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<h1 id="variance-and-phantomdata"><a class="header" href="#variance-and-phantomdata">Variance and PhantomData</a></h1>
<p>如果现在不做，等以后再修，会很麻烦，所以我们现在要做的是硬核布局。</p>
<p>建造 Rust collections 时，有这五个可怕的难题：</p>
<ol>
<li><a href="https://doc.rust-lang.org/nightly/nomicon/subtyping.html">Variance</a></li>
<li><a href="https://doc.rust-lang.org/nightly/nomicon/dropck.html">Drop Check</a></li>
<li><a href="https://doc.rust-lang.org/nightly/std/ptr/struct.NonNull.html">NonNull Optimizations</a></li>
<li><a href="https://doc.rust-lang.org/nightly/nomicon/vec/vec-alloc.html">The isize::MAX Allocation Rule</a></li>
<li><a href="https://doc.rust-lang.org/nightly/nomicon/vec/vec-zsts.html">Zero-Sized Types</a></li>
</ol>
<p>幸好，后面2个对我们来说都不是问题。</p>
<p>我们可以把第三个问题变成我们的问题，但这带来的麻烦比它的价值更多。</p>
<p>第二个问题是我以前一直坚持认为非常重要的，std 也会乱用它，但默认值是安全的，而且你需要非常努力才能注意到默认值的限制，所以不用担心这个问题。</p>
<p>所以只剩下了 Variance(型变)。</p>
<p>Rust 有子类型了。通常，<code>&amp;'big T</code> 是 <code>&amp;'small T</code> 的子类型。因为如果某些代码需要在程序的某个特定区域存活的引用，那么通常完全可以给它一个存在<em>时间更长的</em>引用。直觉上这是正确的，对吧？</p>
<p>为什么这很重要？想象一下，一些代码采用两个具有相同类型的值：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn take_two&lt;T&gt;(_val1: T, _val2: T) { }
<span class="boring">}</span></code></pre></pre>
<p>这是一些非常无聊的代码，并且我们期望它能够很好地与 T=&amp;u32 一起使用，对吧？</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn two_refs&lt;'big: 'small, 'small&gt;(
big: &amp;'big u32,
small: &amp;'small u32,
) {
take_two(big, small);
}
fn take_two&lt;T&gt;(_val1: T, _val2: T) { }
<span class="boring">}</span></code></pre></pre>
<p>是的，编译得很好！</p>
<p>现在让我们找点乐子，把它包起来：<code>std::cell::Cell</code></p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::cell::Cell;
fn two_refs&lt;'big: 'small, 'small&gt;(
// NOTE: these two lines changed
big: Cell&lt;&amp;'big u32&gt;,
small: Cell&lt;&amp;'small u32&gt;,
) {
take_two(big, small);
}
fn take_two&lt;T&gt;(_val1: T, _val2: T) { }
error[E0623]: lifetime mismatch
--&gt; src/main.rs:7:19
|
4 | big: Cell&lt;&amp;'big u32&gt;,
| ---------
5 | small: Cell&lt;&amp;'small u32&gt;,
| ----------- these two types are declared with different lifetimes...
6 | ) {
7 | take_two(big, small);
| ^^^^^ ...but data from `small` flows into `big` here
<span class="boring">}</span></code></pre></pre>
<p>哼？？？我们没有碰过生命周期，为什么编译器现在生气了！？</p>
<p>啊，好吧，生命周期的“子类型”必须非常简单，所以如果你将引用包装在任何东西中，它就会被破坏，看看 Vec：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn two_refs&lt;'big: 'small, 'small&gt;(
big: Vec&lt;&amp;'big u32&gt;,
small: Vec&lt;&amp;'small u32&gt;,
) {
take_two(big, small);
}
fn take_two&lt;T&gt;(_val1: T, _val2: T) { }
Finished dev [unoptimized + debuginfo] target(s) in 1.07s
Running `target/debug/playground`
<span class="boring">}</span></code></pre></pre>
<p>看到它没有编译成功 ——等等???Vec是魔术??????</p>
<p>是的。这种魔力就是✨<em>Variance</em>✨。</p>
<p>如果您想要所有细节，请阅读 <a href="https://doc.rust-lang.org/nightly/nomicon/subtyping.html">nomicon 关于子类型的章节</a>，但基本上子类型<em>并不总是</em>安全的。特别是，当涉及可变引用时，它就更不安全了，。因为你可能会使用诸如<code>mem::swap</code>的东西，突然哎呀，悬空指针！</p>
<p>可变引用是 <em>invariant(不变的)</em>，这意味着它们会阻止对泛型参数子类型化。因此，为了安全起见， <code>&amp;mut T</code> 在 T 上是不变的，并且 <code>Cell&lt;T&gt;</code> 在 T 上也是不变的（因为内部可变性），因为 <code>&amp;Cell&lt;T&gt;</code> 本质上就像 <code>&amp;mut T</code>。</p>
