Type safe state inheritance (#1532)

* Make state type safe

* fix examples

* remove unnecessary `#[track_caller]`s

* Router::into_service -> Router::with_state

* fixup docs

* macro docs

* add missing docs

* fix examples

* format

* changelog

* Update trybuild tests

* Make sure fallbacks are still inherited for opaque services (#1540)

* Document nesting routers with different state

* fix leftover conflicts

examples/todos/src/main.rs

@@ -46,7 +46,7 @@ async fn main() {
     let db = Db::default();
 
     // Compose the routes
-    let app = Router::with_state(db)
+    let app = Router::new()
         .route("/todos", get(todos_index).post(todos_create))
         .route("/todos/:id", patch(todos_update).delete(todos_delete))
         // Add middleware to all routes
@@ -65,7 +65,8 @@ async fn main() {
                 .timeout(Duration::from_secs(10))
                 .layer(TraceLayer::new_for_http())
                 .into_inner(),
-        );
+        )
+        .with_state(db);
 
     let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
     tracing::debug!("listening on {}", addr);