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<main>
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<h1 id="栈上的链表"><a class="header" href="#栈上的链表">栈上的链表</a></h1>
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<p>在之前的章节中，无一例外，我们创建的都是数据存储在堆上的链表，这种链表最常见也最实用：堆内存在动态分配的场景非常好用。</p>
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<p>但是，既然是高级技巧章节，那栈链表也应该拥有一席之地。但与堆内存的简单分配相比，栈内存就没那么友好了，你们猜大名鼎鼎的 C 语言的 <code>alloca</code> 是因为什么而出名的 :)</p>
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<p>限于章节篇幅，这里我们使用一个简单的栈分配方法：调用一个函数，获取一个新的、拥有更多空间的栈帧。说实话，该解决方法要多愚蠢有多愚蠢，但是它确实相当实用，甚至...有用。</p>
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<p>任何时候，当我们在做一些递归的任务时，都可以将当前步骤状态的指针传递给下一个步骤。如果指针本身就是状态的一部分，那恭喜你：你在创建一个栈上分配的链表！</p>
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<p>新的链表类型本身就是一个 Node，并且包含一个引用指向另一个 Node:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>pub struct List<'a, T> {
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pub data: T,
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pub prev: Option<&'a List<'a, T>>,
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}
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<span class="boring">}</span></code></pre></pre>
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<p>该链表只有一个操作 <code>push</code>，需要注意的是，跟其它链表不同，这里的 <code>push</code> 是通过回调的方式来完成新元素推入，并将回调返回的值直接返回给 <code>push</code> 的调用者:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>impl<'a, T> List<'a, T> {
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pub fn push<U>(
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prev: Option<&'a List<'a, T>>,
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data: T,
|
|
|
|
|
|
callback: impl FnOnce(&List<'a, T>) -> U,
|
|
|
|
|
|
) -> U {
|
|
|
|
|
|
let list = List { data, prev };
|
|
|
|
|
|
callback(&list)
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>搞定，提前问一句：你见过回调地狱吗？</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>List::push(None, 3, |list| {
|
|
|
|
|
|
println!("{}", list.data);
|
|
|
|
|
|
List::push(Some(list), 5, |list| {
|
|
|
|
|
|
println!("{}", list.data);
|
|
|
|
|
|
List::push(Some(list), 13, |list| {
|
|
|
|
|
|
println!("{}", list.data);
|
|
|
|
|
|
})
|
|
|
|
|
|
})
|
|
|
|
|
|
})
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>不禁让人感叹，这段回调代码多么的美丽动人😿。</p>
|
|
|
|
|
|
<p>用户还可以简单地使用 <code>while-let</code> 的方式来编译遍历链表，但是为了增加一些趣味，咱们还是继续使用迭代器:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>impl<'a, T> List<'a, T> {
|
|
|
|
|
|
pub fn iter(&'a self) -> Iter<'a, T> {
|
|
|
|
|
|
Iter { next: Some(self) }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<'a, T> Iterator for Iter<'a, T> {
|
|
|
|
|
|
type Item = &'a T;
|
|
|
|
|
|
|
|
|
|
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
|
|
|
|
self.next.map(|node| {
|
|
|
|
|
|
self.next = node.prev;
|
|
|
|
|
|
&node.data
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>测试下：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>#[cfg(test)]
|
|
|
|
|
|
mod test {
|
|
|
|
|
|
use super::List;
|
|
|
|
|
|
|
|
|
|
|
|
#[test]
|
|
|
|
|
|
fn elegance() {
|
|
|
|
|
|
List::push(None, 3, |list| {
|
|
|
|
|
|
assert_eq!(list.iter().copied().sum::<i32>(), 3);
|
|
|
|
|
|
List::push(Some(list), 5, |list| {
