Trait and await in a handler.

[mqsoh]

• I have looked for existing issues (including closed) about this

Is it related to this?

Bug Report

Version

axum_debug v0.1.0 (/home/mason/code/axum_debug)
├── axum v0.6.1
│   ├── axum-core v0.3.0

Platform

Linux pufferfish 5.15.0-56-generic #62-Ubuntu SMP Tue Nov 22 19:54:14 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Description

I'm trying to use a trait with an await in a handler. (Not sure what terminology to use.) It's not working. I tried to narrow down the issue and came up with this.

use axum::http::StatusCode;
use tokio::time::{sleep, Duration};

struct Concrete {}

trait SlowDrying {
    fn hand_print(&self) -> &'static str;
}

impl SlowDrying for Concrete {
    fn hand_print(&self) -> &'static str {
        "🖐️"
    }
}

async fn working1() -> Result<&'static str, (StatusCode, &'static str)> {
    let x = Box::new(Concrete {});
    sleep(Duration::from_secs(1)).await;
    Ok(x.hand_print())
}

async fn working2() -> Result<&'static str, (StatusCode, &'static str)> {
    let x: Box<dyn SlowDrying> = Box::new(Concrete {});
    Ok(x.hand_print())
}

async fn broken() -> Result<&'static str, (StatusCode, &'static str)> {
    let x: Box<dyn SlowDrying> = Box::new(Concrete {});
    sleep(Duration::from_secs(1)).await;
    Ok(x.hand_print())
}

#[cfg(test)]
mod tests {
    use super::*;
    use axum::routing::get;

    #[tokio::test]
    async fn test() {
        get(working1);
        get(working2);
        get(broken);
    }
}

I get this error:

   Compiling axum_debug v0.1.0 (/home/mason/code/axum_debug)
error[E0277]: the trait bound `fn() -> impl Future<Output = Result<&'static str, (StatusCode, &'static str)>> {broken}: Handler<_, _, _>` is not satisfied
   --> src/main.rs:42:13
    |
42  |         get(broken);
    |         --- ^^^^^^ the trait `Handler<_, _, _>` is not implemented for fn item `fn() -> impl Future<Output = Result<&'static str, (StatusCode, &'static str)>> {broken}`
    |         |
    |         required by a bound introduced by this call
    |
    = help: the trait `Handler<T, S, B2>` is implemented for `Layered<L, H, T, S, B, B2>`
note: required by a bound in `axum::routing::get`
   --> /home/mason/.cargo/registry/src/github.com-1ecc6299db9ec823/axum-0.6.1/src/routing/method_routing.rs:403:1
    |
403 | top_level_handler_fn!(get, GET);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `axum::routing::get`
    = note: this error originates in the macro `top_level_handler_fn` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.
error: could not compile `axum_debug` due to previous error

I feel like it's probably something I don't understand about async code... Are you able to point me in the right direction?

[programatik29]

Use debug_handler macro to get a better error message. Rust compiler isn't smart enough to generate better error message yet so we developed a macro for this.

[davidpdrsn]

Does using Box<dyn SlowDrying + Send> work?

[mqsoh]

That was it! The debug_handler made it explicit -- I needed Send. Thank you both so much for your help. I'm really enjoying Axum.

For completeness:

use axum::debug_handler;
use axum::http::StatusCode;
use tokio::time::{sleep, Duration};

struct Concrete {}

trait SlowDrying {
    fn hand_print(&self) -> &'static str;
}

impl SlowDrying for Concrete {
    fn hand_print(&self) -> &'static str {
        "🖐️"
    }
}

async fn working1() -> Result<String, (StatusCode, String)> {
    let x = Box::new(Concrete {});
    sleep(Duration::from_secs(1)).await;
    Ok(x.hand_print().into())
}

async fn working2() -> Result<&'static str, (StatusCode, &'static str)> {
    let x: Box<dyn SlowDrying + Send> = Box::new(Concrete {});
    Ok(x.hand_print())
}

#[debug_handler]
async fn broken() -> Result<&'static str, (StatusCode, &'static str)> {
    let x: Box<dyn SlowDrying + Send> = Box::new(Concrete {});
    sleep(Duration::from_secs(1)).await;
    Ok(x.hand_print())
}

#[cfg(test)]
mod tests {
    use super::*;
    use axum::routing::get;
    use axum::Router;

    #[tokio::test]
    async fn test() {
        let router = Router::new()
            .route("/working1", get(working1))
            .route("/working2", get(working2))
            .route("/now-working", get(broken));
        axum::Server::bind(&std::net::SocketAddr::from(([127, 0, 0, 0], 8765)))
            .serve(router.into_make_service()); // don't actually wait for it
    }
}

[willemvermeer]

Thanks for posting this!

I'd like to add, if you're invoking another method from the original broken(), you need to add Sync too:

#[debug_handler]
async fn broken() -> Result<&'static str, (StatusCode, &'static str)> {
    let x: Box<dyn SlowDrying + Send + Sync> = Box::new(Concrete {});
    use_trait(x).await;
    Ok(x.hand_print())
}
async fn use_trait(x: &Box<dyn SlowDrying + Send + Sync>) -> Result<Uuid, Error> {
    Ok(Uuid::new_v4())
}