结构体组织相关联的数据</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> 结构体的定义和实例化</a></li><li class="chapter-item expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> 结构体示例程序</a></li><li class="chapter-item expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> 方法语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> 枚举和模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> 枚举的定义</a></li><li class="chapter-item expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> match 控制流结构</a></li><li class="chapter-item expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> if let 简洁控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> 使用包、Crate 和模块管理不断增长的项目</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> 包和 Crate</a></li><li class="chapter-item expanded "><a h
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<h2 id="用-panic-处理不可恢复的错误"><a class="header" href="#用-panic-处理不可恢复的错误">用 <code>panic!</code> 处理不可恢复的错误</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch09-01-unrecoverable-errors-with-panic.md">ch09-01-unrecoverable-errors-with-panic.md</a>
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<br>
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commit 2921743516b3e2c0f45a95390e7b536e42f4af7c</p>
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</blockquote>
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<p>突然有一天，代码出问题了，而你对此束手无策。对于这种情况，Rust 有 <code>panic!</code>宏。在实践中有两种方法造成 panic：执行会造成代码 panic 的操作（比如访问超过数组结尾的内容）或者显式调用 <code>panic!</code> 宏。这两种情况都会使程序 panic。通常情况下这些 panic 会打印出一个错误信息，展开并清理栈数据，然后退出。通过一个环境变量，你也可以让 Rust 在 panic 发生时打印调用堆栈（call stack）以便于定位 panic 的原因。</p>
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<blockquote>
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<h3 id="对应-panic-时的栈展开或终止"><a class="header" href="#对应-panic-时的栈展开或终止">对应 panic 时的栈展开或终止</a></h3>
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<p>当出现 panic 时，程序默认会开始 <strong>展开</strong>（<em>unwinding</em>），这意味着 Rust 会回溯栈并清理它遇到的每一个函数的数据，不过这个回溯并清理的过程有很多工作。另一种选择是直接 <strong>终止</strong>（<em>abort</em>），这会不清理数据就退出程序。</p>
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<p>那么程序所使用的内存需要由操作系统来清理。如果你需要项目的最终二进制文件越小越好，panic 时通过在 <em>Cargo.toml</em> 的 <code>[profile]</code> 部分增加 <code>panic = 'abort'</code>，可以由展开切换为终止。例如，如果你想要在 release 模式中 panic 时直接终止：</p>
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<pre><code class="language-toml">[profile.release]
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panic = 'abort'
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</code></pre>
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</blockquote>
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<p>让我们在一个简单的程序中调用 <code>panic!</code>：</p>
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<p><span class="filename">文件名：src/main.rs</span></p>
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<pre><pre class="playground"><code class="language-rust should_panic panics edition2021">fn main() {
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panic!("crash and burn");
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}</code></pre></pre>
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<p>运行程序将会出现类似这样的输出：</p>
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<pre><code class="language-console">$ cargo run
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Compiling panic v0.1.0 (file:///projects/panic)
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Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.25s
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Running `target/debug/panic`
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thread 'main' panicked at src/main.rs:2:5:
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crash and burn
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note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
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</code></pre>
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<p>最后两行包含 <code>panic!</code> 调用造成的错误信息。第一行显示了 panic 提供的信息并指明了源码中 panic 出现的位置：<em>src/main.rs:2:5</em> 表明这是 <em>src/main.rs</em> 文件的第二行第五个字符。</p>
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<p>在这个例子中，被指明的那一行是我们代码的一部分，而且查看这一行的话就会发现 <code>panic!</code> 宏的调用。在其他情况下，<code>panic!</code> 可能会出现在我们的代码所调用的代码中。错误信息报告的文件名和行号可能指向别人代码中的 <code>panic!</code> 宏调用，而不是我们代码中最终导致 <code>panic!</code> 的那一行。我们可以使用 <code>panic!</code> 被调用的函数的 backtrace 来寻找代码中出问题的地方。下面我们会详细介绍 backtrace 是什么。</p>
