Account for negative bounds in E0277 note and suggestion

Do not suggest `#[derive(Copy)]` when we wanted a `!Copy` type.

Do not say "`Copy` is not implemented for `T` but `Copy` is".

Do not talk about `Trait` having no implementations when `!Trait` was desired.

Author: estebank

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -2540,12 +2540,16 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             && self.tcx.trait_impls_of(trait_def_id).is_empty()
             && !self.tcx.trait_is_auto(trait_def_id)
             && !self.tcx.trait_is_alias(trait_def_id)
+            && trait_predicate.polarity() == ty::PredicatePolarity::Positive
         {
             err.span_help(
                 self.tcx.def_span(trait_def_id),
                 crate::fluent_generated::trait_selection_trait_has_no_impls,
             );
-        } else if !suggested && !unsatisfied_const {
+        } else if !suggested
+            && !unsatisfied_const
+            && trait_predicate.polarity() == ty::PredicatePolarity::Positive
+        {
             // Can't show anything else useful, try to find similar impls.
             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
             if !self.report_similar_impl_candidates(

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -3697,6 +3697,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         err: &mut Diag<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) {
+        if trait_pred.polarity() == ty::PredicatePolarity::Negative {
+            return;
+        }
         let Some(diagnostic_name) = self.tcx.get_diagnostic_name(trait_pred.def_id()) else {
             return;
         };

tests/ui/traits/negative-bounds/on-unimplemented.stderr

@@ -6,12 +6,6 @@ LL | fn hello() -> impl !Foo {
 LL |
 LL |     NotFoo
    |     ------ return type was inferred to be `NotFoo` here
-   |
-help: this trait has no implementations, consider adding one
-  --> $DIR/on-unimplemented.rs:4:1
-   |
-LL | trait Foo {}
-   | ^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/traits/negative-bounds/simple.stderr

@@ -28,18 +28,11 @@ error[E0277]: the trait bound `Copyable: !Copy` is not satisfied
 LL |     not_copy::<Copyable>();
    |                ^^^^^^^^ the trait bound `Copyable: !Copy` is not satisfied
    |
-   = help: the trait `Copy` is not implemented for `Copyable`
-           but trait `Copy` is implemented for it
 note: required by a bound in `not_copy`
   --> $DIR/simple.rs:3:16
    |
 LL | fn not_copy<T: !Copy>() {}
    |                ^^^^^ required by this bound in `not_copy`
-help: consider annotating `Copyable` with `#[derive(Copy)]`
-   |
-LL + #[derive(Copy)]
-LL | struct Copyable;
-   |
 
 error[E0277]: the trait bound `NotNecessarilyCopyable: !Copy` is not satisfied
   --> $DIR/simple.rs:37:16
@@ -52,11 +45,6 @@ note: required by a bound in `not_copy`
    |
 LL | fn not_copy<T: !Copy>() {}
    |                ^^^^^ required by this bound in `not_copy`
-help: consider annotating `NotNecessarilyCopyable` with `#[derive(Copy)]`
-   |
-LL + #[derive(Copy)]
-LL | struct NotNecessarilyCopyable;
-   |
 
 error: aborting due to 4 previous errors