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

axum-extra/src/extract/cookie/private.rs

@@ -64,10 +64,11 @@ use std::{convert::Infallible, fmt, marker::PhantomData};
 ///     key: Key::generate(),
 /// };
 ///
-/// let app = Router::with_state(state)
+/// let app = Router::new()
 ///     .route("/set", post(set_secret))
-///     .route("/get", get(get_secret));
-/// # let app: Router<_> = app;
+///     .route("/get", get(get_secret))
+///     .with_state(state);
+/// # let _: axum::routing::RouterService = app;
 /// ```
 pub struct PrivateCookieJar<K = Key> {
     jar: cookie::CookieJar,