Send 估计

有的async fn状态机在线程间传递时是安全的，有的不是。一个async fn的future是否具有Sendtrait是根据 是否有 跨越带.await的语句的 非Send类型来判断的。编译器会尽可能估计这些值可能跨越.await的时点，但是在今天看来，这种分析显得太过保守了。 举个例子，考虑一个简单的非Send类型，比如包含了一个Rc的类型：

#![allow(unused)]
fn main() {
use std::rc::Rc;

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

NotSend类型的变量在async fn中可以作为临时变量短时间出现，即便async fn返回的最终的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());
}

但是，如果改变foo来将NotSend存储在一个变量中，这个例子就不能编译了：

However, if we change foo to store NotSend in a variable, this example no longer compiles:

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`.

这个报错没问题。如果我们将x存储到变量中，直到.await结束后它才应被释放，到那时async fn可能在另一个线程上运行了。既然Rc不是Send，允许其在线程间传递就是不安全的。一个简单的解决方案就是在.await前就把Rc给drop掉，不过这个目前不行。

为了解决这个问题，可以通过一个块作用域来把非Send变量包裹起来，这样编译器就能知道有哪些变量不能越过.await语句了。

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());
}