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What is the validity invariant of Box<T>? If this was just a library type it would be the same as Unique<T>, which is the same as NonNull<T>. But actually most of the compiler treats Box<T> a lot like a reference, so at least de-facto right now, Box<T> likely has the same validity invariant as &T and &mut T (which is discussed in #76 and #77).
Also see some prior discussion in rust-lang/rust-memory-model#3.