GH-50514: [R] read_ipc_stream fails to unify nested Enum fields inside a Struct array across record batches

[thisisnic]

Rationale for this change

When converting list columns to R, each list element was converted independently, so decisions like factor levels (and uint32/int64 integer-vs-double sizing) could differ between elements and from the column's vctrs_list_of ptype, producing malformed factors when unnesting.

What changes are included in this PR?

List, large list, fixed-size list, and map converters now build one converter over the values of all chunks, so factor levels are unified and integer sizing is decided once for the whole column; the ptype is derived from that converter instead of an empty array. Also fixes a missing return() when inferring a list type from a ptype.

Are these changes tested?

Yes, new tests covering dictionaries (including ordered, nested in structs, and across chunks), int64/uint32 consistency, sliced arrays, extension types, and empty/all-null columns.

Are there any user-facing changes?

Yes: elements of converted list columns now all share one R type (e.g. factors with unified levels), documented in NEWS.md.

• GitHub Issue: [R] read_ipc_stream fails to unify nested Enum fields inside a Struct array across record batches #50514

[github-actions]

⚠️ GitHub issue #50514 has been automatically assigned in GitHub to PR creator.

[Copilot]

🟢 Approval recommended

The changes are coherent, well-tested across the reported edge cases (nested dictionaries/structs/chunks/slicing/empty), and the implementation matches the stated rationale without introducing obvious correctness or API risks.

Pull request overview

Fixes inconsistent R type decisions when converting Arrow list-like columns (including nested dictionaries/factors) by unifying conversion decisions across all chunks/values, preventing malformed factors during downstream operations like tidyr::unnest() (GH-50514).

Changes:

• Build one values-level converter per list-like chunked array (list/large_list/fixed_size_list/map) so factor levels and numeric sizing decisions are made once for the whole column.
• Add slice-aware allocation/conversion hooks to avoid altrep vectors that incorrectly shadow the full chunked array when converting list elements (notably for structs and extension types).
• Add targeted tests + user-facing documentation/NEWS updates; fix a missing return() in list type inference from ptype.

File summaries

File	Description
r/tests/testthat/test-chunked-array.R	Adds regression test ensuring chunked list conversion unifies nested factor levels.
r/tests/testthat/test-Array.R	Adds comprehensive tests for list-like conversions across chunks (dictionary/ordered/struct nesting/int64+uint32 behavior/extension types/empty cases/slicing).
r/src/type_infer.cpp	Fixes missing return when inferring list() type from attr(x, "ptype").
r/src/array_to_vector.cpp	Refactors list-like converters to reuse a unified values converter; introduces AllocateSlice()/ConvertSlice() to ensure correct slice semantics and ptype derivation.
r/R/type.R	Documents that int64/uint32 sizing decisions inside lists are made once per column.
r/NEWS.md	Notes the user-facing behavior change for list-column element type consistency (factors/integers) and references #50514.
r/man/data-type.Rd	Updates rendered documentation to match the new list-nested sizing behavior.

Review details

Files not reviewed (1)

• r/man/data-type.Rd: Generated file

• Files reviewed: 6/7 changed files
• Comments generated: 0
• Review effort level: Lite

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[Copilot]

🟢 Approval recommended

The changes are well-scoped to R conversion internals, and the PR adds comprehensive regression tests covering the reported failure mode and key edge cases (chunks, nesting, slicing, extension types, and empty/null inputs).

Review details

Files not reviewed (1)

• r/man/data-type.Rd: Generated file

• Files reviewed: 6/7 changed files
• Comments generated: 0 new
• Review effort level: Lite