fix(traits): clarify conditional impl diagnostics

Author: raushan728

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());

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> {}

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> {

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>

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>