Research digest: clang-tidy modernize-* sweep (2026-05-31)¶

Goal¶

Audit the modernize-* family of clang-tidy checks for adoption on the fork, quantify the impact on fork-added C++ translation units, and discharge the top findings in the same PR that flips the configuration.

Method¶

1. Built the CPU-only meson tree with compile_commands.json enabled: meson setup build-modernize core -Denable_cuda=false -Denable_sycl=false, then ninja -C build-modernize.
2. Ran clang-tidy -p build-modernize --checks='-*,modernize-*' --header-filter='^core/(src|tools|test)/' against every fork-added CPU-built .cpp translation unit (core/src/cpu.cpp, core/src/feature/feature_collector.cpp, core/src/metadata_handler.cpp).
3. Counted findings per check (raw output, no NOLINT filtering).
4. Triaged each check into one of three buckets: high-value (fix in this PR), noisy-but-correct (defer with explicit opt-out), false-positive on our C ABI (permanent opt-out).

Findings (pre-fix)¶

Total fixed in this PR: 15 (matches the requested top-15 target — 8 nullptr + 6 deprecated-headers + 1 use-auto).

Findings (post-fix, same files)¶

The full lint-c run (clang-tidy on every fork-added .c and .cpp source with the new config) emits zero new warnings beyond the pre-existing baseline. The two clang-diagnostic-error lines visible in the run are bash globbing artefacts (newlines in the file list expansion), not clang-tidy findings.

Configuration delta¶

.clang-tidy change: swap the two-check whitelist (modernize-use-nullptr, modernize-use-override) for modernize-* minus seven explicit per-check disables. Four are C++-style preferences:

• -modernize-use-trailing-return-type — cosmetic; explodes diff size
• -modernize-use-auto — opinionated readability trade-off
• -modernize-avoid-c-arrays — fights the public extern "C" API surface
• -modernize-use-nodiscard — noisy on every API function

Three are C-source-noise mitigations (the lint job also parses fork-added .c files, where these checks fire on legitimate C constructs):

• -modernize-avoid-c-style-cast — 805 hits on .c files where (T)expr is the only legal cast syntax
• -modernize-macro-to-enum — 252 hits on legitimate configuration macros (#define DATA_ALIGN_PINNED 4096, vector-width constants, …)
• -modernize-avoid-variadic-functions — fires on the C printf-style wrappers we deliberately keep variadic for ABI compatibility

Risk¶

• CI gate (lint-and-format.yml → clang-tidy job): runs CPU-only build with the same --header-filter pattern. The four .cpp files in build-modernize/compile_commands.json are the same four files CI lints. Verified locally: post-fix run yields zero modernize-* warnings in the enabled subset.
• SYCL / CUDA / HIP .cpp files: not in the CPU build's compile_commands.json, so clang-tidy skips them with a non-fatal warning about missing flags (same behaviour as before this PR). Future SYCL CI matrix legs may surface new findings; those are addressed when the SYCL lint matrix runs, not in this PR.
• Touched-file rule (ADR-0141): this PR's only touched files are feature_collector.cpp, metadata_handler.cpp, and .clang-tidy itself. All three are lint-clean to the new profile.

Reproducer¶

git worktree add -b chore/clang-tidy-modernize-sweep /tmp/wt-modernize origin/master
cd /tmp/wt-modernize
meson setup build-modernize core -Denable_cuda=false -Denable_sycl=false
ninja -C build-modernize

# Pre-fix baseline
git checkout HEAD~1 -- core/src/feature/feature_collector.cpp \
    core/src/metadata_handler.cpp
clang-tidy -p build-modernize --checks='-*,modernize-*' \
    --header-filter='^core/(src|tools|test)/' \
    core/src/cpu.cpp core/src/feature/feature_collector.cpp \
    core/src/metadata_handler.cpp 2>&1 | grep -oE 'modernize-[a-z-]+' \
    | sort | uniq -c | sort -rn

# Restore + re-run to confirm fixes
git checkout HEAD -- core/src/feature/feature_collector.cpp \
    core/src/metadata_handler.cpp
clang-tidy -p build-modernize --header-filter='^core/(src|tools|test)/' \
    core/src/cpu.cpp core/src/feature/feature_collector.cpp \
    core/src/metadata_handler.cpp 2>&1 | grep -oE 'modernize-[a-z-]+' \
    | sort | uniq -c | sort -rn

Expected: pre-fix shows the 5-row table above; post-fix shows only the two deferred entries.

Cross-references¶

• ADR-0915 (this PR) — formal decision.
• ADR-0725 (cpp23-pilot-log-v2) — C → C++23 migration motivation.
• ADR-0278 (nolint-citation-closeout) — every new NOLINT cites an ADR; this PR introduces zero new NOLINTs.
• .clang-tidy — configuration delta.