|
|
|
|
|
|
assert_eq!(list.iter().copied().sum::<i32>(), 5 + 3);
|
|
|
|
|
|
List::push(Some(list), 13, |list| {
|
|
|
|
|
|
assert_eq!(list.iter().copied().sum::<i32>(), 13 + 5 + 3);
|
|
|
|
|
|
})
|
|
|
|
|
|
})
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
|
|
|
|
|
|
|
|
|
running 18 tests
|
|
|
|
|
|
test fifth::test::into_iter ... ok
|
|
|
|
|
|
test fifth::test::iter ... ok
|
|
|
|
|
|
test fifth::test::iter_mut ... ok
|
|
|
|
|
|
test fifth::test::basics ... ok
|
|
|
|
|
|
test fifth::test::miri_food ... ok
|
|
|
|
|
|
test first::test::basics ... ok
|
|
|
|
|
|
test second::test::into_iter ... ok
|
|
|
|
|
|
test fourth::test::peek ... ok
|
|
|
|
|
|
test fourth::test::into_iter ... ok
|
|
|
|
|
|
test second::test::iter_mut ... ok
|
|
|
|
|
|
test fourth::test::basics ... ok
|
|
|
|
|
|
test second::test::basics ... ok
|
|
|
|
|
|
test second::test::iter ... ok
|
|
|
|
|
|
test third::test::basics ... ok
|
|
|
|
|
|
test silly1::test::walk_aboot ... ok
|
|
|
|
|
|
test silly2::test::elegance ... ok
|
|
|
|
|
|
test second::test::peek ... ok
|
|
|
|
|
|
test third::test::iter ... ok
|
|
|
|
|
|
|
|
|
|
|
|
test result: ok. 18 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out;
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>部分读者此时可能会有一些大胆的想法：咦？我能否修改 Node 中的值？大胆但貌似可行，不妨来试试。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>pub struct List<'a, T> {
|
|
|
|
|
|
pub data: T,
|
|
|
|
|
|
pub prev: Option<&'a mut List<'a, T>>,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
pub struct Iter<'a, T> {
|
|
|
|
|
|
next: Option<&'a List<'a, T>>,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<'a, T> List<'a, T> {
|
|
|
|
|
|
pub fn push<U>(
|
|
|
|
|
|
prev: Option<&'a mut List<'a, T>>,
|
|
|
|
|
|
data: T,
|
|
|
|
|
|
callback: impl FnOnce(&mut List<'a, T>) -> U,
|
|
|
|
|
|
) -> U {
|
|
|
|
|
|
let mut list = List { data, prev };
|
|
|
|
|
|
callback(&mut list)
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
pub fn iter(&'a self) -> Iter<'a, T> {
|
|
|
|
|
|
Iter { next: Some(self) }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<'a, T> Iterator for Iter<'a, T> {
|
|
|
|
|
|
type Item = &'a T;
|
|
|
|
|
|
|
|
|
|
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
|
|
|
|
self.next.map(|node| {
|
|
|
|
|
|
self.next = node.prev.as_ref().map(|prev| &**prev);
|
|
|
|
|
|
&node.data
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
|
|
|
|
|
|
|
|
|
error[E0521]: borrowed data escapes outside of closure
|
|
|
|
|
|
--> src\silly2.rs:47:32
|
|
|
|
|
|
|
|
|
|
|
|
|
46 | List::push(Some(list), 13, |list| {
|
|
|
|
|
|
| ----
|
|
|
|
|
|
| |
|
|
|
|
|
|
| `list` declared here, outside of the closure body
|
|
|
|
|
|
| `list` is a reference that is only valid in the closure body
|
|
|
|
|
|
47 | assert_eq!(list.iter().copied().sum::<i32>(), 13 + 5 + 3);
|
|
|
|
|
|
| ^^^^^^^^^^^ `list` escapes the closure body here
|
|
|
|
|
|
|
|
|
|
|
|
error[E0521]: borrowed data escapes outside of closure
|
|
|
|
|
|
--> src\silly2.rs:45:28
|
|
|
|
|
|
|
|
|
|
|
|
|
44 | List::push(Some(list), 5, |list| {
|
|
|
|
|
|
| ----
|
|
|
|
|
|
| |
|
|
|
|
|
|
| `list` declared here, outside of the closure body
|
|
|
|
|
|
| `list` is a reference that is only valid in the closure body