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<h3 id="使用-panic-的-backtrace"><a class="header" href="#使用-panic-的-backtrace">使用 <code>panic!</code> 的 backtrace</a></h3>
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<p>让我们来看看另一个因为我们代码中的 bug 引起的别的库中 <code>panic!</code> 的例子，而不是直接的宏调用。示例 9-1 有一些尝试通过索引访问 vector 中元素的例子：</p>
|
|||
|
|
<p><span class="filename">文件名：src/main.rs</span></p>
|
|||
|
|
<pre><pre class="playground"><code class="language-rust should_panic panics edition2021">fn main() {
|
|||
|
|
let v = vec![1, 2, 3];
|
|||
|
|
|
|||
|
|
v[99];
|
|||
|
|
}</code></pre></pre>
|
|||
|
|
<p><span class="caption">示例 9-1：尝试访问超越 vector 结尾的元素，这会造成 <code>panic!</code></span></p>
|
|||
|
|
<p>这里尝试访问 vector 的第一百个元素（这里的索引是 99 因为索引从 0 开始），不过它只有三个元素。这种情况下 Rust 会 panic。<code>[]</code> 应当返回一个元素，不过如果传递了一个无效索引，就没有可供 Rust 返回的正确的元素。</p>
|
|||
|
|
<p>C 语言中，尝试读取数据结构之后的值是未定义行为（undefined behavior）。你会得到任何对应数据结构中这个元素的内存位置的值，甚至是这些内存并不属于这个数据结构的情况。这被称为 <strong>缓冲区溢出</strong>（<em>buffer overread</em>），并可能会导致安全漏洞，比如攻击者可以像这样操作索引来读取储存在数据结构之后不被允许的数据。</p>
|
|||
|
|
<p>为了保护程序远离这类漏洞，如果尝试读取一个索引不存在的元素，Rust 会停止执行并拒绝继续。尝试运行上面的程序会出现如下：</p>
|
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|
<pre><code class="language-console">$ cargo run
|
|||
|
|
Compiling panic v0.1.0 (file:///projects/panic)
|
|||
|
|
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.27s
|
|||
|
|
Running `target/debug/panic`
|
|||
|
|
thread 'main' panicked at src/main.rs:4:6:
|
|||
|
|
index out of bounds: the len is 3 but the index is 99
|
|||
|
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|||
|
|
</code></pre>
|
|||
|
|
<p>错误指向 <code>main.rs</code> 的第 4 行，这里我们尝试访问索引 99。下面的说明（note）行提醒我们可以设置 <code>RUST_BACKTRACE</code> 环境变量来得到一个 backtrace。<em>backtrace</em> 是一个执行到目前位置所有被调用的函数的列表。Rust 的 backtrace 跟其他语言中的一样：阅读 backtrace 的关键是从头开始读直到发现你编写的文件。这就是问题的发源地。这一行往上是你的代码所调用的代码；往下则是调用你的代码的代码。这些行可能包含核心 Rust 代码，标准库代码或用到的 crate 代码。让我们将 <code>RUST_BACKTRACE</code> 环境变量设置为任何不是 0 的值来获取 backtrace 看看。示例 9-2 展示了与你看到类似的输出：</p>
|
|||
|
|
<pre><code class="language-console">$ RUST_BACKTRACE=1 cargo run
|
|||
|
|
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', src/main.rs:4:5
|
|||
|
|
stack backtrace:
|
|||
|
|
0: rust_begin_unwind
|
|||
|
|
at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/std/src/panicking.rs:584:5
|
|||
|
|
1: core::panicking::panic_fmt
|
|||
|
|
at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/panicking.rs:142:14
|
|||
|
|
2: core::panicking::panic_bounds_check
|
|||
|
|
at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/panicking.rs:84:5
|
|||
|
|
3: <usize as core::slice::index::SliceIndex<[T]>>::index
|
|||
|
|
at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/slice/index.rs:242:10
|
|||
|
|
4: core::slice::index::<impl core::ops::index::Index<I> for [T]>::index
|
|||
|
|
at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/slice/index.rs:18:9
|
|||
|
|
5: <alloc::vec::Vec<T,A> as core::ops::index::Index<I>>::index
|
|||
|
|
at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/alloc/src/vec/mod.rs:2591:9
|
|||
|
|
6: panic::main
|
|||
|
|
at ./src/main.rs:4:5
|
|||
|
|
7: core::ops::function::FnOnce::call_once
|
|||
|
|
at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/ops/function.rs:248:5
|
|||
|
|
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
|
|||
|
|
</code></pre>
|
|||
|
|
<p><span class="caption">示例 9-2：当设置 <code>RUST_BACKTRACE</code> 环境变量时 <code>panic!</code> 调用所生成的 backtrace 信息</span></p>
|
|||
|
|
<p>这里有大量的输出！你实际看到的输出可能因不同的操作系统和 Rust 版本而有所不同。为了获取带有这些信息的 backtrace，必须启用 debug 标识。当不使用 <code>--release</code> 参数运行 cargo build 或 cargo run 时 debug 标识会默认启用，就像这里一样。</p>
|
|||
|
|
<p>示例 9-2 的输出中，backtrace 的 12 行指向了我们项目中造成问题的行：<em>src/main.rs</em> 的第 4 行。如果你不希望程序 panic，第一个提到我们编写的代码行的位置是你应该开始调查的，以便查明是什么值如何在这个地方引起了 panic。在示例 9-1 中，我们故意编写会 panic 的代码来演示如何使用 backtrace，修复这个 panic 的方法就是不要尝试在一个只包含三个项的 vector 中请求索引是 100 的元素。当将来你的代码出现了 panic，你需要搞清楚在这特定的场景下代码中执行了什么操作和什么值导致了 panic，以及应当如何处理才能避免这个问题。</p>
|
|||
|
|
<p>本章后面的小节 <a href="ch09-03-to-panic-or-not-to-panic.html#%E8%A6%81%E4%B8%8D%E8%A6%81-panic">“要不要 panic!”</a> 会再次回到 <code>panic!</code> 并讲解何时应该、何时不应该使用 <code>panic!</code> 来处理错误情况。接下来，我们来看看如何使用 <code>Result</code> 来从错误中恢复。</p>
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