/swift-concurrency

Linting & Concurrency

Use this when:

• SwiftLint flags async_without_await or other concurrency-related warnings.
• You need to decide whether to suppress, fix, or reconfigure a concurrency lint rule.

Skip this file if:

• The issue is a compiler diagnostic, not a lint rule. Use actors.md, sendable.md, or threading.md.

Jump to:

• SwiftLint Concurrency Rules Overview
• async_without_await Rule
• Suppression Strategies

SwiftLint Concurrency Rules Overview

SwiftLint provides several rules targeting async/await and concurrency patterns. Understanding when to fix vs. suppress is critical.

SwiftLint: async_without_await

• Intent: A declaration should not be async if it never awaits.
• Never "fix" by inserting fake suspension (e.g. await Task.yield(), await Task { ... }.value). Those mask the real issue and add meaningless suspension points.
• Legit use of Task.yield(): OK in tests or scheduling control when you truly need a yield; not as a lint workaround.

Diagnose why the declaration is async

1. Protocol requirement — the protocol method/property is async.
2. Override requirement — base class API is async.
3. @concurrent requirement — stays async even without await.
4. Accidental/legacy async — no caller needs async semantics.

Preferred fixes (order)

1. Remove async (and adjust call sites) when no async semantics are needed.
2. If async is required (protocol/override/@concurrent):
   • Re-evaluate the upstream API if you own it (can it be non-async?).
   • If you cannot change it, keep async and narrowly suppress the rule where appropriate (common for mocks/stubs/overrides).

Suppression examples (keep scope tight)

// swiftlint:disable:next async_without_await
func fetch() async { perform() }

// For a block:
// swiftlint:disable async_without_await
func makeMock() async { perform() }
// swiftlint:enable async_without_await

Quick checklist

• Confirm if async is truly required (protocol/override/@concurrent).
• If not required, remove async and update callers.
• If required, prefer localized suppression over dummy awaits.
• Avoid adding new suspension points without intent.

Compiler Warnings: Sendable & Isolation

The Swift compiler generates concurrency-related warnings based on strict concurrency checking level.

Common Warning Patterns

"Capture of non-sendable type"

// Warning: Capture of 'self' with non-sendable type 'MyClass' in a `@Sendable` closure
Task {
    self.doWork() // 'self' is non-Sendable
}

Fixes (in order of preference):

1. Make the type Sendable if it's truly thread-safe
2. Use @MainActor isolation if it's UI-related
3. Capture only Sendable values instead of self
4. Use @unchecked Sendable with documented safety invariant (last resort)

"Non-sendable result returned"

// