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 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).