Pencil: A Microframework Inspired by Flask for Rust(fengsp.github.io)

    struct RouteUser {
        route: &'static str,
        f: fn(_: &mut Request) -> PencilResult
    }

    #[route("/user")]
    fn user(_: &mut Request) -> PencilResult {}

    fn user() -> RouteUser {
        // The actual function the user wrote:
        fn user(_: &mut Request) -> PencilResult {
            // ...
        }

        RouteUser {
            // Generated from the annotation argument
            route: "/user",
            f: user
        }
    }

    trait Route {}

    impl Route for RouteUser {}

    app.route(user());

    fn route<R>(r: R) where R: Route {
        // ...
    }