some comments or dynamic drop handling

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -540,7 +540,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     //                \-------/
                     //
                     let virtual_drop = Instance {
-                        def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0),
+                        def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0), // idx 0: the drop function
                         args: drop_fn.args,
                     };
                     debug!("ty = {:?}", ty);
@@ -581,7 +581,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     //
                     // SO THEN WE CAN USE THE ABOVE CODE.
                     let virtual_drop = Instance {
-                        def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0),
+                        def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0), // idx 0: the drop function
                         args: drop_fn.args,
                     };
                     debug!("ty = {:?}", ty);

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -949,6 +949,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // implementation fail -- a problem shared by rustc.
         let place = self.force_allocation(place)?;
 
+        // We behave a bit different from codegen here.
+        // Codegen creates an `InstanceDef::Virtual` with index 0 (the slot of the drop method) and
+        // then dispatches that to the normal call machinery. However, our call machinery currently
+        // only supports calling `VtblEntry::Method`; it would choke on a `MetadataDropInPlace`. So
+        // instead we do the virtual call stuff ourselves. It's easier here than in `eval_fn_call`
+        // since we can just get a place of the underlying type and use `mplace_to_ref`.
         let place = match place.layout.ty.kind() {
             ty::Dynamic(data, _, ty::Dyn) => {
                 // Dropping a trait object. Need to find actual drop fn.