# discarded-comparison

> A comparison not reduced to a source constant is discarded or kept only by unused immutable aliases.

Canonical: https://nightshade.dicksonwu.xyz/rules/discarded-comparison.html
Release: Nightshade 0.1.1 (v0.1.1)

| Field | Value |
| --- | --- |
| Rule ID | `discarded-comparison` |
| Severity | low |
| Confidence | high |
| Maturity | experimental |
| Selection | Audit default |
| Policy intent | not required |
| Default enabled | yes |
| Category | Audit-default ordinary rules |
| Engine facts used | resolved-source-references, typed-source-constants |
| Release | v0.1.1 |

## Selection and scope

Selected by the default audit profile, so an ordinary scan runs it.

```sh
./.nightshade/node_modules/.bin/nightshade ./contracts --detectors discarded-comparison
./.nightshade/node_modules/.bin/nightshade detector show discarded-comparison
```

The first command runs only this rule; `--detectors` takes a comma-separated list. The second prints the rule's metadata from your installed version.

Severity is review priority. It does not express exploitability or impact. Confidence is detector metadata, not a probability. All built-in rules are experimental.

Compiler scope: the engine supports the pinned profiles 0.28.0, 0.29.0, 0.30.0, 0.31.1, 0.33.0-rc.2, 0.34.0, 0.35.0-rc.1 and 0.35.0, with ZKIR2 by default. Support for this rule can be narrower; the rule documentation below states any narrower scope.

## Corpus checks

Synthetic corpus checks for this rule in the 0.1.1 proof run. A check counts only with compiler acceptance and complete analysis.

| Positive detected | Corrected quiet | Lookalike quiet | Other negative quiet | Incomplete |
| ---: | ---: | ---: | ---: | ---: |
| 1/1 | 1/1 | 1/1 | 0/0 | 0 |

No deviations were recorded for this rule.

These are behavior checks on synthetic fixtures, not a precision or recall estimate. See [evaluation](https://nightshade.dicksonwu.xyz/evaluation.md#synthetic-corpus).

## Rule documentation

From https://github.com/DicksonWu654/nightshade/blob/v0.1.1/docs/detectors/discarded-comparison.md at v0.1.1.

Availability: **default-on**, severity **low**, confidence **high**. Select it
alone with `--detectors discarded-comparison`.

The rule reports a resolved comparison (`==`, `!=`, `<`, `<=`, `>`, `>=`) whose
Boolean outcome is discarded, unless typed scalar source folding resolves the
outcome to a constant. It covers expression statements, unused local bindings
and unused immutable identity aliases. Disclosure and identity casts preserve
this relationship. A sequence discards each prefix result and retains only its
tail; prefixes are checked even inside a returned or otherwise consumed sequence.
Only entry-reachable root-source circuits are linted. Source-dead branches,
empty numeric for ranges and components after a definite rejection are excluded.

### Example and scenario

```compact
export circuit increment(limit: Uint<64>): [] {
  Ownable_assertOnlyOwner();
  count < limit; // evaluates the check but discards its Boolean result
  count.increment(1);
}
```

The owner check still enforces authorization. The comparison does not enforce
the intended limit through this use. Replace it with
`assert(count < limit, "limit reached")`, or consume the outcome in the intended
control flow. A Boolean view that returns the comparison is an intentional API
and stays quiet.

### Recommendation and limits

This is a source-use fact. Operand evaluation, proof constraints and side effects
remain; other guards may enforce the same condition. No feasible failure,
absence of authorization or deployed vulnerability is claimed. Any non-alias
reference conservatively retains a binding, including captures, control flow,
computed predicates, aggregates and assignments. One used alias retains an
entire fan-out. General computed-result liveness, arbitrary predicate helper
forwarding and imported-body linting are outside this rule. Source reachability
is structural through the resolved source call graph: a callback used only by a
zero-length map/fold can still be linted. This does not establish callback
execution or successful execution. Constant folding covers typed scalar source
constants, not every tautology or symbolic relationship.

### Evidence and known false positives

## Suppress a match

If a match is intentional, suppress it on the line before with the exact rule ID and a reason of at least eight characters:

```compact
// nightshade-disable-next-line discarded-comparison -- Reason this match is intentional
```

Diagnostics cannot be suppressed, and an unmatched directive for a selected rule makes analysis incomplete. See [configuration](https://nightshade.dicksonwu.xyz/configuration.md#suppress-an-intentional-match) for fingerprint suppressions and baselines.

## References

- [Rule source document at v0.1.1](https://github.com/DicksonWu654/nightshade/blob/v0.1.1/docs/detectors/discarded-comparison.md)
- [Catalog entry in rules.json](https://nightshade.dicksonwu.xyz/rules.json)
- [All rules](https://nightshade.dicksonwu.xyz/rules.md#audit-default-ordinary-rules)
- [Limitations](https://nightshade.dicksonwu.xyz/limitations.md)