Extract is_auto_eligible predicate; document instrumentation flags gotcha

[lukstafi]

Summary

• Extract a single is_auto_eligible predicate in ppx_minidebug.ml to replace three identical 4-conjunct eligibility checks (in needs_runtime_injection, any_eligible, and the List.map filter)
• Add "Dune Instrumentation Backend" section to README documenting that --auto must be passed on the consumer side via (instrumentation (backend ppx_minidebug --auto)), since dune-project's (instrumentation.backend ...) stanza does not support a flags field

Test plan

• dune runtest passes with no diffs — pure refactor, no behavior change
• Verify is_auto_eligible is used in all three original locations

🤖 Generated with Claude Code

[lukstafi]

@codex review Focus on bugs, correctness issues, and edge cases. Do not check adherence to a spec or plan.

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

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[lukstafi]

Coder Retrospective — task-d3ede2d6

What went well

• Clean, single-commit refactor with no test regressions.
• All three duplication sites were truly identical — no hidden divergence to reconcile.
• README section placed logically and covers the key gotcha clearly.

What I'd do differently next time

1. Verify dune-project stanza syntax against dune docs before documenting. The README example uses (instrumentation.backend (ppx ppx_minidebug)) — this matches the proposal, but I didn't independently verify it against current dune documentation. For documentation-focused changes, a quick WebFetch of the relevant dune docs page would add confidence.
2. Consider adding an inline test for is_auto_eligible. The predicate is pure and testable — a small expect test exercising it against a few synthetic value_binding AST nodes would guard against future regressions if the eligibility criteria evolve. This wasn't in scope but would be cheap insurance.
3. Check for other near-duplicates while in the area. When extracting a predicate from 3 sites, it's worth a quick grep for subsets of the conjuncts (e.g., is_function_expr + is_unit_pattern together) to see if there are 4th or 5th partial duplications that could also benefit from is_auto_eligible.