How to use associated function and associated type to create handler and router?

[benlau6]

I need to implement multiple similar routes, which contains lots of handler. Some of the handler could be reused, while some of the handlers' logic are different. So I try to make a handler trait that can be implemented for some unit structs. I managed to make the following code work

use axum::{
    routing::get,
    Router,
};
use async_trait::async_trait;

#[async_trait]
pub trait TraitHandler: Clone + Send + Sync + 'static {
    async fn get_item() -> String;
    fn create_router() -> Router {
        Router::new().route("/", get(Self::get_item))
    }
}

But then it fails to compile if I add associated type to the handler parameter.

use axum::{
    extract::Json,
    routing::get,
    Router,
};
use async_trait::async_trait;

#[async_trait]
pub trait TraitHandler: Clone + Send + Sync + 'static {
    type Req;
    async fn get_item(Json(payload): Json<Self::Req>) -> String;
    fn create_router() -> Router {
        Router::new().route("/", get(Self::get_item))
    }
}

with compile error

error[E0277]: the trait bound `fn(axum::Json<<Self as trait_handler::TraitHandler>::Req>) -> Pin<Box<dyn futures::Future<Output = std::string::String> + std::marker::Send>> {<Self as trait_handler::TraitHandler>::get_item::<'_>}: Handler<_, _, _>` is not satisfied
   --> src/playground/trait_handler.rs:19:38
    |
19  |         Router::new().route("/", get(Self::get_item))
    |                                  --- ^^^^^^^^^^^^^^ the trait `Handler<_, _, _>` is not implemented for fn item `fn(axum::Json<<Self as trait_handler::TraitHandler>::Req>) -> Pin<Box<dyn futures::Future<Output = std::string::String> + std::marker::Send>> {<Self as trait_handler::TraitHandler>::get_item::<'_>}`
    |                                  |
    |                                  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>>
              <Layered<L, H, T, S, B, B2> as Handler<T, S, B2>>
              <MethodRouter<S, B> as Handler<(), S, B>>
              <Or<L, R, Lt, Rt, S, B> as Handler<(M, Lt, Rt), S, B>>
note: required by a bound in `axum::routing::get`
   --> /Users/benyp/.cargo/registry/src/github.com-1ecc6299db9ec823/axum-0.6.15/src/routing/method_routing.rs:403:1
    |
403 | top_level_handler_fn!(get, GET);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `get`

I don't know what to do next based on the given errors, and #[axum_macros::debug_handler] seems not working in trait. Could someone help me please?

[benlau6]

Finally solved it. T in Json<T> needs to be bounded by Send + DeserializeOwned.
So the associated type should be defined as type Req: Send + DeserializeOwned