tests(mozcpp_grammar_metrics): cyclomatic_sum asserted >= 3, not exact, masking a dropped decision-point regression

[dekobon]

Summary

In tests/mozcpp_grammar_metrics.rs, two tests assert cyclomatic.cyclomatic_sum() >= 3 instead of the exact derivable value. In mozcpp_moz_annotated_class_metrics the true sum is well above 3, so a regression that stops counting the if decision point still satisfies >= 3 and is masked — the same bug-masking pattern as #936 (where a C cognitive assertion was > 0 instead of the exact value).

Location

• tests/mozcpp_grammar_metrics.rs:66-69 (mozcpp_moz_annotated_class_metrics)
• tests/mozcpp_grammar_metrics.rs:90-94 (mozcpp_qm_try_function_metrics)

Evidence

mozcpp_moz_annotated_class_metrics analyzes a MOZ_*-annotated class with two methods, one of which (parse) contains a single if:

assert!(
    m.cyclomatic.cyclomatic_sum() >= 3,
    "cyclomatic includes the method branch: {}",
    m.cyclomatic.cyclomatic_sum()
);

Cyclomatic is seeded at 1 per space (src/metrics/cyclomatic.rs:67, cyclomatic: 1.) and compute_sum runs for every space (src/spaces.rs:442-462), so the aggregate here is:

• unit space: 1
• class Parser: 1
• method parse: 1 + 1 (the if, src/metrics/cyclomatic.rs:298) = 2
• method reset: 1

Total = 5. The if decision point the test claims to guard is only 1 of those 5. If the C-family IfStatement cyclomatic arm regressed (the precise failure the docstring says this fixture protects against), the sum drops to 4 — still >= 3, so the test passes green. The assertion protects nothing against the regression it documents.

mozcpp_qm_try_function_metrics (tests/mozcpp_grammar_metrics.rs:90-94) uses the same >= 3 form on a single function whose exact sum is 3 (unit 1 + function Foo 1 + if 1). Here an if-drop (sum 2) is caught, but an over-count regression (e.g. a QM_TRY_INSPECT macro argument spuriously adding a branch) is not — >= 3 admits any value at or above the true value.

Expected Behavior

Assert the exact derivable cyclomatic sum (assert_eq!(m.cyclomatic.cyclomatic_sum(), 5, ...) and assert_eq!(..., 3, ...) respectively), so both an undercount (dropped decision point) and an overcount (spurious branch) fail the test. cyclomatic_sum() returns u64, so exact equality is safe (no float-magnitude brittleness).

Actual Behavior

>= 3 admits any sum at or above 3. In mozcpp_moz_annotated_class_metrics the true value is 5, leaving a two-point margin that masks a dropped-if regression; in mozcpp_qm_try_function_metrics any overcount escapes.

Impact

Test-only. Mozcpp owns no file extensions and has no integration-snapshot corpus (per the file's own module doc), so these unit tests are the only metric coverage for the fork. A loose threshold here is the only line of defense for the very decision-point counting the tests claim to pin, and it does not hold.

[dekobon]

General Plan for Fix

Root Cause

The tests use assert!(cyclomatic_sum() >= 3, ...) where the exact value
is statically derivable (5 for the class fixture, 3 for the QM_TRY
fixture). The inequality was likely chosen to avoid hard-coding a value,
but cyclomatic_sum() returns u64, so exact equality carries no
float-brittleness cost. The loose bound leaves a margin (2 points in the
class case) that swallows the exact regression class the docstrings claim
to guard.

Implementation Steps

1. In mozcpp_moz_annotated_class_metrics
   (tests/mozcpp_grammar_metrics.rs:66-69), replace the assert!(... >= 3)
   with assert_eq!(m.cyclomatic.cyclomatic_sum(), 5, "...") and update the
   message/comment to spell out the derivation (unit 1 + class 1 +
   parse 2 + reset 1).
2. In mozcpp_qm_try_function_metrics
   (tests/mozcpp_grammar_metrics.rs:90-94), replace assert!(... >= 3)
   with assert_eq!(m.cyclomatic.cyclomatic_sum(), 3, "...") (unit 1 +
   Foo 1 + if 1).
3. Confirm the exact constants against the actual run before committing
   (the derivation above is from src/metrics/cyclomatic.rs defaults and
   the C-family IfStatement arm; verify empirically, do not trust the
   comment).

Tests to Add/Update

• The two existing tests above (tighten assertions in place — do not
  rewrite the file).
• Verify-via-revert: temporarily disable the C-family IfStatement
  cyclomatic arm and confirm BOTH tests now fail (the class test does
  not fail today under >= 3).

Verification

• cargo test -p big-code-analysis --features mozcpp passes
• cargo clippy -p big-code-analysis clean
• Manual verification: revert the IfStatement cyclomatic increment
  and confirm mozcpp_moz_annotated_class_metrics fails (it does not
  today)