6cb69ea790
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.
For more information about how rustc works, see the rustc dev guide.