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<h1 id="send-sync-and-compile-tests"><a class="header" href="#send-sync-and-compile-tests">Send, Sync, and Compile Tests</a></h1>
<p>好吧,其实我们还有一对特征需要考虑,但它们很特别。我们必须对付 Rust 的神圣罗马帝国: unsafe 的 Opt-in Built-out 特征OIBITs Send 和 Sync它们实际上是opt-outbuilt-out3 个中有 1 个已经很不错了!)。</p>
<p>与 Copy 一样这些特征完全没有相关代码只是标记您的类型具有特定属性。Send 表示你的类型可以安全地发送到另一个线程。Sync 表示你的类型可以在线程间安全共享(&amp;Self: Send</p>
<p>关于 LinkedList <em>covariant(协变的)</em> 论点在这里同样适用:一般来说,不使用花哨的内部可变技巧的普通集合可以安全地进行 Send 和 Sync。</p>
<p>But I said they're <em>opt out</em>. So actually, are we already? How would we know?</p>
<p>让我们在代码中添加一些新的魔法:随机的私有垃圾,除非我们的类型具有我们所期望的属性,否则将无法编译:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[allow(dead_code)]
fn assert_properties() {
fn is_send&lt;T: Send&gt;() {}
fn is_sync&lt;T: Sync&gt;() {}
is_send::&lt;LinkedList&lt;i32&gt;&gt;();
is_sync::&lt;LinkedList&lt;i32&gt;&gt;();
is_send::&lt;IntoIter&lt;i32&gt;&gt;();
is_sync::&lt;IntoIter&lt;i32&gt;&gt;();
is_send::&lt;Iter&lt;i32&gt;&gt;();
is_sync::&lt;Iter&lt;i32&gt;&gt;();
is_send::&lt;IterMut&lt;i32&gt;&gt;();
is_sync::&lt;IterMut&lt;i32&gt;&gt;();
is_send::&lt;Cursor&lt;i32&gt;&gt;();
is_sync::&lt;Cursor&lt;i32&gt;&gt;();
fn linked_list_covariant&lt;'a, T&gt;(x: LinkedList&lt;&amp;'static T&gt;) -&gt; LinkedList&lt;&amp;'a T&gt; { x }
fn iter_covariant&lt;'i, 'a, T&gt;(x: Iter&lt;'i, &amp;'static T&gt;) -&gt; Iter&lt;'i, &amp;'a T&gt; { x }
fn into_iter_covariant&lt;'a, T&gt;(x: IntoIter&lt;&amp;'static T&gt;) -&gt; IntoIter&lt;&amp;'a T&gt; { x }
}
cargo build
Compiling linked-list v0.0.3
error[E0277]: `NonNull&lt;Node&lt;i32&gt;&gt;` cannot be sent between threads safely
--&gt; src\lib.rs:433:5
|
433 | is_send::&lt;LinkedList&lt;i32&gt;&gt;();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ `NonNull&lt;Node&lt;i32&gt;&gt;` cannot be sent between threads safely
|
= help: within `LinkedList&lt;i32&gt;`, the trait `Send` is not implemented for `NonNull&lt;Node&lt;i32&gt;&gt;`
= note: required because it appears within the type `Option&lt;NonNull&lt;Node&lt;i32&gt;&gt;&gt;`
note: required because it appears within the type `LinkedList&lt;i32&gt;`
--&gt; src\lib.rs:8:12
|
8 | pub struct LinkedList&lt;T&gt; {
| ^^^^^^^^^^
note: required by a bound in `is_send`
--&gt; src\lib.rs:430:19
|
430 | fn is_send&lt;T: Send&gt;() {}
| ^^^^ required by this bound in `is_send`
&lt;a million more errors&gt;
<span class="boring">}</span></code></pre></pre>
<p>我骗你说原始指针只有一个安全保护:这是另一个。 <code>*const</code><code>*mut</code> explicitly opt out of Send and Sync to be safe, so we do <em>actually</em> have to opt back in:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>unsafe impl&lt;T: Send&gt; Send for LinkedList&lt;T&gt; {}
unsafe impl&lt;T: Sync&gt; Sync for LinkedList&lt;T&gt; {}
unsafe impl&lt;'a, T: Send&gt; Send for Iter&lt;'a, T&gt; {}
unsafe impl&lt;'a, T: Sync&gt; Sync for Iter&lt;'a, T&gt; {}
unsafe impl&lt;'a, T: Send&gt; Send for IterMut&lt;'a, T&gt; {}
unsafe impl&lt;'a, T: Sync&gt; Sync for IterMut&lt;'a, T&gt; {}
<span class="boring">}</span></code></pre></pre>
<p>请注意,我们必须在这里编写不安全的 impl这些是不安全的特征不安全代码如并发库只能依靠我们正确地实现这些特征由于没有实际代码我们所做的保证只是是的我们在线程间发送或共享确实是安全的</p>
<p>别以为这些都是随便说说的,我可是经过认证的专业人士,我在这里要说:是的,这些都是完全没问题的。请注意,我们并不需要为 IntoIter 实现 Send 和 Sync它只是包含 LinkedList所以会自动生成 Send 和 Sync--我告诉过你它们实际上是 opt out</p>
<pre><code class="language-text">cargo build
Compiling linked-list v0.0.3
Finished dev [unoptimized + debuginfo] target(s) in 0.18s
</code></pre>
<p>很好</p>
<p>IterMut 绝对不应该是协变的,因为它 "就像" <code>&amp;mut T</code></p>
<p>用魔术!其实是用 rustdoc好吧我们不一定要使用 rustdoc但这是最有趣的用法。你看如果你写了一个 doccomment 并包含了一个代码块,那么 rustdoc 就会尝试编译并运行它,所以我们可以用它来创建新的匿名 "程序",而这些程序不会影响主程序:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span> /// ```
/// use linked_list::IterMut;
///
/// fn iter_mut_covariant&lt;'i, 'a, T&gt;(x: IterMut&lt;'i, &amp;'static T&gt;) -&gt; IterMut&lt;'i, &amp;'a T&gt; { x }
/// ```
fn iter_mut_invariant() {}
cargo test
...
