Different combination of extractors in a method router/handler causes errors (0.6.0-rc.2)

[gak]

Hi,

I have a method router in 0.6.0-rc.2:

pub async fn set_hero(
    user: AuthenticatedUser,
    Path(config_id): Path<ConfigId>,
    Json(request): Json<SetHeroRequest>,
    State(mut db): State<Db>,
) -> Result<impl IntoResponse, ApiError> {
    Ok(Json(Empty {}))
}

Set up like this:

 let app = Router::with_state(state.clone())
        .route("/api/auth/start/", get(api::oauth::auth_start))
        .route("/api/auth/complete/", get(api::oauth::auth_complete))
        .route(
            "/api/counters/:config_id/set-hero/",
            post(api::config::set_hero),  // <-- This is the fn in question
        )
        .route("/api/counters/strengths/", get(api::config::strengths))
        // Few more similar routes, then a layer for `AuthenticatedUser` to access:
        .layer(Extension(state));

Strangely, if I comment a certain set of combination of extractors, it builds, otherwise when there are these 4 as is it fails to build:

error[E0277]: the trait bound `fn(AuthenticatedUser, axum::extract::Path<ConfigId>, axum::Json<SetHeroRequest>, State<Db>) -> impl Future<Output = Result<impl IntoResponse, ApiError>> {set_hero}: Handler<_, _, _>` is not satisfied
   --> server\src\main.rs:77:18
    |
77  |             post(api::config::set_hero),
    |             ---- ^^^^^^^^^^^^^^^^^^^^^ the trait `Handler<_, _, _>` is not implemented for `fn(AuthenticatedUser, axum::extract::Path<ConfigId>, axum::Json<SetHeroRequest>, State<Db>) -> impl Future<Output = Result<impl IntoResponse, ApiError>> {set_hero}`
    |             |
    |             required by a bound introduced by this call
    |
    = help: the trait `Handler<T, S, B>` is implemented for `axum::handler::Layered<L, H, T, S, B>`
note: required by a bound in `post`
   --> .cargo\registry\src\github.com-1ecc6299db9ec823\axum-0.6.0-rc.2\src\routing\method_routing.rs:404:1
    |
404 | 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 combinations that work seem strange to me. Any two of these combined compile fine. 3 work together only sometimes, i.e. user + path + json works, and user + path + state works, but path + json + state does not work.

I'm having trouble finding clues to what the error means and how I can fix the problem, as I can't pinpoint what extractor might be causing the problem, or if I'm doing something silly otherwise.

AuthenticatedUser is a custom extractor that implements Deref and FromRequestParts.

State<Db> is from a typical (from examples) AppState with a FromRef<AppState> impl.

SetHeroRequest has a serde::Deserialize derive, and nothing apparently changes when I make the request struct empty.

Let me know if you need more code or a full reproduction.

[davidpdrsn]

Try https://docs.rs/axum/0.6.0-rc.2/axum/attr.debug_handler.html

But it most likely that you need to make Json the last argument. See https://docs.rs/axum/0.6.0-rc.2/axum/extract/index.html#the-order-of-extractors

[gak]

Yup that did it. I somehow missed that section when I was looking into it earlier.

Many thanks!