Add candidates for DiscriminantKind builtin

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -81,6 +81,7 @@ pub(super) enum CandidateSource {
     AliasBound,
 }
 
+/// Methods used to assemble candidates for either trait or projection goals.
 pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy + Eq {
     fn self_ty(self) -> Ty<'tcx>;
 
@@ -188,6 +189,11 @@ pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy + Eq {
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> Vec<CanonicalResponse<'tcx>>;
+
+    fn consider_builtin_discriminant_kind_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx>;
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
@@ -320,6 +326,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             G::consider_builtin_generator_candidate(self, goal)
         } else if lang_items.unsize_trait() == Some(trait_def_id) {
             G::consider_builtin_unsize_candidate(self, goal)
+        } else if lang_items.discriminant_kind_trait() == Some(trait_def_id) {
+            G::consider_builtin_discriminant_kind_candidate(self, goal)
         } else {
             Err(NoSolution)
         };

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -581,6 +581,19 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
     ) -> Vec<super::CanonicalResponse<'tcx>> {
         bug!("`Unsize` does not have an associated type: {:?}", goal);
     }
+
+    fn consider_builtin_discriminant_kind_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx> {
+        let discriminant = goal.predicate.self_ty().discriminant_ty(ecx.tcx());
+        let nested_goals = ecx.infcx.eq(
+            goal.param_env,
+            goal.predicate.term.ty().expect("expected ty goal"),
+            discriminant,
+        )?;
+        ecx.evaluate_all_and_make_canonical_response(nested_goals)
+    }
 }
 
 /// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -439,6 +439,14 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
 
         responses
     }
+
+    fn consider_builtin_discriminant_kind_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        _goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx> {
+        // `DiscriminantKind` is automatically implemented for every type.
+        ecx.make_canonical_response(Certainty::Yes)
+    }
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {