arg-swap

Flags a two-argument call site whose argument identifier names match the function’s parameter names in reversed order. The contradiction is between the call’s positional layout and the callee’s parameter contract.

What it flags

fn copy(dst: &mut [u8], src: &[u8]) { /* ... */ }

fn main() {
    let mut dst = vec![0u8; 16];
    let src = b"hello";
    copy(src, &mut dst);   // <- flagged: argument order reversed
}

The detector resolves the call to the same-file definition of copy, notices that the call’s first identifier src matches the second parameter name and vice versa, and emits a finding pointing at the call site with the definition in related. The evidence.raw payload carries callee_name, parameter_names, and argument_names.

What it does not flag

• Calls whose arguments are not bare identifiers (literals, expressions, field accesses). copy(some.dst, src) is out of scope because the AST node is not a simple identifier.
• Single-argument or N ≥ 3 calls. Only binary functions participate.
• Method calls (obj.copy(a, b)) and qualified-path calls (mod::copy(a, b)).
• Cross-file resolution. The call and the definition must live in the same source file.
• Calls where the call-site identifier names are the same as the parameter names but in the right order, or where the names share no overlap at all.

The narrow scope is deliberate: arg-swap trades recall for precision so the signal is actionable without an LLM in the loop.

Language coverage

• Rust: confirmed citation. Pradel & Sen “DeepBugs” (FSE 2018) and Rice et al. “Detecting Argument Selection Defects” (ICSE 2017) ground the AST-level shape used here.
• Python: confirmed citation. Allamanis, Jackson-Flux, Brockschmidt (NeurIPS 2021, PyBugLab / PyPIBugs) covers the same swap pattern on PyPI corpora. PyBugLab also ships as the Q-15 SOTA baseline comparator for arg-swap (baselines/pybuglab/).

Configuration

Suppress in-source:

• Rust: #[cntrdct::allow(arg-swap)] on the call site’s enclosing function or item.
• Python: # cntrdct: allow(arg-swap) as a trailing or preceding comment (Q-9).

Suppress project-wide via cntrdct.toml:

[detectors.arg-swap]
enabled = false

[languages.python]
suppress = ["arg-swap"]

See In-source suppressions and cntrdct.toml reference.

Related

• Citation policy (P1) — why every language addition needs its own grounding.
• Statistical priors (P4) — how the Layer 2 ranker re-orders arg-swap findings by Wilson / Jeffreys lower bound against the calibrated corpus.
• pr-miner — complementary “implicit pairing” rule miner; both detectors target the same family of API-contract violations from different angles.