Dependency Injection question

[Daniel-Xu]

I'm using DI similar to the example in the repo. However，I don't quite understand why we can't use impl Trait as handler parameter? (In the meantime, we can use impl Trait in the return type of the handler)

here is the handler

pub async fn get_films(State(repo): State<impl FilmRepo + Send + Sync>) -> impl IntoResponse {
    StatusCode::OK
}

and here is the error:

I changed the code and use dynamic dispatch, it works fine. However, there's impl IntoResponse in the return type, why didn't the compiler complain about it?

type DynFilmRepo = Arc<dyn FilmRepo + Send + Sync>;
pub async fn get_films(State(repo): State<DynFilmRepo>) -> impl IntoResponse {
    StatusCode::OK
}

[jplatte]

For the return type, the compiler can infer the concrete type based on the function body. For an impl Trait argument, it is simply impossible for the compiler to infer the correct type. You can still have generic handlers, but you have to use a named generic parameter, and specify it when registering the handler:

pub async fn get_films<R>(State(repo): State<R>) -> impl IntoResponse
where
    R: FilmRepo + Send + Sync
{
    StatusCode::OK
}

Router::new()
    .route("/", get_films::<FilmRepoImpl>)