Improve error messages with #[diagnostic::do_not_recommend] (#3588)

axum-core/src/extract/option.rs

@@ -35,6 +35,9 @@ pub trait OptionalFromRequest<S, M = private::ViaRequest>: Sized {
     ) -> impl Future<Output = Result<Option<Self>, Self::Rejection>> + Send;
 }
 
+// Compiler hint just says that there is an impl for Option<T>, not mentioning
+// the bounds, which is not very helpful.
+#[diagnostic::do_not_recommend]
 impl<S, T> FromRequestParts<S> for Option<T>
 where
     T: OptionalFromRequestParts<S>,
@@ -50,6 +53,7 @@ where
     }
 }
 
+#[diagnostic::do_not_recommend]
 impl<S, T> FromRequest<S> for Option<T>
 where
     T: OptionalFromRequest<S>,

axum-core/src/extract/request_parts.rs

@@ -65,6 +65,7 @@ where
     }
 }
 
+#[diagnostic::do_not_recommend] // pretty niche impl
 impl<S> FromRequest<S> for BytesMut
 where
     S: Send + Sync,
@@ -134,6 +135,7 @@ where
     }
 }
 
+#[diagnostic::do_not_recommend] // pretty niche impl
 impl<S> FromRequestParts<S> for Parts
 where
     S: Send + Sync,
@@ -145,6 +147,7 @@ where
     }
 }
 
+#[diagnostic::do_not_recommend] // pretty niche impl
 impl<S> FromRequestParts<S> for Extensions
 where
     S: Send + Sync,

axum-core/src/extract/tuple.rs

@@ -3,6 +3,7 @@ use crate::response::{IntoResponse, Response};
 use http::request::Parts;
 use std::{convert::Infallible, future::Future};
 
+#[diagnostic::do_not_recommend]
 impl<S> FromRequestParts<S> for ()
 where
     S: Send + Sync,
@@ -18,6 +19,7 @@ macro_rules! impl_from_request {
     (
         [$($ty:ident),*], $last:ident
     ) => {
+        #[diagnostic::do_not_recommend]
         #[allow(non_snake_case, unused_mut, unused_variables)]
         impl<S, $($ty,)* $last> FromRequestParts<S> for ($($ty,)* $last,)
         where
@@ -43,6 +45,7 @@ macro_rules! impl_from_request {
 
         // This impl must not be generic over M, otherwise it would conflict with the blanket
         // implementation of `FromRequest<S, Mut>` for `T: FromRequestParts<S>`.
+        #[diagnostic::do_not_recommend]
         #[allow(non_snake_case, unused_mut, unused_variables)]
         impl<S, $($ty,)* $last> FromRequest<S> for ($($ty,)* $last,)
         where