"RecursionAuditor: Infinite Loops That Compile Cleanly"

Catching infinite-recursion bugs that the compiler is perfectly happy with.

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I ran a usage analysis on my recent coding sessions. One bullet jumped out: convenience inits with infinite recursion, flagged only later by SourceKit. It had cost me a session to track down. SourceKit catches some of these eventually; the Swift build catches others; none of them were caught at quality-gate time, which is exactly when I want them caught.

So RecursionAuditor catches the ones that compile cleanly.

What it catches

// ❌ Convenience init forwarding to itself with identical labels
class Foo {
    init(name: String, age: Int) { self.name = name; self.age = age }
    convenience init(name: String) {
        self.init(name: name)   // ← infinite recursion
    }
}

// ❌ Computed property referencing itself
struct Foo { var value: Int { value } }

// ❌ Function with no base case
func loop(_ n: Int) -> Int { return loop(n) }

// ❌ Mutual recursion across files (caught via project-wide call graph)
// A.swift
func a() { b() }
// B.swift
func b() { a() }

How it’s detected

The mutual-cycle detector builds a project-wide call graph keyed by qualified name (Type.method(label:)) and runs Tarjan’s strongly-connected-components algorithm. A cycle is reported only if none of its participants have a guard-driven base case. Both intra-file and cross-file cycles are detected — regex could never do this, because it requires knowing what calls what across the whole package.

The dogfooding story

The base-case heuristic was originally “any guard statement in the body.” That worked for the test fixtures. But the first time I ran the auditor against quality-gate-swift itself, it flagged its own SwiftSyntax visitor — a legitimate recursive-descent walker whose base case is “the AST is finite,” not a syntactic guard.

So I tightened the heuristic to also recognize bare return statements and returns of literal/identifier expressions as evidence of a non-recursing path. The fix took fifteen lines. Re-running: zero diagnostics. Fixing a false positive in the auditor itself, against my own codebase, before shipping it — that’s exactly what dogfooding is for.

8 rule IDs. All tests passing.

Try it

quality-gate --check recursion

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Source: github.com/jpurnell/quality-gate-swift