[rajaryan2007]

Rationale for this change

Casting a list_view to a list (or across offset widths) incorrectly reused the offsets buffer and ignored the sizes buffer. This caused out-of-bounds memory reads (crashes) and generated invalid data whenever views were overlapping or out-of-order.

views = pa.ListViewArray.from_arrays(
    pa.array([2, 0, 1], pa.int32()),   # offsets
    pa.array([2, 2, 1], pa.int32()),   # sizes
    pa.array([1, 2, 3, 4], pa.int32()))
views.cast(pa.list_(pa.int32())).to_pylist()

# Before: [[], [1], []]         (Invalid: negative / non-monotonic offsets)
# After:  [[3, 4], [1, 2], [2]] (Correct)

### What changes are included in this PR?
list_util::internal::ListFromListView: New shared internal helper that materializes a standard list layout by gathering the values referenced by each view. Supports all 4 offset-width combinations.

CastList Update: List-view sources now use this helper before casting the child array.

FromListView Fix: ListArray::FromListView and LargeListArray::FromListView now delegate to this shared helper. This removes duplicated code (-31 lines) and fixes a bug where nulls were handled incorrectly for sliced inputs (GH-51613).

Are these changes tested?

Yes. Added and updated tests in C++ (scalar_cast_test.cc, list_util_test.cc, array_list_test.cc) and Python (test_array.py). Tests cover all offset-width combinations, null views, out-of-order/overlapping views, sliced inputs, zero-length edge cases, and nested list-views.

Are there any user-facing changes?

Yes.

Performance: list_view -> list casts are no longer zero-copy. The referenced values are now always gathered into a new child array. This is a necessary performance cost to guarantee correctness.

This PR includes breaking changes to public APIs.

ListArray::FromListView and LargeListViewArray::FromListView now return correct nulls for sliced inputs. This changes the output for users who may have been relying on the previous bugged behavior.

This PR contains a "Critical Fix".

Yes, this fixes a bug that produced incorrect and invalid data (negative/non-monotonic offsets that fail validation) and prevents Check failed: (off) <= (length) abort crashes caused by out-of-bounds reads.

Was AI used for this PR?

I used AI to help explain concepts, generate some inline code comments, and assist with writing and building tests.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [Python][C++] Casting list_view to list ignores the view sizes and gives wrong results #51612

[github-actions]

⚠️ GitHub issue #51612 has been automatically assigned in GitHub to PR creator.

[jonasdedden]

@pitrou @felipecrv @kou could one of you take a look at this when you have a moment? It touches the list-view code (ListArray::FromListView and the list-view -> list cast) that you wrote or reviewed originally.

Two points that probably need a committer's call:

• list-view -> list casts are no longer zero-copy; the referenced child values are always gathered into a new child array.
• FromListView changes behaviour for sliced inputs (null handling, [C++] ListArray::FromListView gives wrong nulls for sliced list-view arrays #51613).

[dmitry-chirkov-dremio]

In Debug build, casting list_view<run_end_encoded<int32, extension>> aborts with Check failed: array.type->Equals(type_) (builder_run_end.cc:255). The builder has the storage type, but values has the extension type. Please fix it and add a test.
In the description, close the code block and add * Closes: #51613.

[chrikrah]

Nothing here blocks a merge.

# head 0cd374e27bff (base 9b34c4a6a), cmake -DCMAKE_BUILD_TYPE=Debug -DARROW_COMPUTE=ON -DARROW_BUILD_TESTS=ON, gcc 13
$ ./arrow-compute-scalar-cast-test; ./arrow-array-test; ./arrow-utility-test
[  PASSED  ] 114 tests.
[  PASSED  ] 1074 tests.
[  PASSED  ] 854 tests.
# Cast.ListViewToListWithNestedRunEndEncodedChild covers the run_end_encoded<int32, extension> abort from the 10-28 comment, and passes in this Debug build.

# scalar_cast_nested.cc and array_nested.cc reverted to 9b34c4a6a, tests kept
$ ./arrow-compute-scalar-cast-test --gtest_filter=Cast.ListViewToList
data.cc:190:  Check failed: (off) <= (length) Slice offset (1) greater than array length (0)
$ ./arrow-array-test
[  FAILED  ] TestListConversions.ListFromSlicedListView

# not run: pyarrow tests/test_array.py, release build, benchmarks

non-blocking: the gather is stated; pitrou asked in #50976 for contiguous views sliced in the offsets loop, skipping values only null views reference. Tried that?

[kou]

Could you split a fix for #51613 to another PR for easy to review?

list-view -> list casts are no longer zero-copy; the referenced child values are always gathered into a new child array.

@rajaryan2007 Can we keep zero-copy?