feat: add #[diagnostic::on_unimplemented] to IntoResponse and IntoResponseParts (#3723)

axum-core/src/response/into_response.rs

@@ -109,6 +109,9 @@ use std::{
 /// let app = Router::new().route("/", get(|| async { MyBody }));
 /// # let _: Router = app;
 /// ```
+#[diagnostic::on_unimplemented(
+    note = "See `https://docs.rs/axum/0.8/axum/response/trait.IntoResponse.html` for details"
+)]
 pub trait IntoResponse {
     /// Create a response.
     #[must_use]

axum-core/src/response/into_response_parts.rs

@@ -70,6 +70,9 @@ use std::{convert::Infallible, fmt};
 ///     SetHeader("x-foo", "custom")
 /// }
 /// ```
+#[diagnostic::on_unimplemented(
+    note = "See `https://docs.rs/axum/0.8/axum/response/trait.IntoResponseParts.html` for details"
+)]
 pub trait IntoResponseParts {
     /// The type returned in the event of an error.
     ///