24.1-Send 近似

译文 · 基于 Asynchronous Programming in Rust

Send 近似

原文链接: https://rust-lang.github.io/async-book/07_workarounds/03_send_approximation.html

某些 async fn 状态机可以安全地在线程间发送，而其他则不行。async fn Future 是否为 Send 取决于是否在 .await 点持有非 Send 类型。编译器会尽力近似值何时可能在 .await 点被持有，但此分析在当今若干地方过于保守。

例如，考虑一个简单的非 Send 类型，可能包含 Rc 的类型：

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use std::rc::Rc;

#[derive(Default)]
struct NotSend(Rc<()>);

NotSend 类型的变量可能作为临时值短暂出现在 async fn 中，即使 async fn 返回的 Future 类型必须是 Send：

# use std::rc::Rc;
# #[derive(Default)]
# struct NotSend(Rc<()>);
async fn bar() {}
async fn foo() {
    NotSend::default();
    bar().await;
}

fn require_send(_: impl Send) {}

fn main() {
    require_send(foo());
}

然而，若我们将 foo 改为将 NotSend 存储在变量中，此示例将无法编译：

# use std::rc::Rc;
# #[derive(Default)]
# struct NotSend(Rc<()>);
# async fn bar() {}
async fn foo() {
    let x = NotSend::default();
    bar().await;
}
# fn require_send(_: impl Send) {}
# fn main() {
#    require_send(foo());
# }

error[E0277]: `std::rc::Rc<()>` cannot be sent between threads safely
  --> src/main.rs:15:5
   |
15 |     require_send(foo());
   |     ^^^^^^^^^^^^ `std::rc::Rc<()>` cannot be sent between threads safely
   |
   = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<()>`
   = note: required because it appears within the type `NotSend`
   = note: required because it appears within the type `{NotSend, impl std::future::Future, ()}`
   = note: required because it appears within the type `[static generator@src/main.rs:7:16: 10:2 {NotSend, impl std::future::Future, ()}]`
   = note: required because it appears within the type `std::future::GenFuture<[static generator@src/main.rs:7:16: 10:2 {NotSend, impl std::future::Future, ()}]>`
   = note: required because it appears within the type `impl std::future::Future`
   = note: required because it appears within the type `impl std::future::Future`
note: required by `require_send`
  --> src/main.rs:12:1
   |
12 | fn require_send(_: impl Send) {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

此错误是正确的。若我们将 x 存入变量，它要到 .await 之后才会被 drop，此时 async fn 可能运行在不同线程上。由于 Rc 不是 Send，允许其在线程间传递是不安全的。一个简单的解决方案是在 .await 之前 drop Rc，但遗憾的是这在当今行不通。

要成功变通此问题，你可能需要引入块作用域来封装任何非 Send 变量。这使编译器更容易判断这些变量不会跨越 .await 点存活。

# use std::rc::Rc;
# #[derive(Default)]
# struct NotSend(Rc<()>);
# async fn bar() {}
async fn foo() {
    {
        let x = NotSend::default();
    }
    bar().await;
}
# fn require_send(_: impl Send) {}
# fn main() {
#    require_send(foo());
# }