fix(traits): clarify conditional impl diagnostics

raushan728 · raushan728 · commit 893f9fc7e9ee · 2026-06-28T05:24:36.000Z
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
@@ -2332,7 +2332,55 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             (" implemented for fn pointer `", ", cast using `as`")
                         }
                         (ty::FnPtr(..), _) => (" implemented for fn pointer `", ""),
-                        _ => (" implemented for `", ""),
+                        _ => {
+                            let mut has_failing_obligations = false;
+                            if !self.tcx.predicates_of(def_id).predicates.is_empty() {
+                                has_failing_obligations = self.probe(|_| {
+                                    let ocx = ObligationCtxt::new(self);
+                                    self.enter_forall(trait_pred, |obligation_trait_ref| {
+                                        let impl_args = self.fresh_args_for_item(DUMMY_SP, def_id);
+                                        let impl_trait_ref = ocx.normalize(
+                                            &ObligationCause::dummy(),
+                                            param_env,
+                                            ty::EarlyBinder::bind(self.tcx, cand)
+                                                .instantiate(self.tcx, impl_args),
+                                        );
+                                        if ocx
+                                            .eq(
+                                                &ObligationCause::dummy(),
+                                                param_env,
+                                                obligation_trait_ref.trait_ref,
+                                                impl_trait_ref,
+                                            )
+                                            .is_err()
+                                        {
+                                            return false;
+                                        }
+                                        ocx.register_obligations(
+                                            self.tcx
+                                                .predicates_of(def_id)
+                                                .instantiate(self.tcx, impl_args)
+                                                .into_iter()
+                                                .map(|(clause, _)| {
+                                                    Obligation::new(
+                                                        self.tcx,
+                                                        ObligationCause::dummy(),
+                                                        param_env,
+                                                        clause.skip_norm_wip(),
+                                                    )
+                                                }),
+                                        );
+                                        !ocx.try_evaluate_obligations().is_empty()
+                                    })
+                                });
+                            }
+
+                            if has_failing_obligations {
+                                (" conditionally implemented for `", "")
+                            } else {
+                                (" implemented for `", "")
+                            }
+                        }
                     };
                 let trait_ = self.tcx.short_string(cand.print_trait_sugared(), err.long_ty_path());
                 let self_ty = self.tcx.short_string(cand.self_ty(), err.long_ty_path());
diff --git a/tests/ui/derives/redundant-derive-note-on-unimplemented.stderr b/tests/ui/derives/redundant-derive-note-on-unimplemented.stderr
@@ -7,7 +7,7 @@ LL |     println!("{:?}", S(X));
    |               required by this formatting parameter
    |
    = help: the trait `Debug` is not implemented for `X`
-help: the trait `Debug` is implemented for `S<T>`
+help: the trait `Debug` is conditionally implemented for `S<T>`
   --> $DIR/redundant-derive-note-on-unimplemented.rs:8:10
    |
 LL | #[derive(Debug)]
diff --git a/tests/ui/errors/trait-bound-error-spans/blame-trait-error.stderr b/tests/ui/errors/trait-bound-error-spans/blame-trait-error.stderr
@@ -39,7 +39,7 @@ LL |     want(Some(()));
    |     required by a bound introduced by this call
    |
    = help: the trait `Iterator` is not implemented for `()`
-help: the trait `T1` is implemented for `Option<It>`
+help: the trait `T1` is conditionally implemented for `Option<It>`
   --> $DIR/blame-trait-error.rs:21:1
    |
 LL | impl<It: Iterator> T1 for Option<It> {}
diff --git a/tests/ui/impl-trait/nested_impl_trait.stderr b/tests/ui/impl-trait/nested_impl_trait.stderr
@@ -50,7 +50,7 @@ LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                              |
    |                                              the trait `From<impl Into<u32>>` is not implemented for `impl Debug`
    |
-help: the trait `Into<U>` is implemented for `T`
+help: the trait `Into<U>` is conditionally implemented for `T`
   --> $SRC_DIR/core/src/convert/mod.rs:LL:COL
    = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
 
@@ -62,7 +62,7 @@ LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                  |
    |                                  the trait `From<impl Into<u32>>` is not implemented for `impl Debug`
    |
-help: the trait `Into<U>` is implemented for `T`
+help: the trait `Into<U>` is conditionally implemented for `T`
   --> $SRC_DIR/core/src/convert/mod.rs:LL:COL
    = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
 
