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<ol class="chapter"><li class="chapter-item affix "><a href="../../about-book.html">关于本书</a></li><li class="chapter-item affix "><a href="../../into-rust.html">进入 Rust 编程世界</a></li><li class="chapter-item affix "><a href="../../first-try/sth-you-should-not-do.html">避免从入门到放弃</a></li><li class="chapter-item affix "><a href="../../community.html">社区和锈书</a></li><li class="chapter-item affix "><li class="part-title">Rust 语言基础学习</li><li class="spacer"></li><li class="chapter-item "><a href="../../first-try/intro.html"><strong aria-hidden="true">1.</strong> 寻找牛刀，以便小试</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../first-try/installation.html"><strong aria-hidden="true">1.1.</strong> 安装 Rust 环境</a></li><li class="chapter-item "><a href="../../first-try/editor.html"><strong aria-hidden="true">1.2.</strong> 墙推 VSCode!</a></li><li class="chapter-item "><a href="../../first-try/cargo.html"><strong aria-hidden="true">1.3.</strong> 认识 Cargo</a></li><li class="chapter-item "><a href="../../first-try/hello-world.html"><strong aria-hidden="true">1.4.</strong> 不仅仅是 Hello world</a></li><li class="chapter-item "><a href="../../first-try/slowly-downloading.html"><strong aria-hidden="true">1.5.</strong> 下载依赖太慢了？</a></li></ol></li><li class="chapter-item "><a href="../../basic/intro.html"><strong aria-hidden="true">2.</strong> Rust 基础入门</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/variable.html"><strong aria-hidden="true">2.1.</strong> 变量绑定与解构</a></li><li class="chapter-item "><a href="../../basic/base-type/index.html"><strong aria-hidden="true">2.2.</strong> 基本类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/base-type/numbers.html"><strong aria-hidden="true">2.2.1.</strong> 数值类型</a></li><li class="chapter-item "><a href="../../basic/base-type/char-bool.html"><strong aria-hidden="true">2.2.2.</strong> 字符、布尔、单元类型</a></li><li class="chapter-item "><a href="../../basic/base-type/statement-expression.html"><strong aria-hidden="true">2.2.3.</strong> 语句与表达式</a></li><li class="chapter-item "><a href="../../basic/base-type/function.html"><strong aria-hidden="true">2.2.4.</strong> 函数</a></li></ol></li><li class="chapter-item "><a href="../../basic/ownership/index.html"><strong aria-hidden="true">2.3.</strong> 所有权和借用</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/ownership/ownership.html"><strong aria-hidden="true">2.3.1.</strong> 所有权</a></li><li class="chapter-item "><a href="../../basic/ownership/borrowing.html"><strong aria-hidden="true">2.3.2.</strong> 引用与借用</a></li></ol></li><li class="chapter-item "><a href="../../basic/compound-type/intro.html"><strong aria-hidden="true">2.4.</strong> 复合类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/compound-type/string-slice.html"><strong aria-hidden="true">2.4.1.</strong> 字符串与切片</a></li><li class="chapter-item "><a href="../../basic/compound-type/tuple.html"><strong aria-hidden="true">2.4.2.</strong> 元组</a></li><li class="chapter-item "><a href="../../basic/compound-type/struct.html"><strong aria-hidden="true">2.4.3.</strong> 结构体</a></li><li class="chapter-item "><a href="../../basic/compound-type/enum.html"><strong aria-hidden="true">2.4.4.</strong> 枚举</a></li><li class="chapter-item "><a href="../../basic/compound-type/array.html"><strong aria-hidden="true">2.4.5.</strong> 数组</a></li></ol></li><li class="chapter-item "><a href="../../basic/flow-control.html"><strong aria-hidden="true">2.5.</strong> 流程控制</a></li><li class="chapter-item "><a href="../../basic/match-pattern/intro.html"><strong aria-hidde
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<main>
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<h1 id="栈借用-stacked-borrorw"><a class="header" href="#栈借用-stacked-borrorw">栈借用( Stacked Borrorw)</a></h1>
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<p>上一章节中我们运行 miri 时遇到了一个栈借用错误，还给了文档链接，但这些文档主要是给编译器开发者和 Rust 研究者看的，因此就不进行讲解了。</p>
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<p>而这里，我们将从一个更高层次的角度来看看何为栈借用。</p>
