I filed a PR to track this (https://issues.apache.org/jira/browse/ARROW-2521) but thought it was worth raising on the mailing list too.
I am running into limitations now of the way that Array is represented as an enum and I am unable to implement List<List<T>> with the current design. When Krisztian Szucs and I were working on the initial code we had two different approaches and we went with this enum approach at the time because we weren't able to make the other approach (traits + generics) work. Now that I'm further along the Rust learning curve, I can make the trait + generic approach work and I'm currently prototyping in a separate repo, and it is looking good so far. I have been able to create a struct array containing different type fields including List<List<T>>. I think I'm ready to start the refactor for real in my fork. We only have ~1k LOC so I don't think it will take too long, but because I'm doing this in my spare time I am going to estimate that I will have it complete in just over one week, aiming for having it complete by 4/30. I think it's fine to continue merging small PRs in the meanwhile but I think we should hold off any major changes in the coming week. Thanks, Andy.
