Future不是Send,仅当来自异步Trait时

我有一个特点 Serializer :

trait Serializer {
    fn serialize(&self, data: Rc<()>) -> Result<(), Error>;
}

还有一些 Foo 其实现了它。值得注意的是, data 不是 Send .

实际实现需要一个异步上下文来执行,所以我将异步运行时的创建与实际实现分开,如下所示:

struct Foo {}

impl Foo {
    async fn async_serialize(&self, _data: Rc<()>) -> Result<(), Error> {
        unimplemented!();
    }
}

impl Serializer for Foo {
    fn serialize(&self, data: Rc<()>) -> Result<(), Error> {
        let runtime = Builder::new_current_thread().enable_all().build().unwrap();
        runtime.block_on(async move { self.async_serialize(data).await })
    }
}

这是我所期望的编译和工作方式。

• 游乐场链接: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=04171e7fb65a9e158903978fd19ef8ac

如果我重构代码,使异步运行时的创建通过一个特性来完成:

#[async_trait]
trait AsyncSerializer {
    async fn async_serialize(&self, data: Rc<()>) -> Result<(), Error>;
}

#[async_trait]
impl AsyncSerializer for Foo {
    async fn async_serialize(&self, _data: Rc<()>) -> Result<(), Error> {
        unimplemented!();
    }
}

这不编译,现在抱怨 Rc<()> 不是 邮寄 :

error: future cannot be sent between threads safely
  --> src/main.rs:19:73
   |
19 |       async fn async_serialize(&self, _data: Rc<()>) -> Result<(), Error> {
   |  _________________________________________________________________________^
20 | |         unimplemented!();
21 | |     }
   | |_____^ future created by async block is not `Send`
   |
   = help: within `impl Future<Output = Result<(), anyhow::Error>>`, the trait `Send` is not implemented for `Rc<()>`
note: captured value is not `Send`

• 游乐场链接: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=23f61162b5be8384092888635d8daacc

这个错误消息对我来说很有意义, Rc 不是 邮寄 但是

• 为什么以前这不是问题?先前的实现( impl on Foo 对 impl AsyncSerializer for Foo )看起来和我相似。
• 我能包起来吗 数据 以某种方式避免这种情况?