Better error message

[DaliborTrampota]

``Hi, Im not sure whether to report this as bug report or request as a feature but I've spend like 3 hours figuring out what the issue is with my middleware and Extension extractor.

error[E0277]: the trait bound `fn(Extension<User>) -> impl futures::Future<Output = impl IntoResponse> {handlers::auth::logged}: Handler<_, _, _>` is not satisfied
   --> src\handlers.rs:38:30
    |
38  |         .route("/user", post(logged).route_layer(&user_middleware))
    |                         ---- ^^^^^^ the trait `Handler<_, _, _>` is not implemented for fn item `fn(Extension<User>) -> impl futures::Future<Output = impl IntoResponse> {handlers::auth::logged}`
    |                         |
    |                         required by a bound introduced by this call
    |
    = help: the following other types implement trait `Handler<T, S, B>`:
              <IntoHandler<H, T, S, B> as Handler<T, S, B>>
              <MethodRouter<S, B> as Handler<(), S, B>>
              <axum::handler::Layered<L, H, T, S, B, B2> as Handler<T, S, B2>>
              <axum_extra::handler::Or<L, R, Lt, Rt, S, B> as Handler<(M, Lt, Rt), S, B>>
note: required by a bound in `post`
   --> C:\Users\dalib\.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 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)

the issue is my User struct is not deriving Clone trait.. I'd say im intermediate in rust but i still cant read those errors and I tried #[debug_handler] but it didnt do anything

im not even joking when I say that my head hurts. I dont want others to experience this :)
Thank

[davidpdrsn]

Believe me if we could improve this we would. You’ll find other issues saying similar things.

The compiler simply doesn’t generate good errors for the kinds of trait implementations axum uses. The problem is not unique to axum and there is a rustc issue about it.

We provide https://docs.rs/axum/0.6.20/axum/attr.debug_handler.html which helps in a lot of cases.

[DaliborTrampota]

We provide https://docs.rs/axum/0.6.20/axum/attr.debug_handler.html which helps in a lot of cases.

Yes i mentioned that I tried that but it did not change the error message at all

[davidpdrsn]

Ah I see. We probably should add special case for Extension and Clone. We do that for some other know types. I’ll make a separate issue for that!

Update: Done #2294!