These variables are set by the init function in descriptor.pb.go via logic in internal/filetype. In other words, so long as the descriptor proto package is linked in, these variables will be populated.
Each variable is populated with a nil pointer to the options struct.
var (
File pref.ProtoMessage
Enum pref.ProtoMessage
EnumValue pref.ProtoMessage
Message pref.ProtoMessage
Field pref.ProtoMessage
Oneof pref.ProtoMessage
ExtensionRange pref.ProtoMessage
Service pref.ProtoMessage
Method pref.ProtoMessage
)