[FEA] libcudf APIs accepting `std::vector<T> const&` should instead accept a span

[wence-]

Is your feature request related to a problem? Please describe.

Consider, for example, cudf::merge

std::unique_ptr<cudf::table> merge(
   std::vector<table_view> const& tables_to_merge,
   std::vector<cudf::size_type> const& key_cols,
   std::vector<cudf::order> const& column_order,
   std::vector<cudf::null_order> const& null_precedence = {},
   cuda::stream_ref stream = cudf::get_default_stream(),
   rmm::device_async_resource_ref mr  = cudf::get_current_device_resource_ref());

Suppose that I have a perfectly decent array of values for the key_cols argument, perhaps a {cudf::size_type *, std::size_t} pair. I would have this if, for example, I am in a compiled language and trying to interface with the libcudf API: I will do so via a C ABI-shim.

The current libcudf APIs require me to materialise, by copy, a std::vector<cudf::size_type> from my span-like object.

Describe the solution you'd like

A putative signature:

std::unique_ptr<cudf::table> merge(
   std::span<table_view const> tables_to_merge,
   std::span<cudf::size_type const> key_cols,
   std::span<cudf::order const> column_order,
   std::span<cudf::null_order const> null_precedence = {},
   cuda::stream_ref stream = cudf::get_default_stream(),
   rmm::device_async_resource_ref mr  = cudf::get_current_device_resource_ref());

That way, if I have a std::vector I'm good, and equally if I have a T const *, size pair I'm also good.

Describe alternatives you've considered

Just make a local copy.

[wence-]

Here's a partial audit of call sites (not perhaps exhaustive):

Public API

Header	Function or method(s)
aggregation.hpp	make_quantile_aggregation
ast/expressions.hpp	operation constructor
column/column_factories.hpp	make_strings_column_batch
column/column_view.hpp	column_view and mutable_column_view constructors
contiguous_split.hpp	contiguous_split
copying.hpp	scalar scatter and boolean_mask_scatter
groupby.hpp	groupby constructor; groupby::aggregate
io/data_sink.hpp	data_sink::create<T>
io/datasource.hpp	datasource::create<T>
io/types.hpp	source_info and sink_info constructors
io/parquet.hpp	merge_row_group_metadata; chunked_parquet_writer::write; close
merge.hpp	merge
partitioning.hpp	hash_partition
quantiles.hpp	quantile; quantiles
search.hpp	contains and contains_ordered
sorting.hpp	is_sorted, sort, stable_sort, sort_by_key, stable_sort_by_key, segmented_sort, segmented_sort_by_key, stable_segmented_sort, and stable_segmented_sort_by_key
stream_compaction.hpp	drop_nulls, drop_nans, distinct, stable_distinct, unique, stable_unique, drop_duplicates, and stable_drop_duplicates; filter and filter_extended
strings/translate.hpp	translate
table/table.hpp	table::select
table/table_view.hpp	table_view_base constructors, table_view constructor, mutable_table_view constructor, table_view::select, and scatter_columns

Detail API

Header	Function or method
ast/detail/expression_parser.hpp	detail::expression_parser::extract_size_and_pointer
ast/detail/operators.hpp	detail::is_valid
detail/aggregation/aggregation.hpp	quantile_aggregation constructor
detail/contiguous_split.hpp	contiguous_split
detail/gather.cuh	bitmask gather helper maybe
detail/groupby/sort_helper.hpp	sort_helper constructor
detail/merge.hpp	merge
detail/quantiles.hpp	quantile and quantiles
detail/scatter.hpp	scalar scatter overloads
detail/search.hpp	ordered-search helpers
detail/sorting.hpp	all sort/is-sorted/segmented-sort detail entry points
detail/stream_compaction.hpp	null/NaN/distinct/unique helpers
detail/structs/utilities.hpp	table_flattener and flatten_nested_columns
detail/utilities/vector_factories.hpp	make_device_uvector_async and make_device_uvector
io/detail/parquet.hpp	writer write, close, and merge_row_group_metadata
io/text/detail/trie.hpp	trie::create

Internal-linkage source audit (cpp/src)

Source	Description
table/table_view.cpp	concatenate_column_views
copying/contiguous_split.cu, copying/pack.cpp	anonymous packing/splitting helpers
io/comp/compression.cpp, io/comp/decompression.cpp	internal compression/decompression helpers
row_operator/row_operators.cu	replace_child maybe
search/search_ordered.cu	search_ordered
strings/strings_column_factories.cu	make_offsets_child_column_batch_async
io/csv/reader_impl.cu	load_data_and_gather_row_offsets and CSV helpers
io/json/json_column.cu, io/json/read_json.cu	anonymous JSON helpers
io/orc/aggregate_orc_metadata.cpp, io/orc/reader_impl_decode.cu, io/orc/writer_impl.cu	anonymous ORC helpers
io/parquet/arrow_schema_writer.cpp, io/parquet/reader_impl_dict_transcode.cu	anonymous parquet helpers
io/parquet/writer_impl.cu	update_chunk_encodings maybe
io/parquet/writer_impl.cu	additional anonymous schema helpers
transform/row_bit_count.cu, transform/transform.cu	anonymous flattening/transform helpers
strings/regex/glushkov_regcomp.cpp, strings/regex/regcomp.cpp	anonymous regex helpers
merge/merge.cu	anonymous merge-index helpers

[vyasr]

Dupe of #11116 although narrower since this issue is specifically referring to const refs.