Fix bad caching of `~const Drop` bounds

2 files changed, 12 insertions, 3 deletions

diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index f5a4a4ae24c..37d99766da9 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -734,6 +734,15 @@ pub struct TraitPredicate<'tcx> {
 pub type PolyTraitPredicate<'tcx> = ty::Binder<'tcx, TraitPredicate<'tcx>>;
 
 impl<'tcx> TraitPredicate<'tcx> {
+    pub fn remap_constness(&mut self, tcx: TyCtxt<'tcx>, param_env: &mut ParamEnv<'tcx>) {
+        if unlikely!(Some(self.trait_ref.def_id) == tcx.lang_items().drop_trait()) {
+            // remap without changing constness of this predicate.
+            // this is because `T: ~const Drop` has a different meaning to `T: Drop`
+            param_env.remap_constness_with(self.constness)
+        } else {
+            *param_env = param_env.with_constness(self.constness.and(param_env.constness()))
+        }
+    }
     pub fn def_id(self) -> DefId {
         self.trait_ref.def_id
     }
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 85016a701f3..fa88c8ee370 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -730,7 +730,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let mut param_env = obligation.param_env;
 
         fresh_trait_pred = fresh_trait_pred.map_bound(|mut pred| {
-            param_env = param_env.with_constness(pred.constness.and(param_env.constness()));
+            pred.remap_constness(self.tcx(), &mut param_env);
             pred
         });
 
@@ -1269,7 +1269,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
         let tcx = self.tcx();
         let mut pred = cache_fresh_trait_pred.skip_binder();
-        param_env = param_env.with_constness(pred.constness.and(param_env.constness()));
+        pred.remap_constness(tcx, &mut param_env);
 
         if self.can_use_global_caches(param_env) {
             if let Some(res) = tcx.selection_cache.get(&param_env.and(pred), tcx) {
@@ -1322,7 +1322,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let tcx = self.tcx();
         let mut pred = cache_fresh_trait_pred.skip_binder();
 
-        param_env = param_env.with_constness(pred.constness.and(param_env.constness()));
+        pred.remap_constness(tcx, &mut param_env);
 
         if !self.can_cache_candidate(&candidate) {
             debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");