CC: @cagatay-y

I am confused about how this works. When I extract the expression with slice_from_raw_parts_mut into a variable, rust-analyzer reports the type as *mut [u8]. len is not the length of the entire struct, rather the payload, so the slice we create here does not cover the whole struct. I validated this by checking the length of the pointed slice.

The reason I originally used 0 as the slice length was because that was how it was in the previous version of the code, but I think reverting 93fc6cb and using ptr_metadata here may be worth the unstable feature. I included a commit with the reversion in my PR (#1113) for #1111, but I can remove it if you prefer.