clone-drift

Flags a near-duplicate function whose AST has diverged from the strict majority of its siblings in the same scope. The contradiction is between “looks like a copy” and “doesn’t actually do what its copies do”.

Property	Value
Detector ID	`clone-drift`
Languages	Rust, Python
IEEE 1044-2009 class	Logic
Default severity	Warning
Spec	`docs/spec/clone-drift-v0.md`

What it flags

#![allow(unused)]
fn main() {
fn poll_a(cx: &mut Context<'_>) -> Poll<()> {
    if let Some(x) = inner_a(cx) { return Poll::Ready(x); }
    Poll::Pending
}
fn poll_b(cx: &mut Context<'_>) -> Poll<()> {
    if let Some(x) = inner_b(cx) { return Poll::Ready(x); }
    Poll::Pending
}
fn poll_c(cx: &mut Context<'_>) -> Poll<()> {
    if let Some(x) = inner_c(cx) { return Poll::Ready(x); }
    Poll::Pending
}
fn poll_d(cx: &mut Context<'_>) -> Poll<()> {
    // <- flagged: drifted from the other three
    if inner_d(cx).is_some() { break; }
    Poll::Pending
}
}

poll_a / poll_b / poll_c normalise to identical AST node sequences and form a dominant partition of size 3. poll_d belongs to the same Jaccard ≥ 0.5 cluster but has a singleton normalised form that differs by a small surgical edit (return Poll::Ready(x) → break). The finding points at poll_d; related lists the three canonical-form siblings.

How the gates compose

The detector applies four ordered gates so the textbook “one of N copies missed an update” pattern fires while designed N-variant families and library-shape clusters do not:

Gate	Purpose	Default
`MIN_FN_TOKENS = 22`	drop trivially short functions	constant
`SIMILARITY_THRESHOLD = 0.5`	pairwise Jaccard for cluster membership	constant
F5b scope-bounded clustering	clusters never cross crate / package boundaries	provenance-aware
F5c-i strict-majority	dominant partition must be > 50% of the cluster	constant
F5c-ii `NEAR_DUPLICATE_THRESHOLD = 0.7`	drifted singleton must be near-duplicate of the dominant exemplar	constant
F5d-i multi-singleton suppression	≥ 2 singleton partitions ⇒ designed family ⇒ silent	constant
F5d-ii length-imbalance × weak-dominant gate	high length asymmetry under weak canonical evidence ⇒ silent	constants
F5d-iii small-cluster floor	`MIN_GROUP_SIZE` cluster at the resolution limit ⇒ silent	constants

Gates F5b / F5c / F5d landed across P-6 and P-7 in response to the wild β corpus residuals. Wild β clone-drift FP count is 0 in both Rust and Python after F5d.

Language coverage

Rust: grandfathered citation (Juergens et al. ICSE 2009 on inconsistent clone changes); the broader Bettenburg MSR 2009 and Krinke ICSM 2007 citations supply the F5c-ii drift gate’s grounding.
Python: confirmed citation. Assi, Hassan, Zou (TOSEM 2025, DOI 10.1145/3721125) replicate NiCad / SourcererCC on nine Python deep- learning frameworks. SourcererCC also ships as the Q-15 SOTA baseline comparator for clone-drift (baselines/sourcerercc/).

What it does not flag

Clusters smaller than MIN_GROUP_SIZE = 3.
Functions whose normalised AST has fewer than MIN_FN_TOKENS = 22 tokens (small utility functions whose drift signal is too noisy).
Functions inside impl / trait / mod blocks. Only top-level fn (Rust) and function_definition (Python) participate in v0.
Cross-scope siblings. The F5b scope rule keeps clustering within a single crate / package / parent-directory namespace.

Configuration

[detectors.clone-drift]
enabled = false
# Severity overrides apply at SARIF emission time (P5).
severity = "Note"

Per-language disable:

[languages.python]
suppress = ["clone-drift"]

See In-source suppressions for the per-function attribute and comment forms.

arg-swap — different shape of same-file contradiction (parameter contract vs structural-clone consistency).
Statistical priors (P4) — embedded clone-drift Wilson lower bound is 0.676 (8 TP / 0 FP) after P-7.
Severity and anomaly classes (P5) — Logic anomaly class mapping.

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