Tom-Newton opened a new pull request, #44587:
URL: https://github.com/apache/arrow/pull/44587
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### Rationale for this change
Sometimes its useful to add a column full of nulls in a cast. Currently this
is supported in top level columns but not inside structs. Example where this is
important: https://github.com/delta-io/delta-rs/issues/1610
### What changes are included in this PR?
Add support for filling in columns full of null for nullable struct fields.
I've gone for a fairly minimal change that achieves what I needed but I
wonder if there should be a more significant change so that this casting is
done by field name and ignore the field order.
### Are these changes tested?
Yes. The expected behaviour in several existing tests has been altered and a
couple of new tests have been added.
I also rolled out a custom build with this change internally because it
suddenly became a critical problem.
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### Are there any user-facing changes?
Yes. There are scenarios that previously failed with `struct fields don't
match or are in the wrong order` but now succeed after filling in `null`s.
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