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<h1 id="一些疑难问题的解决办法"><a class="header" href="#一些疑难问题的解决办法">一些疑难问题的解决办法</a></h1>
<p><code>async</code> 在 Rust 依然比较新，疑难杂症少不了，而它们往往还处于活跃开发状态，短时间内无法被解决，因此才有了本文。下面一起来看看这些问题以及相应的临时解决方案。</p>
<h2 id="在-async-语句块中使用-"><a class="header" href="#在-async-语句块中使用-">在 async 语句块中使用 ?</a></h2>
<p><code>async</code> 语句块和 <code>async fn</code> 最大的区别就是前者无法显式的声明返回值，在大多数时候这都不是问题，但是当配合 <code>?</code> 一起使用时，问题就有所不同:</p>
<pre><pre class="playground"><code class="language-rust edition2021">async fn foo() -&gt; Result&lt;u8, String&gt; {
Ok(1)
}
async fn bar() -&gt; Result&lt;u8, String&gt; {
Ok(1)
}
pub fn main() {
let fut = async {
foo().await?;
bar().await?;
Ok(())
};
}</code></pre></pre>
<p>以上代码编译后会报错:</p>
<pre><code class="language-shell">error[E0282]: type annotations needed
--&gt; src/main.rs:14:9
|
11 | let fut = async {
| --- consider giving `fut` a type
...
14 | Ok(1)
| ^^ cannot infer type for type parameter `E` declared on the enum `Result`
</code></pre>
<p>原因在于编译器无法推断出 <code>Result&lt;T, E&gt;</code> 中的 <code>E</code> 的类型， 而且编译器的提示 <code>consider giving `fut` a type</code> 你也别傻乎乎的相信，然后尝试半天，最后无奈放弃：目前还没有办法为 <code>async</code> 语句块指定返回类型。</p>
<p>既然编译器无法推断出类型，那咱就给它更多提示，可以使用 <code>::&lt; ... &gt;</code> 的方式来增加类型注释：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let fut = async {
foo().await?;
bar().await?;
Ok::&lt;(), String&gt;(()) // 在这一行进行显式的类型注释
};
<span class="boring">}</span></code></pre></pre>
<p>给予类型注释后此时编译器就知道 <code>Result&lt;T, E&gt;</code> 中的 <code>E</code> 的类型是 <code>String</code>，进而成功通过编译。</p>
<h2 id="async-函数和-send-特征"><a class="header" href="#async-函数和-send-特征">async 函数和 Send 特征</a></h2>
<p>在多线程章节我们深入讲过 <code>Send</code> 特征对于多线程间数据传递的重要性，对于 <code>async fn</code> 也是如此，它返回的 <code>Future</code> 能否在线程间传递的关键在于 <code>.await</code> 运行过程中，作用域中的变量类型是否是 <code>Send</code>。</p>
<p>学到这里，相信大家已经很清楚 <code>Rc</code> 无法在多线程环境使用，原因就在于它并未实现 <code>Send</code> 特征，那咱就用它来做例子:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::rc::Rc;
#[derive(Default)]
struct NotSend(Rc&lt;()&gt;);
<span class="boring">}</span></code></pre></pre>
<p>事实上，未实现 <code>Send</code> 特征的变量可以出现在 <code>async fn</code> 语句块中:</p>
<pre><pre class="playground"><code class="language-rust edition2021">async fn bar() {}
async fn foo() {
NotSend::default();
bar().await;
}
fn require_send(_: impl Send) {}
fn main() {
require_send(foo());
}</code></pre></pre>
<p>即使上面的 <code>foo</code> 返回的 <code>Future</code> 是 <code>Send</code>， 但是在它内部短暂的使用 <code>NotSend</code> 依然是安全的，原因在于它的作用域并没有影响到 <code>.await</code>，下面来试试声明一个变量，然后让 <code>.await</code> 的调用处于变量的作用域中试试:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>async fn foo() {
let x = NotSend::default();
bar().await;
}
<span class="boring">}</span></code></pre></pre>
<p>不出所料，错误如期而至:</p>
<pre><code class="language-shell">error: future cannot be sent between threads safely
--&gt; src/main.rs:17:18
|
17 | require_send(foo());
| ^^^^^ future returned by `foo` is not `Send`
|
= help: within `impl futures::Future&lt;Output = ()&gt;`, the trait `std::marker::Send` is not implemented for `Rc&lt;()&gt;`
note: future is not `Send` as this value is used across an await
--&gt; src/main.rs:11:5
|
10 | let x = NotSend::default();
| - has type `NotSend` which is not `Send`
11 | bar().await;
| ^^^^^^^^^^^ await occurs here, with `x` maybe used later
12 | }
| - `x` is later dropped here
</code></pre>
<p>提示很清晰，<code>.await</code>在运行时处于 <code>x</code> 的作用域内。在之前章节有提到过， <code>.await</code> 有可能被执行器调度到另一个线程上运行，而 <code>Rc</code> 并没有实现 <code>Send</code>，因此编译器无情拒绝了咱们。</p>
<p>其中一个可能的解决方法是在 <code>.await</code> 之前就使用 <code>std::mem::drop</code> 释放掉 <code>Rc</code>，但是很可惜，截止今天，该方法依然不能解决这种问题。</p>
