in rules/starlark_configurations/cc_binary_selectable_copts/lib.cc [3:13]
void runlib() {
#ifdef feature1
std::cout << "Building lib with feature 1!\n";
#elif feature2
std::cout << "Building lib with feature 2!\n";
#else
std::cout << "Building lib without features. But the select() in the BUILD "
<< "file should prevent this case from building in the first "
<< "place.\n";
#endif
}