Support StructArray in Cast Kernel

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

There is currently extremely limited support for StructArray in the cast kernel

Describe the solution you'd like

We should support combinations of:

• Re-ordering columns
• Adding new nullable fields by adding entirely null columns as appropriate
• Casting columns within the array

Describe alternatives you've considered

Additional context

[fansehep]

Hi @tustvold I want to try this. Can you in detail and with specific reference to these three points? I am not very sure what you want.

[tustvold]

Re-ordering columns

Casting from

DataType::Struct(Fields::new(vec![
    Field::new("a", DataType::Int32, false),
    Field::new("b", DataType::Int32, false),
])

To

DataType::Struct(Fields::new(vec![
    Field::new("b", DataType::Int32, false),
    Field::new("a", DataType::Int32, false),
])

Adding new nullable fields by adding entirely null columns as appropriate

Casting from

DataType::Struct(Fields::new(vec![
    Field::new("a", DataType::Int32, false),
    Field::new("b", DataType::Int32, false),
])

To

DataType::Struct(Fields::new(vec![
    Field::new("a", DataType::Int32, false),
    Field::new("b", DataType::Int32, false),
    Field::new("c", DataType::Int32, true),
])

Casting columns within the array

DataType::Struct(Fields::new(vec![
    Field::new("a", DataType::Int32, false),
    Field::new("b", DataType::Int32, false),
])

To

DataType::Struct(Fields::new(vec![
    Field::new("a", DataType::Int32, true),
    Field::new("b", DataType::Int64, false),
])

We should support combinations of:

So the eventual goal would be to support

DataType::Struct(Fields::new(vec![
    Field::new("a", DataType::Int32, false),
    Field::new("b", DataType::Int32, false),
])

To

DataType::Struct(Fields::new(vec![
    Field::new("c", DataType::Float32, true),
    Field::new("b", DataType::Int64, false),
    Field::new("a", DataType::Int32, false),
])

[tustvold]

label_issue.py automatically added labels {'arrow'} from #4902

[tustvold]

Looks like #4985 prematurely closed this

[tustvold]

FYI @my-vegetable-has-exploded this is the broader issue

[alamb]

See #6726 (comment) for a non trivial discussion about this API

[alamb]

For the reasons described in #6735, closing this in favor of a more generic solution