[vm] Support absent 'values' field in enum classes

VM queries the list of enum elements during class finalization
in order to be prepared for the future hot reload. This is done by
reading 'values' field. However, 'values' field can be missing if
running from a dill file which was tree-shaken. This change avoids
crash in that case and handles missing 'values' field as if it was
declared with an empty list.

TEST=Manually tested in g3.

Issue: #47861
Bug: b/213584901
Change-Id: Ic7271476ee115aa93a73a824af3a867784b820eb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/226981
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

runtime/vm/class_finalizer.cc

@@ -1226,15 +1226,16 @@ void ClassFinalizer::AllocateEnumValues(const Class& enum_cls) {
 
   const auto& values_field =
       Field::Handle(zone, enum_cls.LookupStaticField(Symbols::Values()));
-  ASSERT(!values_field.IsNull() && values_field.is_static() &&
-         values_field.is_const());
+  if (!values_field.IsNull()) {
+    ASSERT(values_field.is_static() && values_field.is_const());
 
-  const auto& values =
-      Object::Handle(zone, values_field.StaticConstFieldValue());
-  if (values.IsError()) {
-    ReportError(Error::Cast(values));
+    const auto& values =
+        Object::Handle(zone, values_field.StaticConstFieldValue());
+    if (values.IsError()) {
+      ReportError(Error::Cast(values));
+    }
+    ASSERT(values.IsArray());
   }
-  ASSERT(values.IsArray());
 
   // The enum_cls is the actual declared class.
   // The shared super-class holds the fields for index and name.

runtime/vm/object_reload.cc

@@ -325,9 +325,13 @@ void Class::ReplaceEnum(ProgramReloadContext* reload_context,
 
   {
     field = old_enum.LookupStaticField(Symbols::Values());
-    ASSERT(!field.IsNull() && field.is_static() && field.is_const());
-    old_enum_values ^= field.StaticConstFieldValue();
-    ASSERT(!old_enum_values.IsNull());
+    if (!field.IsNull()) {
+      ASSERT(field.is_static() && field.is_const());
+      old_enum_values ^= field.StaticConstFieldValue();
+      ASSERT(!old_enum_values.IsNull());
+    } else {
+      old_enum_values = Array::empty_array().ptr();
+    }
 
     field = old_enum.LookupStaticField(Symbols::_DeletedEnumSentinel());
     ASSERT(!field.IsNull() && field.is_static() && field.is_const());
@@ -357,9 +361,13 @@ void Class::ReplaceEnum(ProgramReloadContext* reload_context,
   bool enums_deleted = false;
   {
     field = LookupStaticField(Symbols::Values());
-    ASSERT(!field.IsNull() && field.is_static() && field.is_const());
-    enum_values ^= field.StaticConstFieldValue();
-    ASSERT(!enum_values.IsNull());
+    if (!field.IsNull()) {
+      ASSERT(field.is_static() && field.is_const());
+      enum_values ^= field.StaticConstFieldValue();
+      ASSERT(!enum_values.IsNull());
+    } else {
+      enum_values = Array::empty_array().ptr();
+    }
 
     field = LookupStaticField(Symbols::_DeletedEnumSentinel());
     ASSERT(!field.IsNull() && field.is_static() && field.is_const());