keirsalterego commented on issue #6528:
URL: https://github.com/apache/arrow-rs/issues/6528#issuecomment-3724793516

   hey @alamb @tustvold I'd like to pick this up. 
   I read through the thread and I get the safety concern regarding 
`DictionaryArray` and `unmasking null slots`. To avoid hitting any UB or 
reading garbage data, I’m planning to implement `with_nulls` using a subset 
invariant check.
   
   The logic would ensure that the new `NullBuffer` is a subset of the existing 
validity (i.e., we can turn valid slots into nulls, but we cannot turn null 
slots back into valid values). This satisfies the primary use case mentioned 
(combining existing nulls with a new filter) while guaranteeing we never 
typically unmask garbage or undefined data.
   
   Does that approach sound good to you? If so, I’ll start with the primitive 
arrays.


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