Making a generic router

[willburden]

I'm trying to create a generic function (resource_router) that returns a Router, so I can eventually have something like:

// Where Account and Post are structs that implement Resource:

fn router() -> Router {
    Router::new()
        .nest("/accounts", resource_router::<Account>)
        .nest("/posts", resource_router::<Post>)
}

Here is a minimal reproduction of the error I'm encountering:

use axum::{Router, routing::get, Json};
use serde::{de::DeserializeOwned, Deserialize};

trait Resource: DeserializeOwned {}

#[derive(Deserialize)]
struct Account {
    email: String,
}

impl Resource for Account {}

fn resource_router<R: Resource>() -> Router {
    Router::new().route("/", get(get_resource::<R>))
}

async fn get_resource<R: Resource>(Json(_payload): Json<R>) {
    todo!()
}

And the error message:

error[E0277]: the trait bound `fn(axum::Json<R>) -> impl std::future::Future<Output = ()> {routes::get_resource::<R>}: axum::handler::Handler<_, _, _>` is not satisfied
   --> src/routes.rs:13:34
    |
13  |     Router::new().route("/", get(get_resource::<R>))
    |                              --- ^^^^^^^^^^^^^^^^^ the trait `axum::handler::Handler<_, _, _>` is not implemented for fn item `fn(axum::Json<R>) -> impl std::future::Future<Output = ()> {routes::get_resource::<R>}`
    |                              |
    |                              required by a bound introduced by this call
    |
    = help: the following other types implement trait `axum::handler::Handler<T, S, B>`:
              <axum::handler::Layered<L, H, T, S, B, B2> as axum::handler::Handler<T, S, B2>>
              <axum::routing::MethodRouter<S, B> as axum::handler::Handler<(), S, B>>
note: required by a bound in `axum::routing::get`
   --> /home/will/.cargo/registry/src/index.crates.io-6f17d22bba15001f/axum-0.6.20/src/routing/method_routing.rs:403:1
    |
403 | top_level_handler_fn!(get, GET);
    | ^^^^^^^^^^^^^^^^^^^^^^---^^^^^^
    | |                     |
    | |                     required by a bound in this function
    | required by this bound in `get`
    = note: this error originates in the macro `top_level_handler_fn` (in Nightly builds, run with -Z macro-backtrace for more info)

I tried using axum_macros::debug_handler but discovered that it doesn't support generic functions. I believe that my handler here satisfies all the requirements listed in the documentation:

• Are async fns.
• Take no more than 16 arguments that all implement FromRequest.
• Returns something that implements IntoResponse.
• If a closure is used it must implement Clone + Send and be 'static.
• Returns a future that is Send. The most common way to accidentally make a future !Send is to hold a !Send type across an await.

I've found that the error vanishes if I make any of the following changes:

• Make resource_router use a specific Resource:

  fn resource_router() -> Router {
      Router::new().route("/", get(get_resource::<Account>))
  }

• Remove the Json extractor from the handler:

  async fn get_resource<R: Resource>() {
      todo!()
  }

So my guess is that the Json extractor isn't meeting the FromRequest trait bound; it's as though Resource doesn't have a Deserialize trait bound, even though it does.

Any help would be greatly appreciated.

[davidpdrsn]

You need to also make sure that R is Send + 'static:

trait Resource: DeserializeOwned + Send + 'static {}

fn resource_router<R: Resource>() -> Router {
    Router::new().route("/", get(get_resource::<R>))
}

async fn get_resource<R: Resource>(_: Json<R>) {}

The Send bound comes from the impl for Handler for functions that take one argument:

and the 'static bound comes from get: