Avoid redundant note when a #[derive] is already suggested

Dnreikronos · Dnreikronos · commit 8685d8fd39da · 2026-05-29T18:02:10.000-03:00
The `#[rustc_on_unimplemented]` note on `Debug` ("add `#[derive(Debug)]`
to `X` or manually `impl Debug for X`") duplicates the `consider
annotating X with #[derive(..)]` suggestion emitted by `suggest_derive`.

Skip the note when that suggestion will be shown, and keep it otherwise
so types whose derive can't be suggested (e.g. a field isn't `Debug`)
still get actionable guidance.
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
@@ -448,8 +448,21 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         if let Some((msg, span)) = type_def {
                             err.span_label(span, msg);
                         }
+                        // `#[rustc_on_unimplemented]` notes for derivable traits (e.g. `Debug`'s
+                        // "add `#[derive(Debug)]` to `X` or manually `impl Debug for X`") duplicate
+                        // the `consider annotating X with #[derive(..)]` suggestion that
+                        // `suggest_derive` emits below, so skip them when that suggestion will be
+                        // shown. We keep the note otherwise (e.g. when a field isn't `Debug`, so
+                        // the derive can't be suggested) to avoid leaving the diagnostic without
+                        // actionable guidance.
+                        let derive_suggestion_will_be_shown = main_trait_predicate
+                            == leaf_trait_predicate
+                            && self.can_suggest_derive(&obligation, leaf_trait_predicate);
                         for note in notes {
                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
+                            if derive_suggestion_will_be_shown {
+                                continue;
+                            }
                             err.note(note);
                         }
                         if let Some(s) = parent_label {
diff --git a/tests/ui/derives/debug/derives-span-Debug.stderr b/tests/ui/derives/debug/derives-span-Debug.stderr
@@ -7,7 +7,6 @@ LL | #[derive(Debug)]
 LL |         x: Error,
    |         ^^^^^^^^ the trait `Debug` is not implemented for `Error`
    |
-   = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
 help: consider annotating `Error` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
@@ -23,7 +22,6 @@ LL | #[derive(Debug)]
 LL |         Error,
    |         ^^^^^ the trait `Debug` is not implemented for `Error`
    |
-   = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
 help: consider annotating `Error` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
@@ -39,7 +37,6 @@ LL | struct Struct {
 LL |     x: Error,
    |     ^^^^^^^^ the trait `Debug` is not implemented for `Error`
    |
-   = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
 help: consider annotating `Error` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
@@ -55,7 +52,6 @@ LL | struct TupleStruct(
 LL |     Error,
    |     ^^^^^ the trait `Debug` is not implemented for `Error`
    |
-   = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
 help: consider annotating `Error` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
diff --git a/tests/ui/derives/redundant-derive-note-on-unimplemented.rs b/tests/ui/derives/redundant-derive-note-on-unimplemented.rs
@@ -0,0 +1,15 @@
+// Regression test for the redundant `note: add `#[derive(Debug)]` to `X` or manually
+// `impl Debug for X`` that was emitted alongside the `consider annotating X with
+// `#[derive(Debug)]`` suggestion. When the derive suggestion is shown, the note is
+// redundant and should be suppressed.