diff --git a/tests/ui/traits/copy-bounds-impl-type-params.stderr b/tests/ui/traits/copy-bounds-impl-type-params.stderr
@@ -80,7 +80,7 @@ error[E0277]: the trait bound `String: Copy` is not satisfied
 LL |     let a = t as Box<dyn Gettable<String>>;
    |             ^ the trait `Copy` is not implemented for `String`
    |
-help: the trait `Gettable<T>` is implemented for `S<T>`
+help: the trait `Gettable<T>` is conditionally implemented for `S<T>`
   --> $DIR/copy-bounds-impl-type-params.rs:14:1
    |
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
@@ -100,7 +100,7 @@ error[E0277]: the trait bound `Foo: Copy` is not satisfied
 LL |     let a: Box<dyn Gettable<Foo>> = t;
    |                                     ^ the trait `Copy` is not implemented for `Foo`
    |
-help: the trait `Gettable<T>` is implemented for `S<T>`
+help: the trait `Gettable<T>` is conditionally implemented for `S<T>`
   --> $DIR/copy-bounds-impl-type-params.rs:14:1
    |
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
diff --git a/tests/ui/traits/error_reporting/conditionally-implemented-trait-158423.stderr b/tests/ui/traits/error_reporting/conditionally-implemented-trait-158423.stderr
@@ -11,7 +11,7 @@ help: the trait `Foo<'_>` is not implemented for `Test`
    |
 LL | struct Test(i32, i64);
    | ^^^^^^^^^^^
-help: the trait `Foo<'_>` is implemented for `Test`
+help: the trait `Foo<'_>` is conditionally implemented for `Test`
   --> $DIR/conditionally-implemented-trait-158423.rs:20:1
    |
 LL | impl<'a> Foo<'a> for Test where i32: Foo<'a, Assoc = i32>, i64: Foo<'a, Assoc = i64> {
diff --git a/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.with.stderr b/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.with.stderr
@@ -9,7 +9,7 @@ help: the trait `Trait<_, _, _>` is not implemented for `A<X>`
    |
 LL | struct A<T>(*const T);
    | ^^^^^^^^^^^
-help: the trait `Trait<U, V, D>` is implemented for `A<T>`
+help: the trait `Trait<U, V, D>` is conditionally implemented for `A<T>`
   --> $DIR/incompleteness-unstable-result.rs:34:1
    |
 LL | / impl<T, U, V, D> Trait<U, V, D> for A<T>
diff --git a/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.without.stderr b/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.without.stderr
@@ -9,7 +9,7 @@ help: the trait `Trait<_, _, _>` is not implemented for `A<X>`
    |
 LL | struct A<T>(*const T);
    | ^^^^^^^^^^^
-help: the trait `Trait<U, V, D>` is implemented for `A<T>`
+help: the trait `Trait<U, V, D>` is conditionally implemented for `A<T>`
   --> $DIR/incompleteness-unstable-result.rs:34:1
    |
 LL | / impl<T, U, V, D> Trait<U, V, D> for A<T>

Original file line number	Diff line number	Diff line change
`@@ -7,7 +7,7 @@ LL \| println!("{:?}", S(X));`
`7`	`7`	`\| required by this formatting parameter`
`8`	`8`	`\|`
`9`	`9`	= help: the trait `Debug` is not implemented for `X`
`10`		-help: the trait `Debug` is implemented for `S<T>`
	`10`	+help: the trait `Debug` is conditionally implemented for `S<T>`
`11`	`11`	`--> $DIR/redundant-derive-note-on-unimplemented.rs:8:10`
`12`	`12`	`\|`
`13`	`13`	`LL \| #[derive(Debug)]`
Original file line number	Diff line number	Diff line change
`@@ -39,7 +39,7 @@ LL \| want(Some(()));`
`39`	`39`	`\| required by a bound introduced by this call`
`40`	`40`	`\|`
`41`	`41`	= help: the trait `Iterator` is not implemented for `()`
`42`		-help: the trait `T1` is implemented for `Option<It>`
	`42`	+help: the trait `T1` is conditionally implemented for `Option<It>`
`43`	`43`	`--> $DIR/blame-trait-error.rs:21:1`
`44`	`44`	`\|`
`45`	`45`	`LL \| impl<It: Iterator> T1 for Option<It> {}`
Original file line number	Diff line number	Diff line change
@@ -80,7 +80,7 @@ error[E0277]: the trait bound `String: Copy` is not satisfied
`80`	`80`	`LL \| let a = t as Box<dyn Gettable<String>>;`
`81`	`81`	\| ^ the trait `Copy` is not implemented for `String`
`82`	`82`	`\|`
`83`		-help: the trait `Gettable<T>` is implemented for `S<T>`
	`83`	+help: the trait `Gettable<T>` is conditionally implemented for `S<T>`
`84`	`84`	`--> $DIR/copy-bounds-impl-type-params.rs:14:1`
`85`	`85`	`\|`
`86`	`86`	`LL \| impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}`
@@ -100,7 +100,7 @@ error[E0277]: the trait bound `Foo: Copy` is not satisfied
`100`	`100`	`LL \| let a: Box<dyn Gettable<Foo>> = t;`
`101`	`101`	\| ^ the trait `Copy` is not implemented for `Foo`
`102`	`102`	`\|`
`103`		-help: the trait `Gettable<T>` is implemented for `S<T>`
	`103`	+help: the trait `Gettable<T>` is conditionally implemented for `S<T>`
`104`	`104`	`--> $DIR/copy-bounds-impl-type-params.rs:14:1`
`105`	`105`	`\|`
`106`	`106`	`LL \| impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}`
Original file line number	Diff line number	Diff line change
@@ -11,7 +11,7 @@ help: the trait `Foo<'_>` is not implemented for `Test`
`11`	`11`	`\|`
`12`	`12`	`LL \| struct Test(i32, i64);`
`13`	`13`	`\| ^^^^^^^^^^^`
`14`		-help: the trait `Foo<'_>` is implemented for `Test`
	`14`	+help: the trait `Foo<'_>` is conditionally implemented for `Test`
`15`	`15`	`--> $DIR/conditionally-implemented-trait-158423.rs:20:1`
`16`	`16`	`\|`
`17`	`17`	`LL \| impl<'a> Foo<'a> for Test where i32: Foo<'a, Assoc = i32>, i64: Foo<'a, Assoc = i64> {`
Original file line number	Diff line number	Diff line change
@@ -9,7 +9,7 @@ help: the trait `Trait<_, _, _>` is not implemented for `A<X>`
`9`	`9`	`\|`
`10`	`10`	`LL \| struct A<T>(*const T);`
`11`	`11`	`\| ^^^^^^^^^^^`
`12`		-help: the trait `Trait<U, V, D>` is implemented for `A<T>`
	`12`	+help: the trait `Trait<U, V, D>` is conditionally implemented for `A<T>`
`13`	`13`	`--> $DIR/incompleteness-unstable-result.rs:34:1`
`14`	`14`	`\|`
`15`	`15`	`LL \| / impl<T, U, V, D> Trait<U, V, D> for A<T>`