litebox_{platform_lvbs,shim_optee}: simplify box_new_zeroed

[lschuermann]

The existing implementation of box_new_zeroed is unnecessarily complex. We can simplify it to two function calls from the standard library: create a new zeroed heap region, and treat it as an initialized instance of type T. This also weakens this functions preconditions: T no longer strictly needs to be zero-sized (removing a runtime panic), and we can use FromZeros instead of FromBytes: FromZeros. It does not produce stack intermediaries, an important invariant for large types.

This is a minor thing I noticed while reviewing #817; it can go in after that PR to avoid conflicts. I'll be happy to rebase it.

[lschuermann]

I see that the ratchet::ratchet_maybe_uninit test fails, which I assume is just a guard to ensure one is not inadvertently adding a new unsafe transmute? Either way, I think this is cleaner and more idiomatic; should I go ahead and add a path prefix to the ratchet test accordingly?

[jaybosamiya-ms]

Overall seems good to me, thanks Leon! Minor stylistic thing: prefer SAFETY: ... comment rather than # Safety\n\n... comment when discharging a safety obligation.

Also, fwiw, maybe we should use FromZeros::new_box_zeroed instead of the current body? Might need to tweak Cargo.toml to pull in the alloc feature, but since we're depending on alloc anyways, that's not an issue. Thoughts?

If we do not use the new_box_zeroed, then yes, we'd need to update the ratchet for this, thanks for confirming.

[lschuermann]

Agreed, new_box_zeroed seems strictly better. Enabling alloc on zerocopy sounds like the right call and could enable a bunch more cleanups. In that case, it's probably best to get #817 over the finish line first and do the cleanup inline on top of that merged result.

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