That's why I asked if that's an attempt to show that Rust is not yet “done” (is there any language which is actually 100% done?) or some kind of practical issue.
In practice using unsafe to allocate memory directly on heap is cumbersome and error-prone, but doable, and you couldn't have bazillion places in your code where you allocate 100Mb types (1000 times by 100Mb means you use 100Gb and most apps couldn't afford that). And that's nice practical answer.
In theory you may use that an excuse to avoid Rust and using C or C++ — by explaining how you, personally, can't use language which doesn't perform that rare-yet-sometimes-important trick perfectly. And that couldn't be refuted if you are interested in theoretical debate, not in practical work.