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

axum-macros/tests/debug_handler/fail/argument_not_extractor.stderr

@@ -7,14 +7,14 @@ error[E0277]: the trait bound `bool: FromRequest<(), axum_core::extract::private
   = note: Function argument is not a valid axum extractor.
           See `https://docs.rs/axum/0.8/axum/extract/index.html` for details
   = help: the following other types implement trait `FromRequestParts<S>`:
-            `()` implements `FromRequestParts<S>`
+            `ConnectInfo<T>` implements `FromRequestParts<S>`
-            `(T1, T2)` implements `FromRequestParts<S>`
+            `Extension<T>` implements `FromRequestParts<S>`
-            `(T1, T2, T3)` implements `FromRequestParts<S>`
+            `HeaderMap` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4)` implements `FromRequestParts<S>`
+            `MatchedPath` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4, T5)` implements `FromRequestParts<S>`
+            `Method` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4, T5, T6)` implements `FromRequestParts<S>`
+            `OriginalUri` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4, T5, T6, T7)` implements `FromRequestParts<S>`
+            `Query<T>` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4, T5, T6, T7, T8)` implements `FromRequestParts<S>`
+            `RawPathParams` implements `FromRequestParts<S>`
           and $N others
   = note: required for `bool` to implement `FromRequest<(), axum_core::extract::private::ViaParts>`
 note: required by a bound in `__axum_macros_check_handler_0_from_request_check`

axum-macros/tests/debug_handler/fail/extension_not_clone.stderr

@@ -5,15 +5,14 @@ error[E0277]: the trait bound `NonCloneType: Clone` is not satisfied
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `NonCloneType`, which is required by `Extension<NonCloneType>: FromRequest<(), _>`
   |
   = help: the following other types implement trait `FromRequest<S, M>`:
-            (T1, T2)
+            Body
-            (T1, T2, T3)
+            Form<T>
-            (T1, T2, T3, T4)
+            Json<T>
-            (T1, T2, T3, T4, T5)
+            RawForm
-            (T1, T2, T3, T4, T5, T6)
+            Result<T, <T as FromRequest<S>>::Rejection>
-            (T1, T2, T3, T4, T5, T6, T7)
+            String
-            (T1, T2, T3, T4, T5, T6, T7, T8)
+            axum::body::Bytes
-            (T1, T2, T3, T4, T5, T6, T7, T8, T9)
+            axum::http::Request<Body>
-          and $N others
   = note: required for `Extension<NonCloneType>` to implement `FromRequestParts<()>`
   = note: required for `Extension<NonCloneType>` to implement `FromRequest<(), axum_core::extract::private::ViaParts>`
 note: required by a bound in `__axum_macros_check_test_extension_non_clone_0_from_request_check`

axum-macros/tests/from_request/fail/generic_without_via.stderr

@@ -13,9 +13,6 @@ error[E0277]: the trait bound `fn(Extractor<()>) -> impl Future<Output = ()> {fo
    |                                        required by a bound introduced by this call
    |
    = note: Consider using `#[axum::debug_handler]` to improve the error message
-   = help: the following other types implement trait `Handler<T, S>`:
-             `Layered<L, H, T, S>` implements `Handler<T, S>`
-             `MethodRouter<S>` implements `Handler<(), S>`
 note: required by a bound in `axum::routing::get`
   --> $WORKSPACE/axum/src/routing/method_routing.rs
    |

axum-macros/tests/from_request/fail/generic_without_via_rejection.stderr

@@ -13,9 +13,6 @@ error[E0277]: the trait bound `fn(Extractor<()>) -> impl Future<Output = ()> {fo
    |                                        required by a bound introduced by this call
    |
    = note: Consider using `#[axum::debug_handler]` to improve the error message
-   = help: the following other types implement trait `Handler<T, S>`:
-             `Layered<L, H, T, S>` implements `Handler<T, S>`
-             `MethodRouter<S>` implements `Handler<(), S>`
 note: required by a bound in `axum::routing::get`
   --> $WORKSPACE/axum/src/routing/method_routing.rs
    |

axum-macros/tests/from_request/fail/override_rejection_on_enum_without_via.stderr

