[tialaramex]

If you run nightly you can already Box::new_in(thing, my_allocator) if that is what you want. Or indeed, if you want to keep Nathan here happy, you can even Box::try_new_in(thing, my_allocator) and get Errors when your allocator says it can't help you (which you likely won't ever bother handling meaningfully).

In the event you're as desperate for those last dregs of performance as the author of the "Why not rust?" article, (and let's assume you actually measured your performance problem before jumping to the step where you make everything more complicated and more buggy for no good reason?) you can even Box::try_new_uninit_in() and unsafely do all the initialization in place as needed yourself as you might do by default in C++.

Now, I don't know when Allocators will be stabilized, the Working Group has been making steady progress for some years now but this is a tricky business, witness the many languages that get it quite badly wrong...

[ncmncm]

Imagining that any of this has to do with "last dregs of performance" demonstrates that you have completely misunderstood the topic.