The stock-market reaction to Spirit Airlines’ troubles shows that every cloud can indeed have a silver lining. Shares in Spirit plummeted about 27% Friday following The Wall Street Journal’s report that the ultra-low-cost carrier is discussing a potential bankruptcy with bondholders, though the timing of such a filing may not be imminent. Inflation in labor and maintenance costs, as well as engine problems, have left the entire U.S. budget-airline industry in rough shape, but Spirit has been doing particularly badly because of its large refinancing needs.