The common place to use something like this would be to mmap an existing external data structure. There are a number of existing mmap-able 0copy k/v library/db formats that fit the bill here.

> Like needing to allocate a fixed amount of memory for your whole program upfront

Can I not do the normal virtual memory thing and allocate far more memory than I actually have hardware for?

> Or that there is some state you can not carry over even with that and if you try you can easily run into soundness issues.

I mean, fair... you're going to have to be careful what you save either way but making saving things the default does seem like more of a footgun.

> Or that the state saved might be "in between" operations in a very unexpected way and in turn unsound.

This applies even if you only use it for some structs? This whole mechanism seems like it would only be sound if it was only used after clean exits.

> sound if it was only used after clean exits.

yes, except maybe if a Drop constructor is run unexpectedly on the exit or similar

generally I think using it with anything which has pointers and/or runs Drop in it is brittle and prone to bugs

in turn most things which do not have pointers should be fine with a clean exit

and anything which only consists of memory where any bit combination is always valid should always be sound even on a abort (e.g. a `[u8]` allocated directly in that memory region or a `[T]` where `T` only has primitive non-allocating types)