Legal values for a Box<T> where T is zero-sized

On IRC, @mystor asked me whether, given some zero-sized type ZST, it would be legal to represent Box<ZST> using some arbitrary non-null value, or must it be a particular value (e.g., 1)?

For example, if there is a pointer p to some stack variable, can one write unsafe { Box::from_raw(p as *mut ZST) } and have that be a legal box?

[Amanieu]

I’m of the opinion that Box::from_raw should only ever be called with a pointer allocated by Box::new.

Conceptually, we are allocating a zero-sized block of memory (which may have malloc headers, so the real allocation may be bigger than zero), and we should use the same pointer value when we are freeing it.

The fact that Box::new currently always returns a pointer value of 1 is an implementation detail: it’s just an optimization to avoid allocating zero-sized blocks of memory.

[RalfJung]

No 😦 We haven’t really started writing down anything yet… or even found a good structure to put stuff in. I hope to be able to work on that some more this summer.