Don't create references to uninitialized data in List::from_arena
#87268
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Previously
result
andarena_slice
were references pointing to uninitialized data, which is technically UB. They may have been fine because the pointed data isCopy
and and they were only written to, but the semantics of this aren't clearly defined yet, and since we have a sound way to do the same thing I don't think we should keep the possibly-unsound way.