Whether the message is an automatically generated map entry type for the maps field.

For maps fields:

map<KeyType, ValueType> map_field = 1;

The parsed descriptor looks like:

message MapFieldEntry {
    option map_entry = true;
    optional KeyType key = 1;
    optional ValueType value = 2;
repeated MapFieldEntry map_field = 1;

Implementations may choose not to generate the map_entry=true message, but use a native map in the target language to hold the keys and values. The reflection APIs in such implementions still need to work as if the field is a repeated message field.

NOTE: Do not set the option in .proto files. Always use the maps syntax instead. The option should only be implicitly set by the proto compiler parser.