Use the diagnostic namespace (#2246)

axum/src/handler/mod.rs

@@ -127,7 +127,7 @@ pub use self::service::HandlerService;
 /// ```
 #[cfg_attr(
     nightly_error_messages,
-    rustc_on_unimplemented(
+    diagnostic::on_unimplemented(
         note = "Consider using `#[axum::debug_handler]` to improve the error message"
     )
 )]

axum/src/lib.rs

@@ -1,4 +1,4 @@
-#![cfg_attr(nightly_error_messages, allow(internal_features), feature(rustc_attrs))]
+#![cfg_attr(nightly_error_messages, feature(diagnostic_namespace))]
 //! axum is a web application framework that focuses on ergonomics and modularity.
 //!
 //! # Table of contents
@@ -416,7 +416,7 @@
     missing_debug_implementations,
     missing_docs
 )]
-#![deny(unreachable_pub, private_in_public)]
+#![deny(unreachable_pub)]
 #![allow(elided_lifetimes_in_paths, clippy::type_complexity)]
 // can't be `forbid` since we've vendored code from hyper-util that contains `unsafe`
 // when hyper-util is on crates.io we can stop vendoring it and go back to `forbid`