Do not lint for ~const Drop bounds

Author: fee1-dead

compiler/rustc_lint/src/traits.rs

@@ -87,13 +87,18 @@ declare_lint_pass!(
 impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
         use rustc_middle::ty::PredicateKind::*;
+        use rustc_middle::ty;
 
         let predicates = cx.tcx.explicit_predicates_of(item.def_id);
         for &(predicate, span) in predicates.predicates {
             let trait_predicate = match predicate.kind().skip_binder() {
                 Trait(trait_predicate) => trait_predicate,
                 _ => continue,
             };
+            if trait_predicate.constness == ty::BoundConstness::ConstIfConst {
+                // `~const Drop` definitely have meanings so avoid linting here.
+                continue
+            }
             let def_id = trait_predicate.trait_ref.def_id;
             if cx.tcx.lang_items().drop_trait() == Some(def_id) {
                 // Explicitly allow `impl Drop`, a drop-guards-as-Voldemort-type pattern.