bench(functions): group the 63 bench targets into 9

[Dandandan]

Which issue does this PR close?

None. Found while profiling compile times (#24325, #24326, #24329, #24330,
#24338, #24339) — this is the largest remaining item I measured.

Rationale for this change

datafusion-functions declares 63 [[bench]] targets. Each one is a
separate binary that statically links this crate, arrow and criterion, so
building the crate's benchmarks means 63 link steps.

Linking, not compiling, is the bulk of it: rebuilding a single bench target
takes 10.6s, almost all of it link, while the 63 benchmark files together are
only ~10.5k lines of source.

This is paid by cargo bench, by cargo build --all-targets, and by CI's
clippy --all-targets.

What changes are included in this PR?

The targets are grouped by their required-features, which preserves that
semantics exactly — a group can only be built when its feature is on, just as
each individual bench could before:

target	benches	required-features
string_expressions	16	["string_expressions"]
math_expressions	16	["math_expressions"]
unicode_expressions	12	["unicode_expressions"]
datetime_expressions	8	["datetime_expressions"]
regex_expressions	4	["regex_expressions"]
misc	4	none

crypto, encoding and dictionary_encoding stay standalone — they are the
only members of their feature groups, so grouping them would buy nothing.

Each group lives in its own directory, benches/<group>/, with main.rs
declaring the individual benchmarks as modules and listing their criterion
groups in a single criterion_main!. Cargo does not auto-discover targets
inside those directories, so no autobenches = false is needed — which also
keeps cargo machete working (it stops scanning benches/ when autodiscovery
is disabled). benches/helper.rs stays shared at the top level, reached with
#[path = "../helper.rs"].

The benchmark code itself is untouched. There is not a single change to a
bench_function, benchmark_group or bench_with_input line anywhere in the
diff — git reports every moved file as a 94–99% similarity rename. Per file the
change is only:

• drop criterion_main! — a module cannot define main
• drop the now-unused criterion_main import
• drop extern crate criterion (a no-op since edition 2018, present in 4 files)
• point the four users of benches/helper.rs at crate::helper

Are these changes tested?

Interleaved with main so machine drift cancels out:

cargo clean -p datafusion-functions then…	main	this PR
cargo build --benches	43.9s / 42.9s	28.4s / 28.3s
cargo check --benches	15.0s / 14.7s	12.9s / 12.8s

Every benchmark still runs, and the counts show nothing was dropped:

target	benchmarks (-- --test)
string_expressions	252
unicode_expressions	251
datetime_expressions	103
dictionary_encoding	36
regex_expressions	27
misc	21
encoding	9
crypto	10 (with --features crypto_expressions)

Also clean: cargo clippy -p datafusion-functions --all-targets,
cargo fmt --check, and cargo machete --with-metadata.

Filtering a single benchmark still works — cargo bench --bench math_expressions -- power selects exactly one.

cargo metadata reports 10 bench targets, against 64 on main; both counts
include the auto-discovered helper, which is unchanged by this PR.

Are there any user-facing changes?

Only for running an individual benchmark. Where you would previously write

cargo bench -p datafusion-functions --bench lower

you now select the group and filter by name:

cargo bench -p datafusion-functions --bench string_expressions -- lower

The filter is criterion's own, matching on benchmark id, so it also narrows to a
single case within a benchmark. No library changes.

If this approach looks right, roughly 100 more bench targets across the other
crates could get the same treatment — I did this one crate first so the tradeoff
is visible on a real diff before it spreads.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.17%. Comparing base (8c099dc) to head (a6b5f3e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24342      +/-   ##
==========================================
+ Coverage   81.13%   81.17%   +0.03%     
==========================================
  Files        1112     1109       -3     
  Lines      388220   388033     -187     
  Branches   388220   388033     -187     
==========================================
- Hits       314985   314977       -8     
+ Misses      54696    54513     -183     
- Partials    18539    18543       +4     

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