Use the diagnostic namespace (#2246)

axum-core/src/extract/mod.rs

@@ -45,7 +45,7 @@ mod private {
 #[async_trait]
 #[cfg_attr(
     nightly_error_messages,
-    rustc_on_unimplemented(
+    diagnostic::on_unimplemented(
         note = "Function argument is not a valid axum extractor. \nSee `https://docs.rs/axum/latest/axum/extract/index.html` for details",
     )
 )]
@@ -72,7 +72,7 @@ pub trait FromRequestParts<S>: Sized {
 #[async_trait]
 #[cfg_attr(
     nightly_error_messages,
-    rustc_on_unimplemented(
+    diagnostic::on_unimplemented(
         note = "Function argument is not a valid axum extractor. \nSee `https://docs.rs/axum/latest/axum/extract/index.html` for details",
     )
 )]

axum-core/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))]
 //! Core types and traits for [`axum`].
 //!
 //! Libraries authors that want to provide [`FromRequest`] or [`IntoResponse`] implementations
@@ -44,7 +44,7 @@
     missing_debug_implementations,
     missing_docs
 )]
-#![deny(unreachable_pub, private_in_public)]
+#![deny(unreachable_pub)]
 #![allow(elided_lifetimes_in_paths, clippy::type_complexity)]
 #![forbid(unsafe_code)]
 #![cfg_attr(test, allow(clippy::float_cmp))]