Flood aggregate assignments with Top.

Author: cjgillot

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -122,7 +122,10 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
     ) {
         match rvalue {
             Rvalue::Aggregate(kind, operands) => {
-                state.flood_with(target.as_ref(), self.map(), FlatSet::Bottom);
+                // If we assign `target = Enum::Variant#0(operand)`,
+                // we must make sure that all `target as Variant#i` are `Top`.
+                state.flood(target.as_ref(), self.map());
+
                 if let Some(target_idx) = self.map().find(target.as_ref()) {
                     let (variant_target, variant_index) = match **kind {
                         AggregateKind::Tuple | AggregateKind::Closure(..) => {

tests/mir-opt/dataflow-const-prop/enum.multiple.DataflowConstProp.diff

@@ -71,8 +71,7 @@
 
       bb7: {
           StorageLive(_9);                 // scope 2 at $DIR/enum.rs:+11:9: +11:10
--         _9 = _6;                         // scope 2 at $DIR/enum.rs:+11:13: +11:14
-+         _9 = const 0_u8;                 // scope 2 at $DIR/enum.rs:+11:13: +11:14
+          _9 = _6;                         // scope 2 at $DIR/enum.rs:+11:13: +11:14
           _0 = const ();                   // scope 0 at $DIR/enum.rs:+0:29: +12:2
           StorageDead(_9);                 // scope 2 at $DIR/enum.rs:+12:1: +12:2
           StorageDead(_6);                 // scope 1 at $DIR/enum.rs:+12:1: +12:2