+//
+// See https://github.com/rust-lang/rust/issues/157118
+
+#[derive(Debug)]
+struct S<T>(T);
+
+struct X;
+
+fn main() {
+    println!("{:?}", S(X)); //~ ERROR `X` doesn't implement `Debug`
+}
diff --git a/tests/ui/derives/redundant-derive-note-on-unimplemented.stderr b/tests/ui/derives/redundant-derive-note-on-unimplemented.stderr
@@ -0,0 +1,31 @@
+error[E0277]: `X` doesn't implement `Debug`
+  --> $DIR/redundant-derive-note-on-unimplemented.rs:14:22
+   |
+LL |     println!("{:?}", S(X));
+   |               ----   ^^^^ `X` cannot be formatted using `{:?}` because it doesn't implement `Debug`
+   |               |
+   |               required by this formatting parameter
+   |
+   = help: the trait `Debug` is not implemented for `X`
+help: the trait `Debug` is implemented for `S<T>`
+  --> $DIR/redundant-derive-note-on-unimplemented.rs:8:10
+   |
+LL | #[derive(Debug)]
+   |          ^^^^^
+note: required for `S<X>` to implement `Debug`
+  --> $DIR/redundant-derive-note-on-unimplemented.rs:9:8
+   |
+LL | #[derive(Debug)]
+   |          ----- in this derive macro expansion
+LL | struct S<T>(T);
+   |        ^ - type parameter would need to implement `Debug`
+   = help: consider manually implementing `Debug` to avoid undesired bounds
+help: consider annotating `X` with `#[derive(Debug)]`
+   |
+LL + #[derive(Debug)]
+LL | struct X;
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/fmt/format-args-argument-span.stderr b/tests/ui/fmt/format-args-argument-span.stderr
@@ -25,7 +25,6 @@ LL |     println!("{x} {x:?} {x}");
    |                   ^^^^^ `DisplayOnly` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
    = help: the trait `Debug` is not implemented for `DisplayOnly`
-   = note: add `#[derive(Debug)]` to `DisplayOnly` or manually `impl Debug for DisplayOnly`
 help: consider annotating `DisplayOnly` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
@@ -41,7 +40,6 @@ LL |     println!("{x} {x:?} {x}", x = DisplayOnly);
    |                   required by this formatting parameter
    |
    = help: the trait `Debug` is not implemented for `DisplayOnly`
-   = note: add `#[derive(Debug)]` to `DisplayOnly` or manually `impl Debug for DisplayOnly`
 help: consider annotating `DisplayOnly` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
diff --git a/tests/ui/fmt/non-source-literals.stderr b/tests/ui/fmt/non-source-literals.stderr
@@ -31,7 +31,6 @@ LL |     let _ = format!(concat!("{:", "?}"), NonDebug);
    |                                          ^^^^^^^^ `NonDebug` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
    = help: the trait `Debug` is not implemented for `NonDebug`
-   = note: add `#[derive(Debug)]` to `NonDebug` or manually `impl Debug for NonDebug`
 help: consider annotating `NonDebug` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
@@ -45,7 +44,6 @@ LL |     let _ = format!(concat!("{", "0", ":?}"), NonDebug);
    |                                               ^^^^^^^^ `NonDebug` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
    = help: the trait `Debug` is not implemented for `NonDebug`
-   = note: add `#[derive(Debug)]` to `NonDebug` or manually `impl Debug for NonDebug`
 help: consider annotating `NonDebug` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
diff --git a/tests/ui/mismatched_types/method-help-unsatisfied-bound.stderr b/tests/ui/mismatched_types/method-help-unsatisfied-bound.stderr
@@ -4,7 +4,6 @@ error[E0277]: `Foo` doesn't implement `Debug`
 LL |     a.unwrap();
    |       ^^^^^^ the trait `Debug` is not implemented for `Foo`
    |
-   = note: add `#[derive(Debug)]` to `Foo` or manually `impl Debug for Foo`
 note: required by a bound in `Result::<T, E>::unwrap`
   --> $SRC_DIR/core/src/result.rs:LL:COL
 help: consider annotating `Foo` with `#[derive(Debug)]`
