Account for #[rustc_reservation_impl] when pointing at potential impls

Author: estebank

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

@@ -1908,7 +1908,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 // Ignore automatically derived impls and `!Trait` impls.
                 .map(|def_id| (self.tcx.impl_trait_header(def_id), def_id))
                 .filter_map(|(header, def_id)| {
-                    (header.polarity != ty::ImplPolarity::Negative
+                    (header.polarity == ty::ImplPolarity::Positive
                         || self.tcx.is_automatically_derived(def_id))
                     .then(|| (header.trait_ref.instantiate_identity(), def_id))
                 })

tests/ui/traits/reservation-impl/no-use.next.stderr

@@ -3,12 +3,6 @@ error[E0277]: the trait bound `(): MyTrait` is not satisfied
    |
 LL |     <() as MyTrait>::foo(&());
    |      ^^ the trait `MyTrait` is not implemented for `()`
-   |
-help: the trait `MyTrait` is implemented for `()`
-  --> $DIR/no-use.rs:8:1
-   |
-LL | impl MyTrait for () { fn foo(&self) {} }
-   | ^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/traits/reservation-impl/no-use.old.stderr

@@ -3,12 +3,6 @@ error[E0277]: the trait bound `(): MyTrait` is not satisfied
    |
 LL |     <() as MyTrait>::foo(&());
    |      ^^ the trait `MyTrait` is not implemented for `()`
-   |
-help: the trait `MyTrait` is implemented for `()`
-  --> $DIR/no-use.rs:8:1
-   |
-LL | impl MyTrait for () { fn foo(&self) {} }
-   | ^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error