`BitSlice` or a sound way to implement one

robofinch:

Edit: wait, doesn’t this imply that &mut u1: Send cannot be permitted to hold, so u1: !Send?

The painful thing here is that the way auto traits work is that this also would prevent the whole struct or array from being Send by way of the auto trait mechanism if it contains a u1. And it would be rather painful to have to do manual unsafe impls all over the code.