But why does pinning solve the issue? Fundamentally the lifetime of the future is unchanged as far as the compiler is concerned so in theory the callback should be capable of doing the same stashing, no?
The lifetime _is_ changed; this lets you use the lifetime from the HRTB instead of the function generics. It’s not so much the pinning itself that does it, for the type system, but using the trait object enables referring to that HRTB to require true lifetime generic (and then pinning comes along for the ride).
