Replicate live variant drop elaboration during const-checking

src/librustc_mir/transform/check_consts/post_drop_elaboration.rs

@@ -8,6 +8,9 @@ use super::ops;
 use super::qualifs::{NeedsDrop, Qualif};
 use super::validation::Qualifs;
 use super::ConstCx;
+use crate::dataflow::drop_flag_effects::on_all_drop_children_bits;
+use crate::dataflow::move_paths::MoveData;
+use crate::dataflow::{self, Analysis, MoveDataParamEnv, ResultsCursor};
 
 /// Returns `true` if we should use the more precise live drop checker that runs after drop
 /// elaboration.
@@ -31,14 +34,22 @@ pub fn check_live_drops(tcx: TyCtxt<'tcx>, def_id: LocalDefId, body: &mir::Body<
 
     let ccx = ConstCx { body, tcx, def_id, const_kind, param_env: tcx.param_env(def_id) };
 
-    let mut visitor = CheckLiveDrops { ccx: &ccx, qualifs: Qualifs::default() };
+    let mut visitor = CheckLiveDrops { ccx: &ccx, qualifs: Qualifs::default(), maybe_inits: None };
 
     visitor.visit_body(body);
 }
 
+type MaybeInits<'mir, 'tcx> = ResultsCursor<
+    'mir,
+    'tcx,
+    dataflow::impls::MaybeInitializedPlaces<'mir, 'tcx, MoveDataParamEnv<'tcx>>,
+>;
+
 struct CheckLiveDrops<'mir, 'tcx> {
     ccx: &'mir ConstCx<'mir, 'tcx>,
     qualifs: Qualifs<'mir, 'tcx>,
+
+    maybe_inits: Option<MaybeInits<'mir, 'tcx>>,
 }
 
 // So we can access `body` and `tcx`.
@@ -83,7 +94,41 @@ impl Visitor<'tcx> for CheckLiveDrops<'mir, 'tcx> {
                     return;
                 }
 
-                if self.qualifs.needs_drop(self.ccx, dropped_place.local, location) {
+                if !self.qualifs.needs_drop(self.ccx, dropped_place.local, location) {
+                    return;
+                }
+
+                let ConstCx { param_env, body, tcx, def_id, .. } = *self.ccx;
+
+                // Replicate some logic from drop elaboration during const-checking. If we know
+                // that the active variant of an enum does not have drop glue, we can allow it to
+                // be dropped.
+                let maybe_inits = self.maybe_inits.get_or_insert_with(|| {
+                    let move_data = MoveData::gather_moves(body, tcx, param_env).unwrap();
+                    let mdpe = MoveDataParamEnv { move_data, param_env };
+                    dataflow::impls::MaybeInitializedPlaces::new(tcx, body, mdpe)
+                        .mark_inactive_variants_as_uninit(true)
+                        .into_engine(tcx, body, def_id.to_def_id())
+                        .iterate_to_fixpoint()
+                        .into_results_cursor(body)
+                });
+                maybe_inits.seek_before_primary_effect(location);
+                let mdpe = &maybe_inits.analysis().mdpe;
+
+                let dropped_mpi = mdpe
+                    .move_data
+                    .rev_lookup
+                    .find(dropped_place.as_ref())
+                    .expect_exact("All dropped places should have a move path");
+
+                let mut is_live_drop = false;
+                on_all_drop_children_bits(tcx, body, mdpe, dropped_mpi, |mpi| {
+                    if maybe_inits.contains(mpi) {
+                        is_live_drop = true;
+                    }
+                });
+
+                if is_live_drop {
                     // Use the span where the dropped local was declared for the error.
                     let span = self.body.local_decls[dropped_place.local].source_info.span;
                     self.check_live_drop(span);