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<blockquote>
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<p>目前栈借用在 Rust 语义模型中还是试验阶段，因此破坏这些规则不一定说明你的程序错了。但是除非你在做编译器开发，否则最好还是修复这些错误。事前的麻烦总比事后的不安全要好，特别是当涉及到 UB 未定义行为时</p>
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</blockquote>
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<h2 id="指针混叠-pointer-aliasing-"><a class="header" href="#指针混叠-pointer-aliasing-">指针混叠( Pointer Aliasing )</a></h2>
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<p>在开始了解我们破坏的规则之前，首先应该了解为何会有这些规则的存在。这里有多个动机，但是我认为最重要的动机是： 指针混叠.</p>
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<p>当两个指针指向的内存区域存在重叠时，就说这两个指针发生了混叠，这种情况会造成一些问题。例如，编译器使用指针混叠的信息来优化内存的访问，当这些信息出错时，那程序就会被不正确地编译，然后产生一些奇怪的结果。</p>
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<blockquote>
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<p>实际上，混叠更多关心的是内存访问而不是指针本身，而且只有在其中一个访问是可变的时，才可能出问题。之所以说指针，是因为指针这个概念更方便跟一些规则进行关联。</p>
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</blockquote>
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<p>再比如，编译器需要获取一个值时，是该去缓存中查询还是每次都去内存中加载呢？关于这个选择，编译器需要清晰地知道是否有一个指针在背后修改内存，如果内存值被修改了，那缓存显然就失效了。</p>
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<h2 id="安全地栈借用"><a class="header" href="#安全地栈借用">安全地栈借用</a></h2>
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<p>有了之前的铺垫，大家肯定希望编译器能对指针混叠的信息了若指掌，但是可以吗？对于 Rust 正常代码而言，这种情况是可以避免的，因为严格的借用规则是我们的后盾：要么同时存在一个可变引用，要么同时存在多个不可变引用，这种规则简直完美避免了：两个指针指向同一块儿重叠内存区域，而其中一个是可变指针。</p>
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<p>然而实际使用中，有一些情况会较为复杂，例如以下代码中发生了可变引用的再借用( reborrow )：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>let mut data = 10;
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let ref1 = &mut data;
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let ref2 = &mut *ref1;
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*ref2 += 2;
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*ref1 += 1;
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println!("{}", data);
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<span class="boring">}
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</span></code></pre></pre>
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<p>看上去像是违反了借用规则，但是这段代码确实可以正常编译运行，如果交换下引用使用的顺序呢？</p>
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<pre><pre class="playground"><code class="language-rust edition2021">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>let mut data = 10;
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let ref1 = &mut data;
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let ref2 = &mut *ref1;
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// ORDER SWAPPED!
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*ref1 += 1;
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*ref2 += 2;
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println!("{}", data);
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<span class="boring">}
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</span></code></pre></pre>
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<pre><code class="language-shell">error[E0503]: cannot use `*ref1` because it was mutably borrowed
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--> src/main.rs:6:5
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4 | let ref2 = &mut *ref1;
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| ---------- borrow of `*ref1` occurs here
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5 |
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6 | *ref1 += 1;
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| ^^^^^^^^^^ use of borrowed `*ref1`
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7 | *ref2 += 2;
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| ---------- borrow later used here
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For more information about this error, try `rustc --explain E0503`.
