routing with closures rather than async fn

[yatesco]

Hi - first, it's me, not you :-), but I don't know why it's me.

I'm converting a tide web app to Axum and there are 50 odd routes, each of which is a closure on the main app:

let mut app = tide::new();
    app.at("/xyz/something")
        .post(|mut req: Request<()>| async move {
            let result = ...;
            json_and_cors_it_up(result)
        });
    app.at("/xyz/something-else")
        .post(|mut req: Request<()>| async move {
            let result = ...;
            json_and_cors_it_up(result)
        });

I've tried to use each closure as a handler:

let mut app = Router::new().route(
            "/xyz/something",
            post(
                |TypedHeader(context): TypedHeader<RequestContext>, Json(filter): Json<UIState>| -> anyhow::Result<dyn IntoResponse> {
                    async {
                        let result = ...;
                        json_and_cors_it_up(result)
                    }
                },
            ),
        ).route(
            "/xyz/something-else",
            post(
                |TypedHeader(context): TypedHeader<RequestContext>, Json(filter): Json<UIState>| -> anyhow::Result<dyn IntoResponse> {
                    async {
                        let result = ...;
                        json_and_cors_it_up(result)
                    }
                },
            ),
        );

fn json_and_cors_it_up<T>(result: T) -> anyhow::Result<impl IntoResponse>
where
    T: serde::Serialize,
{
    let json = serde_json::to_string_pretty(&result)?;
    let response = (
        StatusCode::OK,
        [
            (header::CONTENT_TYPE, "application/json"),
            (header::ACCESS_CONTROL_ALLOW_ORIGIN, "*"),
        ],
        json,
    );
    Ok(response)
}

but I'm getting the error:

error[E0308]: mismatched types
    --> dashboard_backend_server/src/main.rs:1007:21
     |
1007 | /                     async {
1008 | |                         let filter = sanitise(&context, filter)
1009 | |                             .await
1010 | |                             .expect("cannot sanitise filter");
...    |
1013 | |                         json_and_cors_it_up(result)
1014 | |                     }
     | |_____________________^ expected `Result<dyn IntoResponse, Error>`, found `async` block
...
1031 |   fn json_and_cors_it_up<T>(result: T) -> anyhow::Result<impl IntoResponse>
     |                                                          ----------------- the found opaque type
     |
     = note:       expected enum `Result<dyn IntoResponse, anyhow::Error>`
             found `async` block `{async block@dashboard_backend_server/src/main.rs:1007:21: 1014:22}`

error[E0277]: the trait bound `{closure@dashboard_backend_server/src/main.rs:1004:17: 1006:53}: Handler<_, _, _>` is not satisfied
    --> dashboard_backend_server/src/main.rs:1004:17
     |
1003 |               post(
     |               ---- required by a bound introduced by this call
1004 | /                 |TypedHeader(context): TypedHeader<RequestContext>,
1005 | |                  extract::Json(filter): extract::Json<dashboard_api::filter::UIState>|
1006 | |                  -> anyhow::Result<dyn IntoResponse> {
1007 | |                     async {
...    |
1014 | |                     }
1015 | |                 },
     | |_________________^ the trait `Handler<_, _, _>` is not implemented for closure `{closure@dashboard_backend_server/src/main.rs:1004:17: 1006:53}`
     |
     = help: the following other types implement trait `Handler<T, S, B>`:
               <Layered<L, H, T, S, B, B2> as Handler<T, S, B2>>
               <MethodRouter<S, B> as Handler<(), S, B>>
note: required by a bound in `post`
    --> /Users/coliny/.cargo/registry/src/index.crates.io-6f17d22bba15001f/axum-0.6.20/src/routing/method_routing.rs:407:1
     |
407  | top_level_handler_fn!(post, POST);
     | ^^^^^^^^^^^^^^^^^^^^^^----^^^^^^^
     | |                     |
     | |                     required by a bound in this function
     | required by this bound in `post`
     = note: this error originates in the macro `top_level_handler_fn` (in Nightly builds, run with -Z macro-backtrace for more info)

Any suggestions? Thanks!

[davidpdrsn]

anyhow::Error doesn’t implement IntoResponse. You have to make a wrapper that does.