About asynchronous recursion

[fawdlstty]

Version
└── tokio v1.43.0
└── tokio-macros v2.5.0 (proc-macro)

Platform
Linux fa-pc 6.8.0-52-generic #53~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Jan 15 19:18:46 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

Description
I know asynchronous recursion requires indirect implementation through methods such as BoxFuture, but I don't understand why this code also needs to handle such issues. It seems like a problem

[short summary of the bug]
asynchronous recursion

I tried this code:

[code sample that causes the bug]

async fn test_func() {
    _ = tokio::task::spawn(async move {
        tokio::time::sleep(std::time::Duration::from_secs(1)).await;
        test_func().await;
    });
}

#[tokio::main]
async fn main() {
    test_func().await;
}

I expected to see this happen: [explanation]
compile success

Instead, this happened: [explanation]

error: future cannot be sent between threads safely
   --> src/main.rs:2:9
    |
2   |       _ = tokio::task::spawn(async move {
    |  _________^
3   | |         tokio::time::sleep(std::time::Duration::from_secs(1)).await;
4   | |         test_func().await;
5   | |     });
    | |______^ future created by async block is not `Send`
    |
    = note: cannot satisfy `impl Future<Output = ()>: Send`
note: future is not `Send` as it awaits another future which is not `Send`
   --> src/main.rs:4:9
    |
4   |         test_func().await;
    |         ^^^^^^^^^^^ await occurs here on type `impl Future<Output = ()>`, which is not `Send`
note: required by a bound in `tokio::spawn`
   --> /home/fawdlstty/.cargo/registry/src/mirrors.ustc.edu.cn-04b7754156161b43/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`

error: could not compile `test_rs` (bin "test_rs") due to 1 previous error

[Darksonn]

Unfortunately this is just a bad error message, and Tokio can do nothing to improve it. It's a Rust bug, not a Tokio bug. I've converted your bug report to a discussion so lets discuss workarounds.

The usual workaround is to move the call to tokio::spawn into a non-async function:

async fn test_func() {
    test_func_spawn();
}

fn test_func_spawn() {
    _ = tokio::task::spawn(async move {
        tokio::time::sleep(std::time::Duration::from_secs(1)).await;
        test_func().await;
    });
}

#[tokio::main]
async fn main() {
    test_func().await;
}

or in your case just make test_func non-async:

fn test_func() {
    _ = tokio::task::spawn(async move {
        tokio::time::sleep(std::time::Duration::from_secs(1)).await;
        test_func();
    });
}

#[tokio::main]
async fn main() {
    test_func();
}

[fawdlstty]

Thank you for your answer. And, is there a Rust issue or RFC related to this bug?

[Darksonn]

There's probably a bug.