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<title>Refutability（可反驳性）: 模式是否会匹配失效 - Rust 程序设计语言 简体中文版</title>
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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 href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> 定义模块来控制作用域与私有性</a></li><li class="chapter-item expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> 引用模块项目的路径</a></li><li class="chapter-item expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> 使用 use 关键字将路径引入作用域</a></li><li class="chapter-item expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> 将模块拆分成多个文件</a></li></ol></li><li class="chapter-item expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> 常见集合</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> 使用 Vector 储存列表</a></li><li class="chapter-item expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> 使用字符串储存 UTF-8 编码的文本</a></li><li class="chapter-item expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> 使用 Hash Map 储存键值对</a></li></ol></li><li class="chapter-item expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> 错误处理</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> 用 panic! 处理不可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> 用 Result 处理可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> 要不要 panic!</a></li></ol></li><li class="chapter-item expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> 泛型、Trait 和生命周期</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> 泛型数据类型</a></li><li class="chapter-item expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Trait：定义共同行为</a></li><li class="chapter-item expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> 生命周期确保引用有效</a></li></ol></li><li class="chapter-item expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> 编写自动化测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> 如何编写测试</a></li><li class="chapter-item expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> 控制测试如何运行</a></li><li class="chapter-item expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> 测试的组织结构</a></li></ol></li><li class="chapter-item expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> 一个 I/O 项目：构建命令行程序</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> 接受命令行参数</a></li><li class="chapter-item expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> 读取文件</a></li><li class="chapter-item expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> 重构以改进模块化与错误处理</a></li><li class="chapter-item expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> 采用测试驱动开发完善库的功能</a></li><li class="chapter-item expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> 处理环境变量</a></li><li class="chapter-item expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> 将错误信息输出到标准错误而不是标准输出</a></li></ol></li><li class="chapter-item expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Rust 中的函数式语言功能：迭代器与闭包</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> 闭包：可以捕获其环境的匿名函数</a></li><li class="chapter-item expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> 使用迭代器处理元素序列</a></li><li class="chapter-item expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> 改进之前的 I/O 项目</a></li><li class="chapter-item expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> 性能比较：循环对迭代器</a></li></ol></li><li class="chapter-item expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> 更多关于 Cargo 和 Crates.io 的内容</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> 采用发布配置自定义构建</a></li><li class="chapter-item expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> 将 crate 发布到 Crates.io</a></li><li class="chapter-item expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo 工作空间</a></li><li class="chapter-item expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> 使用 cargo install 安装二进制文件</a></li><li class="chapter-item expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Cargo 自定义扩展命令</a></li></ol></li><li class="chapter-item expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> 智能指针</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> 使用 Box<T> 指向堆上数据</a></li><li class="chapter-item expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> 使用 Deref Trait 将智能指针当作常规引用处理</a></li><li class="chapter-item expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> 使用 Drop Trait 运行清理代码</a></li><li class="chapter-item expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T> 引用计数智能指针</a></li><li class="chapter-item expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> 与内部可变性模式</a></li><li class="chapter-item expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> 引用循环会导致内存泄漏</a></li></ol></li><li class="chapter-item expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> 无畏并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> 使用线程同时地运行代码</a></li><li class="chapter-item expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> 使用消息传递在线程间通信</a></li><li class="chapter-item expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> 共享状态并发</a></li><li class="chapter-item expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> 使用 Sync 与 Send Traits 的可扩展并发</a></li></ol></li><li class="chapter-item expanded "><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Rust 的面向对象编程特性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> 面向对象语言的特点</a></li><li class="chapter-item expanded "><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> 顾及不同类型值的 trait 对象</a></li><li class="chapter-item expanded "><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> 面向对象设计模式的实现</a></li></ol></li><li class="chapter-item expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> 模式与模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> 所有可能会用到模式的位置</a></li><li class="chapter-item expanded "><a href="ch18-02-refutability.html" class="active"><strong aria-hidden="true">18.2.</strong> Refutability（可反驳性）: 模式是否会匹配失效</a></li><li class="chapter-item expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> 模式语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> 高级特征</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> 不安全的 Rust</a></li><li class="chapter-item expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> 高级 trait</a></li><li class="chapter-item expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> 高级类型</a></li><li class="chapter-item expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> 高级函数与闭包</a></li><li class="chapter-item expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> 宏</a></li></ol></li><li class="chapter-item expanded "><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> 最后的项目：构建多线程 web server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> 建立单线程 web server</a></li><li class="chapter-item expanded "><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> 将单线程 server 变为多线程 server</a></li><li class="chapter-item expanded "><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> 优雅停机与清理</a></li></ol></li><li class="chapter-item expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - 关键字</a></li><li class="chapter-item expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - 运算符与符号</a></li><li class="chapter-item expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - 可派生的 trait</a></li><li class="chapter-item expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - 实用开发工具</a></li><li class="chapter-item expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - 版本</a></li><li class="chapter-item expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - 本书译本</a></li><li class="chapter-item expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - Rust 是如何开发的与 “Nightly Rust”</a></li></ol></li></ol>
