returning json - Handler<_, _> is not implemented

[yatesco]

Hi - quite simply, why doesn't the following code work:

use axum::{Json, Router};
use axum::routing::get;
use serde_json::{json, Value};

pub(crate) fn start() -> axum::routing::Router {
    let app = Router::new()
        .route("translations.json", get(handle_locale));
    app
}

fn handle_locale() -> Json<HashMap<String,String>> {
    let m = HashMap::default();
    Json(m)
}

HashMap<String,String> is serialisable by serde - I don't get the problem?

The error is as follows:

error[E0277]: the trait bound `fn() -> axum::Json<HashMap<std::string::String, std::string::String>> {handle_locale}: Handler<_, _>` is not satisfied
   --> src/i18n/mod.rs:19:41
    |
19  |         .route("translations.json", get(handle_locale));
    |                                     --- ^^^^^^^^^^^^^ the trait `Handler<_, _>` is not implemented for `fn() -> axum::Json<HashMap<std::string::String, std::string::String>> {handle_locale}`
    |                                     |
    |                                     required by a bound introduced by this call
    |
note: required by a bound in `axum::routing::get`
   --> /Users/coliny/.cargo/registry/src/github.com-1ecc6299db9ec823/axum-0.4.2/src/routing/method_routing.rs:394:1
    |
394 | 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)

I get the same error if I return serde::Value as well.

Help :-)

[jplatte]

If you run into Handler<_, _> is not implemented errors, enable axum's macros feature and annotate your handler with #[axum::debug_handler]. It will give you much better error messages. In this case it looks like you simply forgot to make your handler function async.

[yatesco]

excellent - thanks!

[jplatte]

Updated my anwer to link to axum-macros instead of axum-debug 🙂

… and again to axum's macros feature.

[hanMaster]

I have handler in my code:
#[axum_macros::debug_handler] pub async fn create_account( Json(user): Json<RequestedUser>, Extension(db): Extension<DatabaseConnection>, ) -> Result<Json<ResponseUser>, StatusCode> { ... }
it cause error:
error[E0277]: the trait bound axum::Json: FromRequestParts<()>is not satisfied --> src/routes/create_account.rs:21:17 | 21 | Json(user): Json<RequestedUser>, | ^^^^ the traitFromRequestParts<()>is not implemented foraxum::Json|

but if I made Json as second parameter, it works fine
#[axum_macros::debug_handler] pub async fn create_account( Extension(db): Extension<DatabaseConnection>, Json(user): Json<RequestedUser>, ) -> Result<Json<ResponseUser>, StatusCode> { ... }

[jplatte]

This is intentional. https://docs.rs/axum/latest/axum/extract/index.html#the-order-of-extractors

[mustafa-ans]

@hanMaster Your solution to that error saved me lol
Thanks a million!