`impl Trait` not allowed outside of function and inherent method return types

Question

trait Responder{
}

struct App;

impl App {
    fn new() -> Self {
        Self {}
    }
    fn service<T, B>(&self, routes: T) -> Self where T: Fn()-> impl Responder {
        Self {}
    }
}

struct Routes {
    data:String,
}

impl Responder for Routes{
}

fn routes() -> impl Responder {
    Routes {
        data: "Hello".to_string()
    }
}

fn main() {
    // let new_routes = routes;
    App::new().service(routes);
}

How do I pass a function a parameter that return impl Trait or in my case impl Responder.

The error that it gives is: impl Trait not allowed outside of function and inherent method return types.

Answer 1

You can't - the docs (and the error) are explicit that the impl trait syntax can only be used when returning a value from a function to help the compiler deduce things (about a value). In your case you are trying to say something about a generic type, which is not the same and thus this syntax cannot be used.

Instead, you have to specify a type, in your case a generic one with a constraint:

fn service<T, B>(&self, routes: T) -> Self where T: Fn()->B, B: Responder

assuming you did not need B for something else. Note routes returns a value with a specific type, not an impl trait, and this type is what you are trying to write out.