使用AsyncFnOnce与tokio::spawn时编译错误的解决方案咨询

初始代码与编译错误 #

尝试编写一个接收AsyncFnOnce并通过tokio::spawn执行的函数：

use std::ops::AsyncFnOnce;
use tokio::task::JoinHandle;

fn spawn_asyncfn<F>(f: F) -> JoinHandle<()>
where
    F: 'static + AsyncFnOnce() -> (),
{
    let task = f();
    tokio::spawn(task)
}

出现如下编译错误：

error[E0277]: `<F as AsyncFnOnce<()>>::CallOnceFuture` cannot be sent between threads safely
   --> src\lib.rs:6:18
    |
6   |     tokio::spawn(task)
    |     ------------ ^^^^ `<F as AsyncFnOnce<()>>::CallOnceFuture` cannot be sent between threads safely
    |     |
    |     required by a bound introduced by this call
    |
= help: the trait `Send` is not implemented for `<F as AsyncFnOnce<()>>::CallOnceFuture`
note: required by a bound in `tokio::spawn`
   --> C:\Users\Martin\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\tokio-1.43.0\src\task\spawn.rs:168:21
    |
166 |     pub fn spawn<F>(future: F) -> JoinHandle<F::Output>
    |            ----- required by a bound in this function
167 |     where
168 |         F: Future + Send + 'static,
    |                     ^^^^ required by this bound in `spawn`
help: consider further restricting the associated type
    |
3   |     F: 'static + AsyncFnOnce() -> (), <F as AsyncFnOnce<()>>::CallOnceFuture: Send
    |                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

按提示修改后的代码与新错误 #

按照编译器提示添加关联类型约束后：

use std::ops::AsyncFnOnce;
use tokio::task::JoinHandle;

fn spawn_asyncfn<F>(f: F) -> JoinHandle<()>
where
    F: 'static + AsyncFnOnce() -> (),
    <F as AsyncFnOnce<()>>::CallOnceFuture: Send,
{
    let task = f();
    tokio::spawn(task)
}

又出现新的错误：

error[E0658]: use of unstable library feature `async_fn_traits`
 --> src\lib.rs:4:5
  |
4 |     <F as AsyncFnOnce<()>>::CallOnceFuture: Send,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
 --> src\lib.rs:4:5
  |
4 |     <F as AsyncFnOnce<()>>::CallOnceFuture: Send,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `AsyncFnOnce() -> ()`
  |
= note: see issue #29625 for more information

解决方案 #

在async_fn_traits稳定前，直接通过关联类型添加Send约束确实不可行，但有两种可行的处理方式：

方案1：用async闭包包裹调用（稳定方案） #

将AsyncFnOnce的调用逻辑包裹在一个async闭包中，闭包生成的Future会自动满足tokio::spawn的Send约束（只要F本身是Send）：

use std::ops::AsyncFnOnce;
use tokio::task::JoinHandle;

fn spawn_asyncfn<F>(f: F) -> JoinHandle<()>
where
    F: 'static + AsyncFnOnce() -> () + Send,
{
    tokio::spawn(async move {
        f().await;
    })
}

这里给F添加Send约束是因为闭包需要跨线程移动f，而async闭包会自动将内部的Future调用包装成符合Send要求的类型。

方案2：启用不稳定特性（仅限开发/测试） #

如果必须使用关联类型的方式，只能在nightly Rust环境下启用async_fn_traits特性：

1. 在代码开头添加特性启用声明：

#![feature(async_fn_traits)]

2. 使用nightly版本的Rust工具链（可通过rustup default nightly切换）。

这种方式依赖不稳定特性，不适合生产环境，仅用于实验性开发。

总结：无需等待async_fn_traits稳定，使用async闭包包裹的方案是通用且稳定的解决办法。

内容的提问来源于stack exchange，提问作者Finomnis