Fix back-porting drop-livess from Polonius to tracing

compiler/rustc_borrowck/src/type_check/liveness/polonius.rs

@@ -14,7 +14,7 @@ struct UseFactsExtractor<'me, 'tcx> {
     var_defined_at: &'me mut VarPointRelation,
     var_used_at: &'me mut VarPointRelation,
     location_table: &'me LocationTable,
-    var_dropped_at: &'me mut VarPointRelation,
+    var_dropped_at: &'me mut Vec<(Local, Location)>,
     move_data: &'me MoveData<'tcx>,
     path_accessed_at_base: &'me mut PathPointRelation,
 }
@@ -37,7 +37,7 @@ impl<'tcx> UseFactsExtractor<'_, 'tcx> {
 
     fn insert_drop_use(&mut self, local: Local, location: Location) {
         debug!("UseFactsExtractor::insert_drop_use()");
-        self.var_dropped_at.push((local, self.location_to_index(location)));
+        self.var_dropped_at.push((local, location));
     }
 
     fn insert_path_access(&mut self, path: MovePathIndex, location: Location) {
@@ -87,8 +87,12 @@ pub(super) fn populate_access_facts<'a, 'tcx>(
     body: &Body<'tcx>,
     location_table: &LocationTable,
     move_data: &MoveData<'tcx>,
-    //FIXME: this is not mutated, but expected to be modified as
-    // out param, bug?
+    // FIXME: this is an inelegant way of squirreling away a
+    // copy of `var_dropped_at` in the original `Location` format
+    // for later use in `trace::trace()`, which updates some liveness-
+    // internal data based on what Polonius saw.
+    // Ideally, that part would access the Polonius facts directly, and this
+    // would be regular facts gathering.
     dropped_at: &mut Vec<(Local, Location)>,
 ) {
     debug!("populate_access_facts()");
@@ -97,7 +101,7 @@ pub(super) fn populate_access_facts<'a, 'tcx>(
         let mut extractor = UseFactsExtractor {
             var_defined_at: &mut facts.var_defined_at,
             var_used_at: &mut facts.var_used_at,
-            var_dropped_at: &mut facts.var_dropped_at,
+            var_dropped_at: dropped_at,
             path_accessed_at_base: &mut facts.path_accessed_at_base,
             location_table,
             move_data,