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PR Review: fix(ci): force clang for Linux x86_64 native builds

Todo List

• Read and analyze changed workflow files
• Perform root cause analysis
• Check for configuration change risks
• Evaluate test integrity and approach
• Assess backlog compliance
• Provide final review recommendation

Root Cause Analysis

Problem Being Solved: The Linux x86_64 native binary built with gcc 14.2.0 silently drops 4 HCL fixture files in tests/benchmarks/resolution/fixtures/hcl/. This triggers WASM backfill on every build call, causing performance regression detected by pre-publish gates.

Why This Approach: The PR switches from gcc to clang for x86_64-linux-gnu builds based on local Docker verification showing gcc-built binaries drop .tf files while clang-built ones work correctly. This is a C compiler optimization issue in tree-sitter-hcl's bundled parser.c that cc-rs compiles.

Risk Assessment: Low risk. The change is targeted to the specific problematic target (x86_64-unknown-linux-gnu) and has been verified locally. Cross-compile targets already use their own toolchains and are unaffected.

Configuration Change Detection (LOW RISK)

This PR makes targeted CI configuration changes to address a specific C compiler bug:

1. build-native.yml: Adds clang for matrix.target == 'x86_64-unknown-linux-gnu' only
2. ci.yml: Adds clang for matrix.os == 'ubuntu-latest' in native-host-build job
3. publish.yml: Adds clang in two places - preflight job (all Linux) and build-native matrix (x86_64-linux-gnu only)

✅ No rules relaxed or thresholds lowered - this fixes a genuine compiler-level bug
✅ Surgical scope - only affects specific Linux x86_64 targets, preserves existing cross-compile toolchains
✅ Consistent application - applied to all jobs that build the affected target

Test Integrity (EXCELLENT)

✅ This PR follows the project's strict engine parity policy - per CLAUDE.md: "Both engines must produce identical results. If they diverge, the less-accurate engine has a bug — fix it, don't document the gap"

✅ Addresses root cause, not symptoms - fixes the gcc-compiled native engine to match WASM behavior, rather than documenting the divergence as acceptable

✅ Backed by local verification - author reproduced issue in Docker with both gcc 14.2.0 (drops files) and clang 14 (works correctly)

✅ Parity tests will validate the fix - existing tests/engines/parity.test.ts compares WASM vs native extraction of HCL files

Backlog Compliance

• Zero-dep: ✓ - No new runtime dependencies, uses existing clang in CI runners
• Foundation-aligned: ✓ - Maintains dual-engine architecture principle, fixes engine parity bug
• Problem-fit: 4/5 - Directly prevents native/WASM engine divergence that would confuse AI agents with inconsistent results
• Breaking: No - Pure CI change, no API or behavior changes for end users
• Tier: 1 (zero-dep ✓ + foundation-aligned ✓)

Critical Concerns

None identified. This is a well-researched fix for a compiler-specific bug.

Final Recommendation

• Rating: ⭐⭐⭐⭐⭐ (5/5)
• Action: APPROVE
• Reasoning: Exemplary PR that identifies root cause, provides targeted fix, maintains project principles, and includes proper verification. The compiler switch is well-justified and addresses a genuine engine parity bug that would degrade AI agent experience.

Minor suggestion: Consider adding the companion PR #1058 mentioned in the description to make WASM backfill cheaper as defense-in-depth, but this PR stands on its own merit.

Greptile Summary

This PR fixes a silent HCL file drop in the Linux x86_64 native binary by forcing CC=clang/CXX=clang++ ahead of the napi build step in all four relevant CI jobs (build-native.yml, ci.yml native-host-build, publish.yml preflight, and publish.yml build-native). The fix is well-scoped: each job correctly guards the clang step to Linux x86_64 targets only (or omits the guard where the job is already ubuntu-only), and a clang --version preflight check ensures the step fails loudly if clang is absent rather than silently falling back to gcc.

Confidence Score: 5/5

Safe to merge — changes are limited to CI workflow files, correctly scoped to Linux x86_64 targets, and carry no risk of regression on other platforms.

No P0 or P1 findings. All four affected jobs are correctly guarded (or need no guard). The clang --version preflight check provides an explicit failure signal if clang is unavailable rather than silently falling back to gcc. Cross-compile and non-Linux targets are unaffected.

No files require special attention.

Important Files Changed

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[CI / publish triggered] --> B{OS?}
    B -->|ubuntu-latest| C[Run: clang --version\nSet CC=clang / CXX=clang++]
    B -->|macOS / Windows| D[Use default toolchain\nNo CC override]
    C --> E{Target?}
    E -->|x86_64-unknown-linux-gnu| F[napi build --release\nwith clang]
    E -->|aarch64-unknown-linux-gnu| G[Set CC=aarch64-linux-gnu-gcc\nnapi build --release cross]
    E -->|x86_64-unknown-linux-musl| H[Set CC=musl-gcc\nnapi build --release cross]
    F --> I[✅ HCL parser works\nno file drops]
    G --> J[✅ Not affected\nown toolchain]
    H --> K[✅ Not affected\nown toolchain]
    D --> L[macOS/Windows build\nnot affected]

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