Make GenericDistinctBuffer generic over both Hashable and native types #18670

Is your feature request related to a problem or challenge?

GenericDistinctBuffer was introduced by #18348 to make it easier to write distinct accumulators. Currently it uses Hashable to represent the items being stored:

Hashable is mainly required because f32/f64 aren't hashable natively, so this wrapper lets us hash them + all other native types (e.g. i32, u64). However I wonder if there is a way to use Hashable only for floats, whilst using the native Hash implementation for other native types (potential performance benefit?).

Describe the solution you'd like

Be able to use GenericDistinctBuffer generically over both f32 and i32 types without needing to force both through Hashable interface. This could be done by having duplicated versions, one that takes Hashable and one that takes natively hashable types, but ideally want a single solution.

• If we had specialization this would be much easier 😅

Describe alternatives you've considered

No response

Additional context

If we could get this to work, we could fold PrimitiveDistinctCountAccumulator together with FloatDistinctCountAccumulator:

Also when making these changes we'd need to benchmark as likely there are performance implications; I initially tried forcing PrimitiveDistinctCountAccumulator to use Hashable but I kept hitting regressions on micro benchmark with count(distinct) queries, so need to keep that in mind.