<p>不知道有多少同学还记得语句块 <code>{ ... }</code> 在 Rust 中其实具有非常重要的作用(特别是相比其它大多数语言来说时)：可以将变量声明在语句块内，当语句块结束时，变量会自动被 Drop，这个规则可以帮助我们解决很多借用冲突问题，特别是在 <code>NLL</code> 出来之前。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>async fn foo() {
{
let x = NotSend::default();
}
bar().await;
}
<span class="boring">}</span></code></pre></pre>
<p>是不是很简单？最终我们还是通过 Drop 的方式解决了这个问题，当然，还是期待未来 <code>std::mem::drop</code> 也能派上用场。</p>
<h2 id="递归使用-async-fn"><a class="header" href="#递归使用-async-fn">递归使用 async fn</a></h2>
<p>在内部实现中，<code>async fn</code> 被编译成一个状态机，这会导致递归使用 <code>async fn</code> 变得较为复杂， 因为编译后的状态机还需要包含自身。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// foo函数:
async fn foo() {
step_one().await;
step_two().await;
}
// 会被编译成类似下面的类型：
enum Foo {
First(StepOne),
Second(StepTwo),
}
// 因此 recursive 函数
async fn recursive() {
recursive().await;
recursive().await;
}
// 会生成类似以下的类型
enum Recursive {
First(Recursive),
Second(Recursive),
}
<span class="boring">}</span></code></pre></pre>
<p>这是典型的<a href="https://course.rs/advance/into-types/sized.html#%E5%8A%A8%E6%80%81%E5%A4%A7%E5%B0%8F%E7%B1%BB%E5%9E%8B-dst">动态大小类型</a>，它的大小会无限增长，因此编译器会直接报错:</p>
<pre><code class="language-shell">error[E0733]: recursion in an `async fn` requires boxing
--&gt; src/lib.rs:1:22
|
1 | async fn recursive() {
| ^ an `async fn` cannot invoke itself directly
|
= note: a recursive `async fn` must be rewritten to return a boxed future.
</code></pre>
<p>如果认真学过之前的章节，大家应该知道只要将其使用 <code>Box</code> 放到堆上而不是栈上，就可以解决，在这里还是要称赞下 Rust 的编译器，给出的提示总是这么精确 <code>recursion in an `async fn` requires boxing</code>。</p>
<p>就算是使用 <code>Box</code>，这里也大有讲究。如果我们试图使用 <code>Box::pin</code> 这种方式去包裹是不行的，因为编译器自身的限制限制了我们(刚夸过它。。。)。为了解决这种问题，我们只能将 <code>recursive</code> 转变成一个正常的函数，该函数返回一个使用 <code>Box</code> 包裹的 <code>async</code> 语句块：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use futures::future::{BoxFuture, FutureExt};
fn recursive() -&gt; BoxFuture&lt;'static, ()&gt; {
async move {
recursive().await;
recursive().await;
}.boxed()
}
<span class="boring">}</span></code></pre></pre>
<h2 id="在特征中使用-async"><a class="header" href="#在特征中使用-async">在特征中使用 async</a></h2>
<p>在目前版本中，我们还无法在特征中定义 <code>async fn</code> 函数，不过大家也不用担心，目前已经有计划在未来移除这个限制了。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Test {
async fn test();
}
<span class="boring">}</span></code></pre></pre>
<p>运行后报错:</p>
<pre><code class="language-shell">error[E0706]: functions in traits cannot be declared `async`
--&gt; src/main.rs:5:5
|
5 | async fn test();
| -----^^^^^^^^^^^
| |
| `async` because of this
|
= note: `async` trait functions are not currently supported
= note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
</code></pre>
<p>好在编译器给出了提示，让我们使用 <a href="https://github.com/dtolnay/async-trait"><code>async-trait</code></a> 解决这个问题:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use async_trait::async_trait;
#[async_trait]
trait Advertisement {
async fn run(&amp;self);
}
struct Modal;
#[async_trait]
impl Advertisement for Modal {
async fn run(&amp;self) {
self.render_fullscreen().await;
for _ in 0..4u16 {
remind_user_to_join_mailing_list().await;
}
self.hide_for_now().await;
}
}
struct AutoplayingVideo {
media_url: String,
}
#[async_trait]
impl Advertisement for AutoplayingVideo {
async fn run(&amp;self) {
let stream = connect(&amp;self.media_url).await;
stream.play().await;
// 用视频说服用户加入我们的邮件列表
Modal.run().await;
}
}
<span class="boring">}</span></code></pre></pre>
<p>不过使用该包并不是免费的，每一次特征中的 <code>async</code> 函数被调用时，都会产生一次堆内存分配。对于大多数场景，这个性能开销都可以接受，但是当函数一秒调用几十万、几百万次时，就得小心这块儿代码的性能了！</p>
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