have been declared yet on the router,
since the new layer will have no effect, and this is typically a bug.
In generic code, you can test if that is the case first, by calling [`Router::has_routes`].

# Example

```rust
use axum::{
    routing::get,
    Router,
};
use tower_http::validate_request::ValidateRequestHeaderLayer;

let app = Router::new()
    .route("/foo", get(|| async {}))
    .route_layer(ValidateRequestHeaderLayer::bearer("password"));

// `GET /foo` with a valid token will receive `200 OK`
// `GET /foo` with a invalid token will receive `401 Unauthorized`
// `GET /not-found` with a invalid token will receive `404 Not Found`
# let _: Router = app;
```