test(acir_components_check): fix out-of-bounds abort in DetectsUnconstrainedWitnesses under debug build

[AztecBot]

Problem

The Nightly Debug Build failed with exit code 134 (SIGABRT) in acir_components_check_tests:

AcirComponentsCheckTest.DetectsUnconstrainedWitnesses
/usr/include/c++/13/debug/vector:442:
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)

The nightly build compiles with -D_GLIBCXX_DEBUG (libstdc++ debug mode), which bounds-checks std::vector::operator[]. Regular release/default builds do not, so this defect was silent there — the test "passed" on undefined behavior.

Root cause

DetectsUnconstrainedWitnesses corrupted the builder with:

builder.real_variable_index.resize(9);

This left the builder internally inconsistent: its gates still reference variable indices ≥ 9, but real_variable_index was truncated to 9 elements. When ComponentsChecker::check() runs the static analyzer (build_circuit_component_map → StaticAnalyzer::find_connected_components), the analyzer walks the builder's gate wires and calls to_real(wire) = real_variable_index[wire]. A gate wire referencing variable index 10 then reads real_variable_index[10] on a 9-element vector — out of bounds. Release builds read adjacent memory (UB) and happen to produce the expected result; the debug build correctly aborts.

The truncation was a blunt way to simulate "an ACIR witness with no circuit-side component". It can't occur with a real builder — real_variable_index is always sized to hold every variable the gates reference.

Fix

Exercise the same UNCONSTRAINED detection path with a consistent builder — the scenario the checker actually exists to catch (the ACIR graph expects a constraint the circuit does not carry):

• The ACIR view links witnesses 8 ↔ 9 into one component.
• The circuit links witness 8 ↔ 10 instead, leaving witness 9 allocated but touched by no gate.

Witness 9 therefore legitimately maps to no circuit component (hitting the NO_CIRCUIT_CC path in build_circuit_component_map), and the checker reports exactly one UNCONSTRAINED error. No real_variable_index truncation, no out-of-bounds read.

Testing

Built and ran the target with the debug preset (-D_GLIBCXX_DEBUG), matching the nightly:

cmake --preset debug && ninja -C build-debug acir_components_check_tests

• Before: DetectsUnconstrainedWitnesses aborts (index 10, 9 elements).
• After: all 10 AcirComponentsCheckTest tests pass under the debug build.

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Created by claudebox · group: slackbot · Slack thread