Add hasbit to proto3 fields even when 'optional' is missing.

PiperOrigin-RevId: 516687577

src/google/protobuf/compiler/java/message_field.cc

@@ -322,8 +322,10 @@ void ImmutableMessageFieldGenerator::GenerateBuilderMembers(
 
       "$name$Builder_.mergeFrom(value);\n",
 
-      "$set_has_field_bit_builder$\n"
-      "$on_changed$\n"
+      "if ($name$_ != null) {\n"
+      "  $set_has_field_bit_builder$\n"
+      "  $on_changed$\n"
+      "}\n"
       "return this;\n");
 
   // Message.Builder clearField()

src/google/protobuf/descriptor.cc

@@ -8426,19 +8426,7 @@ bool HasPreservingUnknownEnumSemantics(const FieldDescriptor* field) {
 }
 
 bool HasHasbit(const FieldDescriptor* field) {
-  // This predicate includes proto3 message fields only if they have "optional".
-  //   Foo submsg1 = 1;           // HasHasbit() == false
-  //   optional Foo submsg2 = 2;  // HasHasbit() == true
-  // This is slightly odd, as adding "optional" to a singular proto3 field does
-  // not change the semantics or API. However whenever any field in a message
-  // has a hasbit, it forces reflection to include hasbit offsets for *all*
-  // fields, even if almost all of them are set to -1 (no hasbit). So to avoid
-  // causing a sudden size regression for ~all proto3 messages, we give proto3
-  // message fields a hasbit only if "optional" is present. If the user is
-  // explicitly writing "optional", it is likely they are writing it on
-  // primitive fields also.
-  return (FieldDescriptorLegacy(field).has_optional_keyword() ||
-          field->is_required()) &&
+  return field->has_presence() && !field->real_containing_oneof() &&
          !field->options().weak();
 }