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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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<h2 id="可反驳性模式是否可能匹配失败"><a class="header" href="#可反驳性模式是否可能匹配失败">可反驳性：模式是否可能匹配失败</a></h2>
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<p><a href="https://github.com/rust-lang/book/blob/f78ab89d7545ac17780e6a367055cc089f4cd2ec/src/ch19-02-refutability.md">ch19-02-refutability.md</a></p>
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<p>模式分为两种形式：可反驳（<em>refutable</em>）和不可反驳（<em>irrefutable</em>）。对传入的任意可能值都能匹配的模式，称为<strong>不可反驳</strong>模式。比如语句 <code>let x = 5;</code> 中的 <code>x</code> 就是一个例子，因为 <code>x</code> 可以匹配任何值，因此不可能匹配失败。对某些可能值会匹配失败的模式，称为<strong>可反驳</strong>模式。比如表达式 <code>if let Some(x) = a_value</code> 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>，那么模式 <code>Some(x)</code> 就不会匹配。</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模式，因为当值不匹配时，程序无法做出有意义的事情。<code>if let</code> 和 <code>while let</code> 表达式，以及 <code>let...else</code> 语句既接受可反驳模式，也接受不可反驳模式；不过编译器会对其中的不可反驳模式发出警告，因为根据定义，这些结构本来就是为处理可能失败的情况而设计的：条件判断的意义就在于它可以根据成功或失败执行不同的逻辑。</p>
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<p>一般来说，你不必时刻担心可反驳模式和不可反驳模式的区别；不过你确实需要熟悉“可反驳性”这个概念，这样当你在错误信息里看到它时，就知道该如何应对。遇到这类情况时，你需要根据代码想要表达的行为，修改模式本身，或者修改与之搭配使用的语法结构。</p>
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<p>让我们来看一个例子：当我们试图在 Rust 要求使用不可反驳模式的地方使用可反驳模式，以及反过来时，会发生什么。示例 19-8 展示了一个 <code>let</code> 语句，不过我们给它写了模式 <code>Some(x)</code>，这是一个可反驳模式。正如你可能已经猜到的，这段代码不会通过编译。</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 19-8: 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>，它就无法匹配模式 <code>Some(x)</code>，这说明该模式是可反驳的。然而，<code>let</code> 语句只能接受不可反驳模式，因为当值是 <code>None</code> 时，这段代码并没有什么合法的后续操作可做。在编译期，Rust 会抱怨我们试图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/ch19-02-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>因为我们没有覆盖模式 <code>Some(x)</code> 所对应的所有合法值（而且也不可能覆盖全部），所以 Rust 理所当然地给出了编译错误。</p>
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<p>如果在某个需要不可反驳模式的地方却有一个可反驳模式，我们可以通过修改使用该模式的代码来修复这个问题：不用 <code>let</code>，而改用 <code>let...else</code>。这样一来，如果模式不匹配，大括号中的代码就会处理该值。示例 19-9 展示了如何修复示例 19-8 中的代码。</p>
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<pre class="playground"><code class="language-rust edition2024"><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 else {
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return;
|
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};
|
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<span class="boring">}</span></code></pre>
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<p><span class="caption">示例 19-9: 用 <code>let...else</code> 和一个带可反驳模式的代码块来代替 <code>let</code></span></p>
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<p>我们给这段代码留出了一条退路！现在这段代码是完全合法的。不过这也意味着，我们不能在这里使用不可反驳模式而不收到警告。如果我们给 <code>let...else</code> 一个总能匹配的模式，比如示例 19-10 中的 <code>x</code>，编译器就会发出警告。</p>
|
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
|
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</span> let x = 5 else {
|
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return;
|
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};
|
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<span class="boring">}</span></code></pre>
|
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<p><span class="caption">示例 19-10: 尝试在 <code>let...else</code> 中使用不可反驳模式</span></p>
|
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<p>Rust 会抱怨说，在 <code>let...else</code> 中使用不可反驳模式没有意义：</p>
|
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|
<pre><code class="language-console">$ cargo run
|
|
|
Compiling patterns v0.1.0 (file:///projects/patterns)
|
|
|
warning: irrefutable `let...else` pattern
|
|
|
--> src/main.rs:2:5
|
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2 | let x = 5 else {
|
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| ^^^^^^^^^
|
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|
|
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= note: this pattern will always match, so the `else` clause is useless
|
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= help: consider removing the `else` clause
|
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|
= note: `#[warn(irrefutable_let_patterns)]` on by default
|
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|
warning: `patterns` (bin "patterns") generated 1 warning
|
|
|
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s
|
|
|
Running `target/debug/patterns`
|
|
|
</code></pre>
|
|
|
<p>基于这个原因，<code>match</code> 分支必须使用可反驳模式，只有最后一个分支例外，它应该用一个不可反驳模式来匹配所有剩余值。Rust 允许我们在一个只有单个分支的 <code>match</code> 中使用不可反驳模式，不过这种写法并没有太大用处，而且完全可以被更简单的 <code>let</code> 语句替代。</p>
|
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<p>既然现在你已经知道模式该用在什么地方，以及可反驳模式和不可反驳模式之间的区别，接下来我们就来看看所有可以用来创建模式的语法。</p>
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