MutableListArray from an existing MutableArray

[cjermain]

Is there a way to directly nest a non-empty MutableArray in a MutableListArray?

In the majority of MutableArray implementations there is a from_trusted_len_iter method that allows the array to be constructed directly from an iterator. In the case of a MutableStructArray, the new method takes a vector of the inner MutableArrays, so you can directly construct it. Is there an analogy for MutableListArray?

It looks like the new_from has an assert that prevents non-empty MutableArrays from being passed through:

arrow2/src/array/list/mutable.rs

Line 118 in fb5e4d5

assert_eq!(values.len(), 0);

Along with the MutableArray I also have all of the lengths in a Vec<Option<usize>>. I had been hoping to use the following, but am failing on the assert during runtime:

let mut array = MutableListArray::<i32, _>::new_with_capacity(inner_array, lengths.len());
array.try_extend_from_lengths(lengths).unwrap();

I'm trying to accomplish the same thing in this function, without the pre-allocation of an empty inner MutableArray ahead of time:

arrow2/src/io/json/read/deserialize.rs

Lines 254 to 277 in fb5e4d5

	fn deserialize_list_into<'a, O: Offset, A: Borrow<Value<'a>>>(
	target: &mut MutableListArray<O, Box<dyn MutableArray>>,
	rows: &[A],
	) {
	let empty = vec![];
	let inner: Vec<_> = rows
	.iter()
	.flat_map(|row| match row.borrow() {
	Value::Array(value) => value.iter(),
	_ => empty.iter(),
	})
	.collect();
	deserialize_into(target.mut_values(), &inner);
	let lengths = rows.iter().map(|row| match row.borrow() {
	Value::Array(value) => Some(value.len()),
	_ => None,
	});
	target
	.try_extend_from_lengths(lengths)
	.expect("Offsets overflow");
	}

[cjermain]

@jorgecarleitao, what do you think?