|
|
|
|
|
|
45 | assert_eq!(list.iter().copied().sum::<i32>(), 5 + 3);
|
|
|
|
|
|
| ^^^^^^^^^^^ `list` escapes the closure body here
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<ad infinitum>
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>嗯，没想到是浓眉大眼的迭代器背叛了我们，为了验证到底是哪里出了问题，我们来修改下测试:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>#[test]
|
|
|
|
|
|
fn elegance() {
|
|
|
|
|
|
List::push(None, 3, |list| {
|
|
|
|
|
|
assert_eq!(list.data, 3);
|
|
|
|
|
|
List::push(Some(list), 5, |list| {
|
|
|
|
|
|
assert_eq!(list.data, 5);
|
|
|
|
|
|
List::push(Some(list), 13, |list| {
|
|
|
|
|
|
assert_eq!(list.data, 13);
|
|
|
|
|
|
})
|
|
|
|
|
|
})
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
|
|
|
|
|
|
|
|
|
error[E0521]: borrowed data escapes outside of closure
|
|
|
|
|
|
--> src\silly2.rs:46:17
|
|
|
|
|
|
|
|
|
|
|
|
|
44 | List::push(Some(list), 5, |list| {
|
|
|
|
|
|
| ----
|
|
|
|
|
|
| |
|
|
|
|
|
|
| `list` declared here, outside of the closure body
|
|
|
|
|
|
| `list` is a reference that is only valid in the closure body
|
|
|
|
|
|
45 | assert_eq!(list.data, 5);
|
|
|
|
|
|
46 | / List::push(Some(list), 13, |list| {
|
|
|
|
|
|
47 | | assert_eq!(list.data, 13);
|
|
|
|
|
|
48 | | })
|
|
|
|
|
|
| |______^ `list` escapes the closure body here
|
|
|
|
|
|
|
|
|
|
|
|
error[E0521]: borrowed data escapes outside of closure
|
|
|
|
|
|
--> src\silly2.rs:44:13
|
|
|
|
|
|
|
|
|
|
|
|
|
42 | List::push(None, 3, |list| {
|
|
|
|
|
|
| ----
|
|
|
|
|
|
| |
|
|
|
|
|
|
| `list` declared here, outside of the closure body
|
|
|
|
|
|
| `list` is a reference that is only valid in the closure body
|
|
|
|
|
|
43 | assert_eq!(list.data, 3);
|
|
|
|
|
|
44 | / List::push(Some(list), 5, |list| {
|
|
|
|
|
|
45 | | assert_eq!(list.data, 5);
|
|
|
|
|
|
46 | | List::push(Some(list), 13, |list| {
|
|
|
|
|
|
47 | | assert_eq!(list.data, 13);
|
|
|
|
|
|
48 | | })
|
|
|
|
|
|
49 | | })
|
|
|
|
|
|
| |______________^ `list` escapes the closure body here
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>原因在于我们的链表不小心依赖了<ruby>型变<rt>variance</rt></ruby>。型变是一个<a href="https://doc.rust-lang.org/nomicon/subtyping.html">相当复杂的概念</a>，下面来简单了解下。</p>
|
|
|
|
|
|
<p>每一个节点( Node )都包含一个引用，该引用指向另一个节点， 且这两个节点是同一个类型。如果从最里面的节点角度来看，那所有外部的节点都在使用和它一样的生命周期，但这个显然是不对的：链表中的每一个节点都会比它指向的节点活得更久，因为它们的作用域是嵌套存在的。</p>
|
|
|
|
|
|
<p>那之前的不可变引用版本为何可以正常工作呢？原因是在大多数时候，编译器都能自己判断：虽然某些东东活得太久了，但是这是安全的。当我们把一个 List 塞入另一个时，编译器会迅速将生命周期进行收缩以满足新的 List 的需求，<strong>这种生命周期收缩就是一种型变</strong>。</p>
|
|
|
|
|
|
<p>如果大家还是觉得不太理解，我们来考虑下其它拥有继承特性的编程语言。在该语言中，当你将一个 <code>Cat</code> 传递给需要 <code>Animal</code> 的地方时( <code>Animal</code> 是 <code>Cat</code> 的父类型)，型变就发生了。从字面来说，将一只猫传给需要动物的地方，也是合适的，毕竟猫确实是动物的一种。</p>
|
|
|
|
|
|
<p>总之，可以看出无论是从大的生命周期收缩为小的生命周期，还是从 <code>Cat</code> 到 <code>Animal</code>，型变的典型特征就是：范围在减小，毕竟子类型的功能肯定是比父类型多的。</p>
|
|
|
|
|
|
<p>既然有型变，为何可变引用的版本会报错呢？其实在于型变不总是安全的，假如之前的代码可以编译，那我们可以写出<ruby>释放后再使用<rt>use-after-free</rt></ruby> 的代码:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>List::push(None, 3, |list| {
|
|
|
|
|
|
List::push(Some(list), 5, |list| {
|
|
|
|
|
|
List::push(Some(list), 13, |list| {
|
|
|
|
|
|
// 哈哈，好爽，由于所有的生命周期都是相同的，因此编译器允许我重写父节点，并让它持有一个可变指针指向我自己。
|
|
|
|
|
|
// 我将创建所有的 use-after-free !
