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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 class = "playground" > < code class = "language-rust edition2021" > async fn foo() -> Result< u8, String> {
Ok(1)
}
async fn bar() -> Result< u8, String> {
Ok(1)
}
pub fn main() {
let fut = async {
foo().await?;
bar().await?;
Ok(())
};
}< / code > < / pre >
< p > 以上代码编译后会报错:< / p >
< pre > < code class = "language-shell" > error[E0282]: type annotations needed
--> 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< T, E> < / 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 > ::< ... > < / code > 的方式来增加类型注释：< / p >
< 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::< (), String> (()) // 在这一行进行显式的类型注释
};
< span class = "boring" > }< / span > < / code > < / pre >
< p > 给予类型注释后此时编译器就知道 < code > Result< T, E> < / 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 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< ()> );
< span class = "boring" > }< / span > < / code > < / 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 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 >
< p > 即使上面的 < code > foo< / code > 返回的 < code > Future< / code > 是 < code > Send< / code > ， < 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 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 >
< 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
--> src/main.rs:17:18
|
17 | require_send(foo());
| ^^^^^ future returned by `foo` is not `Send`
|
= help: within `impl futures::Future< Output = ()> `, the trait `std::marker::Send` is not implemented for `Rc< ()> `
note: future is not `Send` as this value is used across an await
--> 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， ， < code > NLL< / code > 出来之前。< / p >
< 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 >
< 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 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 >
< p > 这是典型的< a href = "https://course.rs/advance/into-types/sized.html#动态大小类型-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
--> 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 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() -> BoxFuture< 'static, ()> {
async move {
recursive().await;
recursive().await;
}.boxed()
}
< span class = "boring" > }< / span > < / code > < / 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 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 >
< p > 运行后报错:< / p >
< pre > < code class = "language-shell" > error[E0706]: functions in traits cannot be declared `async`
--> 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 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(& self);
}
struct Modal;
#[async_trait]
impl Advertisement for Modal {
async fn run(& 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(& self) {
let stream = connect(& 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 >
< 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 >
< / main >
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