diff --git a/tests/ui/modules/issue-107649.stderr b/tests/ui/modules/issue-107649.stderr
@@ -4,7 +4,6 @@ error[E0277]: `Dummy` doesn't implement `Debug`
 105 |     dbg!(lib::Dummy);
     |     ^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `Dummy`
     |
-    = note: add `#[derive(Debug)]` to `Dummy` or manually `impl Debug for Dummy`
 help: consider annotating `Dummy` with `#[derive(Debug)]`
    --> $DIR/auxiliary/dummy_lib.rs:2:1
     |
diff --git a/tests/ui/on-unimplemented/no-debug.stderr b/tests/ui/on-unimplemented/no-debug.stderr
@@ -7,7 +7,6 @@ LL |     println!("{:?} {:?}", Foo, Bar);
    |               required by this formatting parameter
    |
    = help: the trait `Debug` is not implemented for `Foo`
-   = note: add `#[derive(Debug)]` to `Foo` or manually `impl Debug for Foo`
 help: consider annotating `Foo` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
diff --git a/tests/ui/rfcs/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr b/tests/ui/rfcs/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr
@@ -4,7 +4,6 @@ error[E0277]: `NotDebug` doesn't implement `Debug`
 LL |     let _: NotDebug = dbg!(NotDebug);
    |                       ^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `NotDebug`
    |
-   = note: add `#[derive(Debug)]` to `NotDebug` or manually `impl Debug for NotDebug`
 help: consider annotating `NotDebug` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]
diff --git a/tests/ui/span/issue-71363.stderr b/tests/ui/span/issue-71363.stderr
@@ -18,7 +18,6 @@ error[E0277]: `MyError` doesn't implement `Debug`
 4 | impl std::error::Error for MyError {}
   |                            ^^^^^^^ the trait `Debug` is not implemented for `MyError`
   |
-  = note: add `#[derive(Debug)]` to `MyError` or manually `impl Debug for MyError`
 note: required by a bound in `std::error::Error`
  --> $SRC_DIR/core/src/error.rs:LL:COL
 help: consider annotating `MyError` with `#[derive(Debug)]`
diff --git a/tests/ui/suggestions/derive-macro-missing-bounds.stderr b/tests/ui/suggestions/derive-macro-missing-bounds.stderr
@@ -6,7 +6,6 @@ LL |     #[derive(Debug)]
 LL |     struct Outer<T>(Inner<T>);
    |                     ^^^^^^^^ the trait `Debug` is not implemented for `a::Inner<T>`
    |
-   = note: add `#[derive(Debug)]` to `a::Inner<T>` or manually `impl Debug for a::Inner<T>`
 help: consider annotating `a::Inner<T>` with `#[derive(Debug)]`
    |
 LL +     #[derive(Debug)]

Original file line number	Diff line number	Diff line change
`@@ -7,7 +7,6 @@ LL \| #[derive(Debug)]`
`7`	`7`	`LL \| x: Error,`
`8`	`8`	\| ^^^^^^^^ the trait `Debug` is not implemented for `Error`
`9`	`9`	`\|`
`10`		- = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
`11`	`10`	help: consider annotating `Error` with `#[derive(Debug)]`
`12`	`11`	`\|`
`13`	`12`	`LL + #[derive(Debug)]`
`@@ -23,7 +22,6 @@ LL \| #[derive(Debug)]`
`23`	`22`	`LL \| Error,`
`24`	`23`	\| ^^^^^ the trait `Debug` is not implemented for `Error`
`25`	`24`	`\|`
`26`		- = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
`27`	`25`	help: consider annotating `Error` with `#[derive(Debug)]`
`28`	`26`	`\|`
`29`	`27`	`LL + #[derive(Debug)]`
`@@ -39,7 +37,6 @@ LL \| struct Struct {`
`39`	`37`	`LL \| x: Error,`
`40`	`38`	\| ^^^^^^^^ the trait `Debug` is not implemented for `Error`
`41`	`39`	`\|`
`42`		- = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
`43`	`40`	help: consider annotating `Error` with `#[derive(Debug)]`
`44`	`41`	`\|`
`45`	`42`	`LL + #[derive(Debug)]`
`@@ -55,7 +52,6 @@ LL \| struct TupleStruct(`
`55`	`52`	`LL \| Error,`
`56`	`53`	\| ^^^^^ the trait `Debug` is not implemented for `Error`
`57`	`54`	`\|`
`58`		- = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
`59`	`55`	help: consider annotating `Error` with `#[derive(Debug)]`
`60`	`56`	`\|`
`61`	`57`	`LL + #[derive(Debug)]`
