Fix unsound optimization with explicit variant discriminants

compiler/rustc_mir_transform/src/simplify_try.rs

@@ -713,6 +713,8 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
  ty::Adt(adt, _) if adt.is_enum() => adt,
  _ => return StatementEquality::NotEqual,
  };
+ // We need to make sure that the switch value that targets the bb with
+ // SetDiscriminant is the same as the variant discriminant.
  let variant_discr = adt.discriminant_for_variant(self.tcx, *variant_index).val;
  if variant_discr != switch_value {
  trace!(
@@ -750,20 +752,28 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
  (
  StatementKind::Assign(box (_, rhs)),
  StatementKind::SetDiscriminant { place, variant_index },
- )
- // we need to make sure that the switch value that targets the bb with SetDiscriminant (y), is the same as the variant index
- if y_target_and_value.value.is_some() => {
+ ) if y_target_and_value.value.is_some() => {
  // choose basic block of x, as that has the assign
- helper(rhs, place, variant_index, y_target_and_value.value.unwrap(), x_target_and_value.target)
+ helper(
+ rhs,
+ place,
+ variant_index,
+ y_target_and_value.value.unwrap(),
+ x_target_and_value.target,
+ )
  }
  (
  StatementKind::SetDiscriminant { place, variant_index },
  StatementKind::Assign(box (_, rhs)),
- )
- // we need to make sure that the switch value that targets the bb with SetDiscriminant (x), is the same as the variant index
- if x_target_and_value.value.is_some() => {
+ ) if x_target_and_value.value.is_some() => {
  // choose basic block of y, as that has the assign
- helper(rhs, place, variant_index, x_target_and_value.value.unwrap(), y_target_and_value.target)
+ helper(
+ rhs,
+ place,
+ variant_index,
+ x_target_and_value.value.unwrap(),
+ y_target_and_value.target,
+ )
  }
  _ => {
  trace!("NO: statements `{:?}` and `{:?}` not considered equal", x, y);