SAN FRANCISCO :A key driver of Tesla's profit is disappearing fast as the U.S. government changes policies on an environmental asset known as regulatory credits. Investors are likely to have a number of questions for Chief Executive Elon Musk when Tesla reports second-quarter results on Wednesday. Among them are how fast the EV maker can turn a trial robotaxi program into a money-making business, how to avoid a decline in sales for the second year in a row, and Musk's possible political plans.