Memory leak with LocalFutureObj when not using Boxed futures

[EricRnR]

When using LocalFutureObj directly, the drop on LocalFutureObj here refers to the one of the empty drop function implementations such as here. The future will never be dropped, unlike the Boxed counterparts.

[taiki-e]

cc @Nemo157

[Nemo157]

@EricRnR do you have a code example? The empty drop implementations are for borrowed instances, so they don't need to do anything, the owner will drop the values at some point after the LocalFutureObj stops borrowing them. Other owning types are expected to implement UnsafeFutureObj themselves like Box does.

[EricRnR]

Yes, this makes sense. For some reason I had convinced myself UnsafeFutureObj was not part of the public API and I had no hook into the drop. Crisis averted!