can't compile when async function block has variable not implement Send

[anonymous5l]

Summary

#[derive(Debug)]
pub struct NotSend;
impl !Send for NotSend {}
pub async fn empty_func() {}

#[debug_handler]
pub async fn weird_func() -> impl IntoResponse {
    let a = NotSend;
    println!(
        "it will be compile when `async` function not use below {:?}",
        a
    );
    empty_func().await;
    ()
}

#[tokio::main]
async fn main() {
    let app = Router::new().route("/", get(weird_func));
    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    axum::serve(listener, app).await.unwrap();
}

the debug_handler tell me this error message.

  --> src/main.rs:17:18
   |
15 |     let a = NotSend;
   |         - has type `NotSend` which is not `Send`
16 |     println!("i can print it {:?}", a);
17 |     empty_func().await;
   |                  ^^^^^ await occurs here, with `a` maybe used later

that's weird i can even print it or do some operation but can't use async func after that variable.

can anyone help me dig in why it happens. also my rust version is 1.83.0.

axum version

0.7.9

[anonymous5l]

fix compile ! symbol can't use problem.

the following code should not compile, but it can.

#[derive(Debug)]
pub struct NotSend {
    _not_send_sync: PhantomData<*const ()>,
}

pub async fn empty_func() {}

#[debug_handler]
pub async fn weird_func() -> impl IntoResponse {
    empty_func().await;

    let a = NotSend {
        _not_send_sync: PhantomData,
    };

    println!(
        "it will be compile when `async` function not use below {:?}",
        a
    );
    ()
}

#[tokio::main]
async fn main() {
    let app = Router::new().route("/", get(weird_func));
    let listener = tokio::net::TcpListener::bind("0.0.0.0:4000").await.unwrap();
    axum::serve(listener, app).await.unwrap();
}

[anonymous5l]

there may have some help
rust-lang/rust#57478
rust-lang/rust#63768