Cache the result of FieldDescriptor::is_map().

This is queried from reflection frequently.

PiperOrigin-RevId: 756861989

Author: sbenzaquen

src/google/protobuf/descriptor.cc

@@ -7035,6 +7035,7 @@ void DescriptorBuilder::BuildFieldOrExtension(const FieldDescriptorProto& proto,
   result->is_extension_ = is_extension;
   result->is_oneof_ = false;
   result->in_real_oneof_ = false;
+  result->is_map_ = false;
   result->proto3_optional_ = proto.proto3_optional();
   result->legacy_proto_ctype_ = FieldOptions::CType_MAX + 1;
   // We initialize to STRING because descriptor.proto needs it for
@@ -7982,6 +7983,13 @@ void DescriptorBuilder::CrossLinkField(FieldDescriptor* field,
       }
     }
 
+    // Map entries must be in the same file, so we can populate it directly if
+    // the descriptor is already known. If it is not known, then it must not be
+    // a map entry.
+    if (auto* sub_message = type.descriptor()) {
+      field->is_map_ = sub_message->options().map_entry();
+    }
+
     if (!type.IsVisibleFrom(file_)) {
       AddError(field->full_name(), proto, DescriptorPool::ErrorCollector::TYPE,
                [&] { return type.GetVisibilityError(file_); });

src/google/protobuf/descriptor.h

@@ -1182,6 +1182,10 @@ class PROTOBUF_EXPORT FieldDescriptor : private internal::SymbolBase,
   // Located here for bitpacking.
   bool in_real_oneof_ : 1;
 
+  // We could calculate as `message_type()->options().map_entry()`, but that is
+  // way more expensive and can potentially force load extra lazy files.
+  bool is_map_ : 1;
+
   // Actually an optional `CType`, but stored as uint8_t to save space.  This
   // contains the original ctype option specified in the .proto file.
   uint8_t legacy_proto_ctype_ : 2;
@@ -2937,7 +2941,8 @@ inline bool FieldDescriptor::is_packable() const {
 }
 
 inline bool FieldDescriptor::is_map() const {
-  return type() == TYPE_MESSAGE && is_map_message_type();
+  ABSL_DCHECK_EQ(is_map_, type() == TYPE_MESSAGE && is_map_message_type());
+  return is_map_;
 }
 
 inline const OneofDescriptor* FieldDescriptor::real_containing_oneof() const {