|
|
|
|
|
|
*list.prev.as_mut().unwrap().prev = Some(list);
|
|
|
|
|
|
})
|
|
|
|
|
|
})
|
|
|
|
|
|
})
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>一旦引入可变性，型变就会造成这样的隐患：意外修改了不该被修改的代码，但这些代码的调用者还在期待着和往常一样的结果！例如以下例子：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>let mut my_kitty = Cat; // Make a Cat (long lifetime)
|
|
|
|
|
|
let animal: &mut Animal = &mut my_kitty; // Forget it's a Cat (shorten lifetime)
|
|
|
|
|
|
*animal = Dog; // Write a Dog (short lifetime)
|
|
|
|
|
|
my_kitty.meow(); // Meowing Dog! (Use After Free)
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>我们将长生命周期的猫转换成短生命周期的动物，可变的！然后通过短生命周期的动物将指针重新指向一只狗。此时我们想去撸软萌猫的时候，就听到：<code>旺旺...呜嗷嗷嗷</code>，对，你没听错，不仅没有了猫叫，甚至于狗还没叫完，就可能在某个地方又被修改成狼了。</p>
|
|
|
|
|
|
<p>因此，<strong>虽然你可以修改可变引用的生命周期，但是一旦开始嵌套，它们就将失去型变，变成<code>不变( invariant )</code></strong>。此时，就再也无法对生命周期进行收缩了。</p>
|
|
|
|
|
|
<p>具体来说: <code>&mut &'big mut T</code> 无法被转换成 <code>&mut &'small mut T</code>，这里 <code>'big</code> 代表比 <code>'small</code> 更大的生命周期。或者用更正式的说法：<code>&'a mut T</code> 对于 <code>'a</code> 来说是协变( <code>covariant</code> )的，但是对于 <code>T</code> 是不变的( <code>invariant</code> )。</p>
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<hr />
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<p>说了这么多高深的理论，那么该如何改变链表的数据呢？答案就是：使用老本行 - 内部可变性。</p>
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<p>下面让我们回滚到之前的不可变版本，然后使用 <code>Cell</code> 来替代 <code>&mut</code>。</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>#[test]
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fn cell() {
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use std::cell::Cell;
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List::push(None, Cell::new(3), |list| {
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List::push(Some(list), Cell::new(5), |list| {
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List::push(Some(list), Cell::new(13), |list| {
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// Multiply every value in the list by 10
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for val in list.iter() {
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val.set(val.get() * 10)
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}
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let mut vals = list.iter();
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assert_eq!(vals.next().unwrap().get(), 130);
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assert_eq!(vals.next().unwrap().get(), 50);
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assert_eq!(vals.next().unwrap().get(), 30);
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assert_eq!(vals.next(), None);
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assert_eq!(vals.next(), None);
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})
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})
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})
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}
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<span class="boring">}</span></code></pre></pre>
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<pre><code class="language-shell">$ cargo test
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running 19 tests
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test fifth::test::into_iter ... ok
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test fifth::test::basics ... ok
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test fifth::test::iter_mut ... ok
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test fifth::test::iter ... ok
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test fourth::test::basics ... ok
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test fourth::test::into_iter ... ok
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test second::test::into_iter ... ok
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test first::test::basics ... ok
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test fourth::test::peek ... ok
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test second::test::basics ... ok
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test fifth::test::miri_food ... ok
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test silly2::test::cell ... ok
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test third::test::iter ... ok
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test second::test::iter_mut ... ok
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test second::test::peek ... ok
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test silly1::test::walk_aboot ... ok
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test silly2::test::elegance ... ok
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test third::test::basics ... ok
|
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test second::test::iter ... ok
|
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test result: ok. 19 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out;
|
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</code></pre>
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<p>简简单单搞定，虽然之前我们嫌弃内部可变性，但是在这里：真香！</p>
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