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<h2 id="refutability可反驳性-模式是否会匹配失效"><a class="header" href="#refutability可反驳性-模式是否会匹配失效">Refutability（可反驳性）: 模式是否会匹配失效</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch18-02-refutability.md">ch18-02-refutability.md</a>
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<br>
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commit 0c2d41e2afce734825c3a12087d423e8c2f0ae53</p>
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</blockquote>
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<p>模式有两种形式：refutable（可反驳的）和 irrefutable（不可反驳的）。能匹配任何传递的可能值的模式被称为是 <strong>不可反驳的</strong>（<em>irrefutable</em>）。一个例子就是 <code>let x = 5;</code> 语句中的 <code>x</code>，因为 <code>x</code> 可以匹配任何值所以不可能会失败。对某些可能的值进行匹配会失败的模式被称为是 <strong>可反驳的</strong>（<em>refutable</em>）。一个这样的例子便是 <code>if let Some(x) = a_value</code> 表达式中的 <code>Some(x)</code>；如果变量 <code>a_value</code> 中的值是 <code>None</code> 而不是 <code>Some</code>，那么 <code>Some(x)</code> 模式不能匹配。</p>
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<p>函数参数、<code>let</code> 语句和 <code>for</code> 循环只能接受不可反驳的模式，因为当值不匹配时，程序无法进行有意义的操作。<code>if let</code> 和 <code>while let</code> 表达式可以接受可反驳和不可反驳的模式，但编译器会对不可反驳的模式发出警告，因为根据定义它们旨在处理可能的失败：条件表达式的功能在于它能够根据成功或失败来执行不同的操作。</p>
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<p>通常我们无需担心可反驳和不可反驳模式的区别，不过确实需要熟悉可反驳性的概念，这样当在错误信息中看到时就知道如何应对。遇到这些情况，根据代码行为的意图，需要修改模式或者使用模式的结构。</p>
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<p>让我们看看一个尝试在 Rust 要求不可反驳模式的地方使用可反驳模式以及相反情况的例子。在示例 18-8 中，有一个 <code>let</code> 语句，不过模式被指定为可反驳模式 <code>Some(x)</code>。如你所见，这不能编译：</p>
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<pre><code class="language-rust ignore does_not_compile"><span class="boring">fn main() {
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</span><span class="boring"> let some_option_value: Option<i32> = None;
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</span> let Some(x) = some_option_value;
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<span class="boring">}</span></code></pre>
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<p><span class="caption">示例 18-8: 尝试在 <code>let</code> 中使用可反驳模式</span></p>
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<p>如果 <code>some_option_value</code> 的值是 <code>None</code>，其不会成功匹配模式 <code>Some(x)</code>，表明这个模式是可反驳的。然而，因为 <code>let</code> 对于 <code>None</code> 匹配不能产生任何合法的代码，所以 <code>let</code> 语句只能接受不可反驳模式。Rust 会在编译时抱怨我们尝试在要求不可反驳模式的地方使用可反驳模式：</p>
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<pre><code class="language-console">$ cargo run
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Compiling patterns v0.1.0 (file:///projects/patterns)
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error[E0005]: refutable pattern in local binding
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--> src/main.rs:3:9
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3 | let Some(x) = some_option_value;
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| ^^^^^^^ pattern `None` not covered
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= note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
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= note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
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= note: the matched value is of type `Option<i32>`
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help: you might want to use `let else` to handle the variant that isn't matched
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3 | let Some(x) = some_option_value else { todo!() };
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| ++++++++++++++++
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For more information about this error, try `rustc --explain E0005`.
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error: could not compile `patterns` (bin "patterns") due to 1 previous error
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</code></pre>
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<p>因为我们没有覆盖（也不可能覆盖！）到模式 <code>Some(x)</code> 的每一个可能的值，所以 Rust 会合理地抗议。</p>
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<p>为了修复在需要不可反驳模式的地方使用可反驳模式的情况，可以修改使用模式的代码：不同于使用 <code>let</code>，可以使用 <code>if let</code>。如此，如果模式不匹配，大括号中的代码将被忽略，其余代码保持有效。示例 18-9 展示了如何修复示例 18-8 中的代码。</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
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</span><span class="boring"> let some_option_value: Option<i32> = None;
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</span> if let Some(x) = some_option_value {
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println!("{x}");
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}
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<span class="boring">}</span></code></pre></pre>
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<p><span class="caption">示例 18-9: 使用 <code>if let</code> 和一个带有可反驳模式的代码块来代替 <code>let</code></span></p>
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<p>我们给了代码一个得以继续的出路！虽然我们没办法在避免产生错误的情况下使用不可反驳模式，但这段使用可反驳模式的代码是完全有效的。如果为 <code>if let</code> 提供了一个总是会匹配的模式，比如示例 18-10 中的 <code>x</code>，编译器会给出一个警告：</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
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</span> if let x = 5 {
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println!("{x}");
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};
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<span class="boring">}</span></code></pre></pre>
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<p><span class="caption">示例 18-10: 尝试把不可反驳模式用到 <code>if let</code> 上</span></p>
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<p>Rust 会抱怨将不可反驳模式用于 <code>if let</code> 是没有意义的：</p>
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<pre><code class="language-console">$ cargo run
|
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Compiling patterns v0.1.0 (file:///projects/patterns)
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warning: irrefutable `if let` pattern
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--> src/main.rs:2:8
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2 | if let x = 5 {
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| ^^^^^^^^^
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= note: this pattern will always match, so the `if let` is useless
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= help: consider replacing the `if let` with a `let`
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= note: `#[warn(irrefutable_let_patterns)]` on by default
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warning: `patterns` (bin "patterns") generated 1 warning
|
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Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s
|
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Running `target/debug/patterns`
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5
|
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</code></pre>
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<p>基于此，<code>match</code>匹配分支必须使用可反驳模式，除了最后一个分支需要使用能匹配任何剩余值的不可反驳模式。Rust 允许我们在只有一个匹配分支的<code>match</code>中使用不可反驳模式，不过这么做不是特别有用，并可以被更简单的 <code>let</code> 语句替代。</p>
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<p>目前我们已经讨论了所有可以使用模式的地方，以及可反驳模式与不可反驳模式的区别，下面让我们一起去把可以用来创建模式的语法过目一遍吧。</p>
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