<p>几乎所有不是 <em>invariant</em> 的东西都是 <em>covariant(协变的)</em> ，这意味着子类型可以正常工作（也有 <em>contravariant(逆变的)</em> 的类型使子类型倒退，但它们真的很少见，没有人喜欢它们，所以我不会再提到它们）。</p>
<p>集合通常包含指向其数据的可变指针，因此你可能希望它们也是不变的，但事实上，它们并不需要不变！由于 Rust 的所有权系统，<code>Vec&lt;T&gt;</code> 在语义上等同于 <code>T</code>，这意味着它可以安全地保持<em>covariant(协变的)</em> ！</p>
<p>不幸的的是，下面的定义是 <em>invariant(不变的)</em>:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct LinkedList&lt;T&gt; {
front: Link&lt;T&gt;,
back: Link&lt;T&gt;,
len: usize,
}
type Link&lt;T&gt; = *mut Node&lt;T&gt;;
struct Node&lt;T&gt; {
front: Link&lt;T&gt;,
back: Link&lt;T&gt;,
elem: T,
}
<span class="boring">}</span></code></pre></pre>
<p>所以我们的类型定义中哪里惹 Rust 编译器不高兴了? <code>*mut</code>！</p>
<p>Rust 中的裸指针其实就是让你可以做任何事情，但它们只有一个安全特性：因为大多数人都不知道 Rust 中还有 <em>Variance(型变)</em> 和子类型，而错误地使用 <em>covariant(协变的)</em> 会非常危险，所以 <code>*mut T</code> 是<em>invariant(不变的)</em>，因为它很有可能被 &quot;作为&quot; <code>&amp;mut T</code> 使用。</p>
<p>作为一个花了大量时间在 Rust 中编写集合的人，这让我感到厌烦。这就是为什么我在制作 <a href="https://doc.rust-lang.org/std/ptr/struct.NonNull.html">std::ptr::NonNull</a>, 时添加了这个小魔法：</p>
<blockquote>
<p>与 <em>mut T 不同，NonNull<T> 在 T 上是 <em>covariant(协变的)</em>。这使得使用 NonNull<T> 构建</em>covariant(协变的)*类型成为可能，但如果在不应该是 <em>covariant(协变的)</em> 的地方中使用，则会带来不健全的风险。</p>
</blockquote>
<p>这是一个围绕着 <code>*mut T</code> 构建的类型。真的是魔法吗？让我们来看一下：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct NonNull&lt;T&gt; {
pointer: *const T,
}
impl&lt;T&gt; NonNull&lt;T&gt; {
pub unsafe fn new_unchecked(ptr: *mut T) -&gt; Self {
// SAFETY: the caller must guarantee that `ptr` is non-null.
unsafe { NonNull { pointer: ptr as *const T } }
}
}
<span class="boring">}</span></code></pre></pre>
<p>不，这里没有魔法！NonNull 只是滥用了 *const T 是 <em>covariant(协变的)</em> 这一事实，并将其存储起来。这就是 Rust 中集合的协变方式！这可真是惨不忍睹！所以我为你做了这个 Good Pointer Type ！不客气好好享受子类型吧</p>
<p>解决你所有问题的办法就是使用 NonNull，然后如果你想再次使用可空指针，就使用 Option&lt;NonNull<T>&gt;。我们真的要这么做吗？</p>
<p>是的！这很糟糕，但我们要做的是生产级的链表，所以我们要吃尽千辛万苦（我们可以直接使用裸*const T，然后在任何地方都进行转换，但我真的想看看这样做有多痛苦......为了人体工程学科学）。</p>
<p>下面就是我们最终的类型定义：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::ptr::NonNull;
// !!!This changed!!!
pub struct LinkedList&lt;T&gt; {
front: Link&lt;T&gt;,
back: Link&lt;T&gt;,
len: usize,
}
type Link&lt;T&gt; = Option&lt;NonNull&lt;Node&lt;T&gt;&gt;&gt;;
struct Node&lt;T&gt; {
front: Link&lt;T&gt;,
back: Link&lt;T&gt;,
elem: T,
}
<span class="boring">}</span></code></pre></pre>
<p>...等等，不，最后一件事。每当你使用裸指针时，你都应该添加一个 Ghost 来保护你的指针：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::marker::PhantomData;
pub struct LinkedList&lt;T&gt; {
front: Link&lt;T&gt;,
back: Link&lt;T&gt;,
len: usize,
/// We semantically store values of T by-value.
_boo: PhantomData&lt;T&gt;,
}
<span class="boring">}</span></code></pre></pre>
<p>在这种情况下，我认为我们<em>实际上</em>不需要 <a href="https://doc.rust-lang.org/std/marker/struct.PhantomData.html">PhantomData</a>，但每当你使用 NonNull（或一般的裸指针）时，为了安全起见，你都应该始终添加它，并向编译器和其他人清楚地表明你的想法，你在做什么。</p>
<p>PhantomData 是我们给编译器提供一个额外的 &quot;示例 &quot;字段的方法，这个字段在概念上存在于你的类型中，但由于各种原因（间接、类型擦除......）并不存在。在本例中，我们使用 NonNull 是因为我们声称我们的类型 &quot;好像 &quot;存储了一个值 T，所以我们添加了一个 PhantomData 来明确这一点。</p>
<p>...好吧，我们现在已经完成了布局！进入实际的基本功能！</p>
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