swift-concurrency-guard

Swift Concurrency Guard

Overview

The Concurrency Guard is a SwiftSyntax-based static analyzer that blocks edits introducing concurrency anti-patterns. It runs as a Claude Code PreToolUse hook, preventing regressions before they happen.

Blocker Rules (Exit Code 2)

These rules block edits when violated. Claude receives the error and must fix the code.

CC-CONC-001 — No Task.detached

Why: Unstructured tasks cause priority inversions and are hard to cancel. SSE reconnection loops using Task.detached became unbounded/unowned.

Detect: Task.detached anywhere outside allowlisted paths.

Fix: Replace with structured Task {} owned by a lifecycle object; store handle + cancel.

// BAD
Task.detached { await fetchData() }

// GOOD
private var fetchTask: Task<Void, Never>?

func startFetch() {
    fetchTask = Task { await fetchData() }
}

deinit { fetchTask?.cancel() }

---

CC-CONC-002 — No Task in init for App-Critical Types

Why: Actor hops during sync init cause launch hangs ("actor hop during sync init").

Scope: App, Scene, AppShellState, ViewModels, Coordinators, Services created at launch.

Detect: In init() { ... } block, forbid Task {, Task(, await, withTaskGroup.

Fix: Move to .task {} in the view, or an explicit async start() called from .task.

// BAD
init() {
    Task { await loadData() }  // Actor hop in sync init!
}

// GOOD
init() {
    // Synchronous setup only
}

var body: some View {
    ContentView()
        .task { await viewModel.start() }
}

---

CC-CONC-003 — No Async Work in Render/Layout Paths

Why: Tasks launched during layoutSubviews → CA::Transaction::commit hangs.

Detect: Forbid Task creation inside:

• var body: some View { ... } (and nested builders)
• layoutSubviews, updateUIView, updateNSView, draw(_:), viewDidLayoutSubviews, updateConstraints

Fix: Use .task(id:), onAppear (guarded), or a coordinator started after first frame.