Doc-tests linked-list
running 1 test
test src\lib.rs - assert_properties::iter_mut_invariant (line 458) ... FAILED
failures:
---- src\lib.rs - assert_properties::iter_mut_invariant (line 458) stdout ----
error[E0308]: mismatched types
--&gt; src\lib.rs:461:86
|
6 | fn iter_mut_covariant&lt;'i, 'a, T&gt;(x: IterMut&lt;'i, &amp;'static T&gt;) -&gt; IterMut&lt;'i, &amp;'a T&gt; { x }
| ^ lifetime mismatch
|
= note: expected struct `linked_list::IterMut&lt;'_, &amp;'a T&gt;`
found struct `linked_list::IterMut&lt;'_, &amp;'static T&gt;`
<span class="boring">}</span></code></pre></pre>
<p>好吧我们已经证明了它是不变的但现在我们的测试失败了。不用担心rustdoc 会让你在栅栏上注释 compile_fail说明这是意料之中的</p>
<p>(实际上,我们只证明了它 "不是<em>covariant(协变的)</em>",但老实说,如果你能让一个类型 "意外地、错误地<em>contravariant(逆变的)</em> ",那么,恭喜你。)</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span> /// ```compile_fail
/// use linked_list::IterMut;
///
/// fn iter_mut_covariant&lt;'i, 'a, T&gt;(x: IterMut&lt;'i, &amp;'static T&gt;) -&gt; IterMut&lt;'i, &amp;'a T&gt; { x }
/// ```
fn iter_mut_invariant() {}
cargo test
Compiling linked-list v0.0.3
Finished test [unoptimized + debuginfo] target(s) in 0.49s
Running unittests src\lib.rs
...
Doc-tests linked-list
running 1 test
test src\lib.rs - assert_properties::iter_mut_invariant (line 458) - compile fail ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.12s
<span class="boring">}</span></code></pre></pre>
<p>是的!我建议在进行测试时不要使用 compile_fail这样你可以看到错误是不是和你预期的一致。例如你忘记了使用 use 关键字,这是错误的,但因为 compile_fail通过了测试。如果不使用compile_fail测试会因为没有使用 use 失败,这不是我们想要的, 我们想要的是:测试因为 <code>mut</code> 是*covariant(协变的)*的而失败!</p>
<p>(哦,等等,我们其实可以在 compile_fail 旁边指定我们想要的错误代码,但这只适用于 nightly而且由于上述原因依赖它是个坏主意。在 not-nightly 版本运行时,它将被默默忽略)。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span> /// ```compile_fail,E0308
/// use linked_list::IterMut;
///
/// fn iter_mut_covariant&lt;'i, 'a, T&gt;(x: IterMut&lt;'i, &amp;'static T&gt;) -&gt; IterMut&lt;'i, &amp;'a T&gt; { x }
/// ```
fn iter_mut_invariant() {}
<span class="boring">}</span></code></pre></pre>
<p>......还有,你注意到我们实际上把 IterMut 变成*invariant(不变的)*的那部分了吗?这很容易被忽略,因为我 "只是 "复制粘贴了 Iter 并把它放在了最后。这是最后一行:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct IterMut&lt;'a, T&gt; {
front: Link&lt;T&gt;,
back: Link&lt;T&gt;,
len: usize,
_boo: PhantomData&lt;&amp;'a mut T&gt;,
}
<span class="boring">}</span></code></pre></pre>
<p>我们试着去掉 PhantomData:</p>
<pre><code class="language-text"> cargo build
Compiling linked-list v0.0.3 (C:\Users\ninte\dev\contain\linked-list)
error[E0392]: parameter `'a` is never used
--&gt; src\lib.rs:30:20
|
30 | pub struct IterMut&lt;'a, T&gt; {
| ^^ unused parameter
|
= help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
</code></pre>
<p>哈!编译器在背后支持我们,提示我们未使用的 lifetime。让我们试着用一个错误的例子来代替</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span> _boo: PhantomData&lt;&amp;'a T&gt;,
cargo build
Compiling linked-list v0.0.3 (C:\Users\ninte\dev\contain\linked-list)
Finished dev [unoptimized + debuginfo] target(s) in 0.17s
<span class="boring">}</span></code></pre></pre>
<p>它可以构建!我们的测试可以发现问题吗?</p>
<pre><code class="language-text">cargo test
...
Doc-tests linked-list
running 1 test
test src\lib.rs - assert_properties::iter_mut_invariant (line 458) - compile fail ... FAILED
failures:
---- src\lib.rs - assert_properties::iter_mut_invariant (line 458) stdout ----
Test compiled successfully, but it's marked `compile_fail`.
failures:
src\lib.rs - assert_properties::iter_mut_invariant (line 458)
test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.15s
</code></pre>
<p>Eyyy!!.!这个系统真管用!我喜欢那些能真正完成任务的测试,这样我就不必为那些若隐若现的错误而感到恐惧了!</p>
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