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0004 — Enforce structure with meta-tests, not convention

  • Status: Accepted
  • Deciders: rhiza-tools maintainers

Context

Two structural properties tend to erode quietly over a project's life:

  • Command modules accrete code until they are hard to navigate.
  • Test suites accumulate "coverage bucket" files whose only purpose is to execute lines for the coverage number, divorced from behavior.

Conventions and code review catch these inconsistently. We prefer guarantees that fail a build.

Decision

Encode the structural rules as ordinary tests that run with the suite:

  • tests/test_module_size.py — every src/rhiza_tools/commands/*.py must stay at or below a line ceiling (currently 750; see ADR-0005). Growth must be split into a new module rather than accreted.
  • tests/test_suite_hygiene.py — there must be no test_coverage*.py catch-all bucket, and no test module may declare its purpose as achieving a coverage number. Coverage honesty is the job of mutation testing (see ADR-0003), not of line-chasing files.

Consequences

  • The "keep modules small" and "tests assert behavior" rules are enforced, not hoped for; violations fail CI with an actionable message.
  • The ceiling is a deliberate, reviewable number that moves only with an accompanying decision.