test(bb): fix out-of-bounds read in acir_components_check DetectsUnconstrainedWitnesses

[AztecBot]

Problem

The Nightly Debug Build failed (exit 134 / SIGABRT). The failing test is AcirComponentsCheckTest.DetectsUnconstrainedWitnesses:

build-debug/.../acir_components_check_tests
Error: attempt to subscript container with out-of-bounds index 10, but container only holds 9 elements.
reference std::vector<unsigned int>::operator[](size_type) [_Tp = unsigned int]
timeout: the monitored command dumped core

Root cause

The test deliberately corrupted the builder to simulate an unconstrained witness:

auto builder = create_circuit<AcirComponentsCheckBuilder>(program);
builder.real_variable_index.resize(9);   // "Corrupt the circuit"

create_circuit produces a builder with many more than 9 variables (the AssertZero over witnesses 8/9, plus pairing-point accumulators). Shrinking real_variable_index to 9 entries leaves it inconsistent with the builder's own gates: those gates still reference variable indices ≥ 9. When ComponentsChecker::check() runs the connected-components analysis, cdg::StaticAnalyzer_::to_real() does real_variable_index[variable_index] over every gate wire and reads past the shrunk vector.

In a release build that unchecked operator[] is a silent out-of-bounds read that happens to leave the test passing; the _GLIBCXX_DEBUG nightly build correctly traps it and aborts. This is undefined behavior in the test, not a defect in the production checker — which is only ever given an internally-consistent builder.

Fix

Test-only change. Instead of corrupting real_variable_index, keep the builder fully consistent (built from a constraint over witnesses 0/1) and feed the checker an ACIR component whose witness indices sit beyond real_variable_index.size(). That witness has no circuit-side variable, so it exercises the same NO_CIRCUIT_CC → UNCONSTRAINED path the test intends — which is also the real security property (an ACIR-connected witness missing from the synthesized circuit) — with no out-of-bounds access. The connected-components analysis now runs over an in-bounds builder.

Verification

Reproduced and fixed in a local _GLIBCXX_DEBUG build (cmake --preset debug):

• Before: DetectsUnconstrainedWitnesses aborts with the out-of-bounds message above.
• After: DetectsUnconstrainedWitnesses passes; the full acir_components_check_tests suite is green (10/10).

No production code changed.

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Created by claudebox · group: slackbot