@@ -13,9 +13,6 @@ error[E0277]: the trait bound `fn(MyExtractor) -> impl Future<Output = ()> {hand
    |                                              required by a bound introduced by this call
    |
    = note: Consider using `#[axum::debug_handler]` to improve the error message
-   = help: the following other types implement trait `Handler<T, S>`:
-             `Layered<L, H, T, S>` implements `Handler<T, S>`
-             `MethodRouter<S>` implements `Handler<(), S>`
 note: required by a bound in `axum::routing::get`
   --> $WORKSPACE/axum/src/routing/method_routing.rs
    |
@@ -35,9 +32,6 @@ error[E0277]: the trait bound `fn(Result<MyExtractor, MyRejection>) -> impl Futu
    |                                                           required by a bound introduced by this call
    |
    = note: Consider using `#[axum::debug_handler]` to improve the error message
-   = help: the following other types implement trait `Handler<T, S>`:
-             `Layered<L, H, T, S>` implements `Handler<T, S>`
-             `MethodRouter<S>` implements `Handler<(), S>`
 note: required by a bound in `MethodRouter::<S>::post`
   --> $WORKSPACE/axum/src/routing/method_routing.rs
    |

axum-macros/tests/from_request/fail/parts_extracting_body.stderr

@@ -7,12 +7,12 @@ error[E0277]: the trait bound `String: FromRequestParts<_>` is not satisfied
   = note: Function argument is not a valid axum extractor.
           See `https://docs.rs/axum/0.8/axum/extract/index.html` for details
   = help: the following other types implement trait `FromRequestParts<S>`:
-            `()` implements `FromRequestParts<S>`
+            `ConnectInfo<T>` implements `FromRequestParts<S>`
-            `(T1, T2)` implements `FromRequestParts<S>`
+            `Extension<T>` implements `FromRequestParts<S>`
-            `(T1, T2, T3)` implements `FromRequestParts<S>`
+            `Extractor` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4)` implements `FromRequestParts<S>`
+            `HeaderMap` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4, T5)` implements `FromRequestParts<S>`
+            `MatchedPath` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4, T5, T6)` implements `FromRequestParts<S>`
+            `Method` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4, T5, T6, T7)` implements `FromRequestParts<S>`
+            `OriginalUri` implements `FromRequestParts<S>`
-            `(T1, T2, T3, T4, T5, T6, T7, T8)` implements `FromRequestParts<S>`
+            `Query<T>` implements `FromRequestParts<S>`
           and $N others

axum/src/extract/nested_path.rs

@@ -47,6 +47,7 @@ impl NestedPath {
     }
 }
 
+#[diagnostic::do_not_recommend] // pretty niche type
 impl<S> FromRequestParts<S> for NestedPath
 where
     S: Send + Sync,

axum/src/handler/mod.rs

@@ -204,6 +204,7 @@ pub trait Handler<T, S>: Clone + Send + Sync + Sized + 'static {
     }
 }
 
+#[diagnostic::do_not_recommend]
 impl<F, Fut, Res, S> Handler<((),), S> for F
 where
     F: FnOnce() -> Fut + Clone + Send + Sync + 'static,
@@ -221,6 +222,7 @@ macro_rules! impl_handler {
     (
         [$($ty:ident),*], $last:ident
     ) => {
+        #[diagnostic::do_not_recommend]
         #[allow(non_snake_case, unused_mut)]
         impl<F, Fut, S, Res, M, $($ty,)* $last> Handler<(M, $($ty,)* $last,), S> for F
         where
@@ -265,6 +267,7 @@ mod private {
     pub enum IntoResponseHandler {}
 }
 
+#[diagnostic::do_not_recommend]
 impl<T, S> Handler<private::IntoResponseHandler, S> for T
 where
     T: IntoResponse + Clone + Send + Sync + 'static,
@@ -310,6 +313,7 @@ where
     }
 }
 
+#[diagnostic::do_not_recommend]
 impl<H, S, T, L> Handler<T, S> for Layered<L, H, T, S>
 where
     L: Layer<HandlerService<H, T, S>> + Clone + Send + Sync + 'static,

axum/src/routing/method_routing.rs

@@ -1306,6 +1306,7 @@ where
     }
 }
 
+#[diagnostic::do_not_recommend]
 impl<S> Handler<(), S> for MethodRouter<S>
 where
     S: Clone + 'static,