If/when you bring this to the cargo team, it would be good to include reproduction steps as well as characterize the performance impact (when it happens, how slow it can be, and end-user impact). In addition to the problems you gave, note that the function you linked to specifically calls out that it is blocking and why. Git source updates use progress bars, there is no support for concurrent progress bars in cargo, so these must be disabled for concurrent source updates. Should I add support...
