fix host param for PartialEq

Author: fee1-dead

Diff for: compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -3628,17 +3628,19 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             is_derivable_trait &&
                 // Ensure all fields impl the trait.
                 adt.all_fields().all(|field| {
-                    let field_ty = field.ty(self.tcx, args);
+                    let field_ty = ty::GenericArg::from(field.ty(self.tcx, args));
                     let trait_args = match diagnostic_name {
                         sym::PartialEq | sym::PartialOrd => {
                             Some(field_ty)
                         }
                         _ => None,
                     };
+                    // Also add host param, if present
+                    let host = self.tcx.generics_of(trait_pred.def_id()).host_effect_index.map(|idx| trait_pred.skip_binder().trait_ref.args[idx]);
                     let trait_pred = trait_pred.map_bound_ref(|tr| ty::TraitPredicate {
                         trait_ref: ty::TraitRef::new(self.tcx,
                             trait_pred.def_id(),
-                            [field_ty].into_iter().chain(trait_args),
+                            [field_ty].into_iter().chain(trait_args).chain(host),
                         ),
                         ..*tr
                     });
@@ -3659,6 +3661,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     trait_pred.skip_binder().self_ty(),
                     diagnostic_name,
                 ),
+                // FIXME(effects, const_trait_impl) derive_const as suggestion?
                 format!("#[derive({diagnostic_name})]\n"),
                 Applicability::MaybeIncorrect,
             );