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 )