Rollup merge of #87268 - SkiFire13:fix-uninit-ref-list, r=nagisa

Don't create references to uninitialized data in `List::from_arena`

Previously `result` and `arena_slice` were references pointing to uninitialized data, which is technically UB. They may have been fine because the pointed data is `Copy` 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.