Add try_zeroed_vec and zeroed_vec

[fu5ha]

So, I believe this is sound as it essentially exactly mimics the internal implementation of Vec::with_capacity except using alloc_zeroed instead. However, Vec::from_raw_parts_mut is scary so would like to get someone else to review it and make sure I'm not going crazy.

[5225225]

I believe vec's allocation strategy is unspecified, so using from_raw_parts on something vec didn't allocate itself is questionable.

I'd do

let mut v = Vec::new();
v.try_reserve(length)?;
// somehow zero the vec, maybe make a zeroed item with T::zeroed() and write it to all items? or use `resize_with(T::zeroed)`
Ok(v)

[Noratrieb]

I believe vec's allocation strategy is unspecified

Sounds like a bug in vecs documentation. Vec<T> should always be allocated using the global allocator (as it currently works with the unstable allocator API), so in my opinion this should be fine.

[notgull]

This should be core::ptr::slice_from_raw_parts_mut() to avoid creating an intermediate reference (also also reduces the amount of unsafe by one line).

[Lokathor]

That's a possible change, but in this case the intermediate reference is fine, because the allocation is initialized anyway.

[fu5ha]

I think try_zeroed_boxed_slice().into_vec() is the best strategy here :) Didn't realize into_vec existed on a boxed slice

[fu5ha]

@Lokathor I think this is a pretty easy change with the new strategy. Thoughts/hesitations?