Original file line number	Diff line number	Diff line change
`@@ -25,7 +25,6 @@ LL \| println!("{x} {x:?} {x}");`
`25`	`25`	\| ^^^^^ `DisplayOnly` cannot be formatted using `{:?}` because it doesn't implement `Debug`
`26`	`26`	`\|`
`27`	`27`	= help: the trait `Debug` is not implemented for `DisplayOnly`
`28`		- = note: add `#[derive(Debug)]` to `DisplayOnly` or manually `impl Debug for DisplayOnly`
`29`	`28`	help: consider annotating `DisplayOnly` with `#[derive(Debug)]`
`30`	`29`	`\|`
`31`	`30`	`LL + #[derive(Debug)]`
`@@ -41,7 +40,6 @@ LL \| println!("{x} {x:?} {x}", x = DisplayOnly);`
`41`	`40`	`\| required by this formatting parameter`
`42`	`41`	`\|`
`43`	`42`	= help: the trait `Debug` is not implemented for `DisplayOnly`
`44`		- = note: add `#[derive(Debug)]` to `DisplayOnly` or manually `impl Debug for DisplayOnly`
`45`	`43`	help: consider annotating `DisplayOnly` with `#[derive(Debug)]`
`46`	`44`	`\|`
`47`	`45`	`LL + #[derive(Debug)]`
Original file line number	Diff line number	Diff line change
`@@ -31,7 +31,6 @@ LL \| let _ = format!(concat!("{:", "?}"), NonDebug);`
`31`	`31`	\| ^^^^^^^^ `NonDebug` cannot be formatted using `{:?}` because it doesn't implement `Debug`
`32`	`32`	`\|`
`33`	`33`	= help: the trait `Debug` is not implemented for `NonDebug`
`34`		- = note: add `#[derive(Debug)]` to `NonDebug` or manually `impl Debug for NonDebug`
`35`	`34`	help: consider annotating `NonDebug` with `#[derive(Debug)]`
`36`	`35`	`\|`
`37`	`36`	`LL + #[derive(Debug)]`
`@@ -45,7 +44,6 @@ LL \| let _ = format!(concat!("{", "0", ":?}"), NonDebug);`
`45`	`44`	\| ^^^^^^^^ `NonDebug` cannot be formatted using `{:?}` because it doesn't implement `Debug`
`46`	`45`	`\|`
`47`	`46`	= help: the trait `Debug` is not implemented for `NonDebug`
`48`		- = note: add `#[derive(Debug)]` to `NonDebug` or manually `impl Debug for NonDebug`
`49`	`47`	help: consider annotating `NonDebug` with `#[derive(Debug)]`
`50`	`48`	`\|`
`51`	`49`	`LL + #[derive(Debug)]`
Original file line number	Diff line number	Diff line change
@@ -4,7 +4,6 @@ error[E0277]: `Foo` doesn't implement `Debug`
`4`	`4`	`LL \| a.unwrap();`
`5`	`5`	\| ^^^^^^ the trait `Debug` is not implemented for `Foo`
`6`	`6`	`\|`
`7`		- = note: add `#[derive(Debug)]` to `Foo` or manually `impl Debug for Foo`
`8`	`7`	note: required by a bound in `Result::<T, E>::unwrap`
`9`	`8`	`--> $SRC_DIR/core/src/result.rs:LL:COL`
`10`	`9`	help: consider annotating `Foo` with `#[derive(Debug)]`
Original file line number	Diff line number	Diff line change
@@ -4,7 +4,6 @@ error[E0277]: `Dummy` doesn't implement `Debug`
`4`	`4`	`105 \| dbg!(lib::Dummy);`
`5`	`5`	\| ^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `Dummy`
`6`	`6`	`\|`
`7`		- = note: add `#[derive(Debug)]` to `Dummy` or manually `impl Debug for Dummy`
`8`	`7`	help: consider annotating `Dummy` with `#[derive(Debug)]`
`9`	`8`	`--> $DIR/auxiliary/dummy_lib.rs:2:1`
`10`	`9`	`\|`
Original file line number	Diff line number	Diff line change
`@@ -7,7 +7,6 @@ LL \| println!("{:?} {:?}", Foo, Bar);`
`7`	`7`	`\| required by this formatting parameter`
`8`	`8`	`\|`
`9`	`9`	= help: the trait `Debug` is not implemented for `Foo`
`10`		- = note: add `#[derive(Debug)]` to `Foo` or manually `impl Debug for Foo`
`11`	`10`	help: consider annotating `Foo` with `#[derive(Debug)]`
`12`	`11`	`\|`
`13`	`12`	`LL + #[derive(Debug)]`
Original file line number	Diff line number	Diff line change
@@ -4,7 +4,6 @@ error[E0277]: `NotDebug` doesn't implement `Debug`
`4`	`4`	`LL \| let _: NotDebug = dbg!(NotDebug);`
`5`	`5`	\| ^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `NotDebug`
`6`	`6`	`\|`
`7`		- = note: add `#[derive(Debug)]` to `NotDebug` or manually `impl Debug for NotDebug`
`8`	`7`	help: consider annotating `NotDebug` with `#[derive(Debug)]`
`9`	`8`	`\|`
`10`	`9`	`LL + #[derive(Debug)]`