Use trait select logic instead of query

2 files changed, 33 insertions, 10 deletions

diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
index 13345183124..f1c7d3035b9 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -988,14 +988,12 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
 
                 let mut err_span = self.span;
 
-                // Check to see if the type of this place can ever have a drop impl. If not, this
-                // `Drop` terminator is frivolous.
-                let ty_needs_drop = dropped_place
-                    .ty(self.body, self.tcx)
-                    .ty
-                    .needs_non_const_drop(self.tcx, self.param_env);
-
-                if !ty_needs_drop {
+                let ty_needs_non_const_drop = qualifs::NeedsNonConstDrop::in_any_value_of_ty(
+                    self.ccx,
+                    dropped_place.ty(self.body, self.tcx).ty,
+                );
+
+                if !ty_needs_non_const_drop {
                     return;
                 }
 
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
index 50b691dffe8..ea8f0a29181 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
@@ -3,10 +3,14 @@
 //! See the `Qualif` trait for more info.
 
 use rustc_errors::ErrorReported;
+use rustc_hir as hir;
+use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
 use rustc_span::DUMMY_SP;
-use rustc_trait_selection::traits;
+use rustc_trait_selection::traits::{
+    self, ImplSource, Obligation, ObligationCause, SelectionContext,
+};
 
 use super::ConstCx;
 
@@ -108,7 +112,28 @@ impl Qualif for NeedsNonConstDrop {
     }
 
     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
-        ty.needs_drop(cx.tcx, cx.param_env)
+        let trait_ref = ty::TraitRef {
+            def_id: cx.tcx.require_lang_item(hir::LangItem::Drop, None),
+            substs: cx.tcx.mk_substs_trait(ty, &[]),
+        };
+        let obligation = Obligation::new(
+            ObligationCause::dummy(),
+            cx.param_env,
+            ty::Binder::dummy(ty::TraitPredicate {
+                trait_ref,
+                constness: ty::BoundConstness::ConstIfConst,
+            }),
+        );
+
+        let implsrc = cx.tcx.infer_ctxt().enter(|infcx| {
+            let mut selcx = SelectionContext::with_constness(&infcx, hir::Constness::Const);
+            selcx.select(&obligation)
+        });
+        match implsrc {
+            Ok(Some(ImplSource::ConstDrop(_)))
+            | Ok(Some(ImplSource::Param(_, ty::BoundConstness::ConstIfConst))) => false,
+            _ => true,
+        }
     }
 
     fn in_adt_inherently(cx: &ConstCx<'_, 'tcx>, adt: &'tcx AdtDef, _: SubstsRef<'tcx>) -> bool {