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error: could not compile `playground` due to previous error
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</code></pre>
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<p>果不其然，编译器抛出了错误，当我们再借用了一个可变引用时，那原始的引用就不能再被使用，直到借用者完成了任务：借用者的借用有效范围并不是看作用域，而是看最后一次使用的位置，正因为如此，第一段代码可以编译通过，而第二段不行，这是著名的生命周期 <a href="https://course.rs/advance/lifetime/advance.html#nll-non-lexical-lifetime">NLL 规则</a>。</p>
|
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<p>以上就是我们拥有再借用但是还拥有混叠信息的原因：所有的再借用都在清晰地进行嵌套，因此每个再借用都不会与其它的冲突。那大家知道什么方法可以很好的展现嵌套的事物吗？答案就是使用栈来存放这些嵌套的借用。</p>
|
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<p>嘿，这不就是栈借用吗？</p>
|
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|
<p>这个栈的顶部借用就是当前正在使用( live )的借用，而它清晰的知道在它使用的期间不会发生混叠。当对一个指针进行再借用时，新的借用会被插入到栈的顶部，并变成 live 状态。如果要将一个旧的指针变成 live，就需要将借用栈上在它之前的借用全部弹出( pop )。</p>
|
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<p>通过栈借用的方式，我们保证了尽管存在多个再借用，但是在同一个时间，只会有一个可变引用访问目标内存，再也不用担心指针混叠的问题了。只要不去访问一个已经被弹出借用栈的指针，就会非常安全！</p>
|
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<p>从表述方式来说，与其说使用 <code>ref1</code> 会让 <code>ref2</code> 不合法，不如说 <code>ref2</code> 必须要在所有使用情况下合法，<code>ref1</code> 恰恰是其中一种情况，会破坏 <code>ref2</code> 的合法性。而编译器的报错也是选择了第二种表述方式：无法使用 <code>*ref1</code>，原因是它已经被可变借用了，可以看出，第二种表述方式比第一种要更加符合直觉。</p>
|
|
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|
<p><strong>但是，当使用 <code>unsafe</code> 指针时，借用检查器就无法再帮助我们了！</strong></p>
|
|
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|
|
|
<h2 id="不安全地栈借用"><a class="header" href="#不安全地栈借用">不安全地栈借用</a></h2>
|
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|
|
|
|
<p>所以，我们现在需要一个方式让 unsafe 指针也可以参与到栈借用系统中来，即使编译器无法正确地跟踪它们。同时我们也希望这个系统能宽松一些，不要很容易就产生 UB。</p>
|
|
|
|
|
|
<p>这是一个困难的问题，我也不知道该如何解决，但是目前在编写栈借用系统的开发者显然是有想法的，例如 miri 就是其中一个产物。</p>
|
|
|
|
|
|
<p>从一个高抽象层次来看，当我们将一个引用转换成裸指针时，就是一种再借用。那么随后，裸指针就可以对目标内存进行操作，当再借用结束时，发生的事情跟正常的再借用结束也没有区别。</p>
|
|
|
|
|
|
<p>但是问题是，你还可以将一个裸指针转变成引用，最重要的是，还可以对裸指针进行拷贝！如果发生了以下转换 <code>&mut -> *mut -> &mut -> *mut</code>，然后去访问第一个 <code>*mut</code>，这种见鬼的情况下，栈借用该如何发挥作用？</p>
|
|
|
|
|
|
<p>反正我不知道，只能求助于 miri 了。事实上，正因为这种情况，miri 还提供了试验性的模式: <code>-Zmiri-tag-raw-pointers</code>。可以通过环境的方式来开启该模式：</p>
|
|
|
|
|
|
<pre><code class="language-shell">MIRIFLAGS="-Zmiri-tag-raw-pointers" cargo +nightly-2022-01-21 miri test
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>如果是 Windows，你需要设置全局变量:</p>
|
|
|
|
|
|
<pre><code class="language-shell">$env:MIRIFLAGS="-Zmiri-tag-raw-pointers"
|
|
|
|
|
|
cargo +nightly-2022-01-21 miri test
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<h2 id="管理栈借用"><a class="header" href="#管理栈借用">管理栈借用</a></h2>
|
|
|
|
|
|
<p>因为之前的问题，使用裸指针，应该遵守一个原则：<strong>一旦开始使用裸指针，就要尝试着只使用它</strong>。</p>
|
|
|
|
|
|
<p>现在，我们依然希望在接口中使用安全的引用去构建一个安全的抽象，例如在函数参数中使用引用而不是裸指针，这样我们的用户就无需操心 unsafe 的问题。</p>
|
|
|
|
|
|
<p>为此，我们需要做以下事情：</p>
|
|
|
|
|
|
<ol>
|
|
|
|
|
|
<li>在开始时，将输入参数中的引用转换成裸指针</li>
|
|
|
|
|
|
<li>在函数体中只使用裸指针</li>
|
|
|
|
|
|
<li>返回之前，将裸指针转换成安全的指针</li>
|
|
|
|
|
|
</ol>
|
|
|
|
|
|
<p>但是由于数据结构中的字段都是私有的，无需暴露给用户，因此无需这么麻烦，直接使用裸指针即可。</p>
|
|
|
|
|
|
<p>事实上，一个依然存在的问题就是还在继续使用 <code>Box</code>, 它会告诉编译器：hey，这个看上去很像是 <code>&mut</code> ，因为它唯一的持有那个指针。</p>
|
|
|
|
|
|
<p>但是我们在链表中一直使用的裸指针是指向 Box 的内部，所以无论何时我们通过正常的方式访问 Box，我们都有可能让该裸指针的再借用变得不合法。</p>
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