swift-concurrency-pro

1. Scan for known-dangerous patterns using

   references/hotspots.md

   to prioritize what to inspect.
2. Check for recent Swift 6.2 concurrency behavior using

   references/new-features.md

   .
3. Validate actor usage for reentrancy and isolation correctness using

   references/actors.md

   .
4. Ensure structured concurrency is preferred over unstructured where appropriate using

   references/structured.md

   .
5. Check unstructured task usage for correctness using

   references/unstructured.md

   .
6. Verify cancellation is handled correctly using

   references/cancellation.md

   .
7. Validate async stream and continuation usage using

   references/async-streams.md

   .
8. Check bridging code between sync and async worlds using

   references/bridging.md

   .
9. Review any legacy concurrency migrations using

   references/interop.md

   .
10. Cross-check against common failure modes using

    references/bug-patterns.md

    .
11. If the project has strict-concurrency errors, map diagnostics to fixes using

    references/diagnostics.md

    .
12. If reviewing tests, check async test patterns using

    references/testing.md

    .

Core Instructions

• Target Swift 6.2 or later with strict concurrency checking.
• If code spans multiple targets or packages, compare their concurrency build settings before assuming behavior should match.
• Prefer structured concurrency (task groups) over unstructured (

  Task {}

  ).
• Prefer Swift concurrency over Grand Central Dispatch for new code. GCD is still acceptable in low-level code, framework interop, or performance-critical synchronous work where queues and locks are the right tool – don't flag these as errors.
• If an API offers both

  async

  /

  await

  and closure-based variants, always prefer

  async

  /

  await

  .
• Do not introduce third-party concurrency frameworks without asking first.
• Do not suggest

  @unchecked Sendable

  to fix compiler errors. It silences the diagnostic without fixing the underlying race. Prefer actors, value types, or

  sending

  parameters instead. The only legitimate use is for types with internal locking that are provably thread-safe.

Output Format

1. State the file and relevant line(s).
2. Name the rule being violated.
3. Show a brief before/after code fix.

DataLoader.swift

await

// Before
actor Cache {
    var items: [String: Data] = [:]

    func fetch(_ key: String) async throws -> Data {
        if items[key] == nil {
            items[key] = try await download(key)
        }
        return items[key]!
    }
}

// After
actor Cache {
    var items: [String: Data] = [:]

    func fetch(_ key: String) async throws -> Data {
        if let existing = items[key] { return existing }
        let data = try await download(key)
        items[key] = data
        return data
    }
}

withTaskGroup

// Before
for url in urls {
    Task { try await fetch(url) }
}

// After
try await withThrowingTaskGroup(of: Data.self) { group in
    for url in urls {
        group.addTask { try await fetch(url) }
    }

    for try await result in group {
        process(result)
    }
}

Summary

1. Correctness (high): Actor reentrancy bug on line 18 may cause duplicate downloads and a force-unwrap crash.
2. Structure (medium): Unstructured tasks in loop on line 34 lose cancellation propagation.

References

• references/hotspots.md

  - Grep targets for code review: known-dangerous patterns and what to check for each.

• references/new-features.md

  - Swift 6.2 changes that alter review advice: default actor isolation, isolated conformances, caller-actor async behavior,

  @concurrent

  ,

  Task.immediate

  , task naming, and priority escalation.

• references/actors.md

  - Actor reentrancy, shared-state annotations, global actor inference, and isolation patterns.

• references/structured.md

  - Task groups over loops, discarding task groups, concurrency limits.

• references/unstructured.md

  - Task vs Task.detached, when Task {} is a code smell.

• references/cancellation.md

  - Cancellation propagation, cooperative checking, broken cancellation patterns.

• references/async-streams.md

  - AsyncStream factory, continuation lifecycle, back-pressure.

• references/bridging.md

  - Checked continuations, wrapping legacy APIs,

  @unchecked Sendable

  .

• references/interop.md

  - Migrating from GCD,

  Mutex

  /locks, completion handlers, delegates, and Combine.

• references/bug-patterns.md

  - Common concurrency failure modes and their fixes.

• references/diagnostics.md

  - Strict-concurrency compiler errors, protocol conformance fixes, and likely remedies.

• references/testing.md

  - Async test strategy with Swift Testing, race detection, avoiding timing-based tests.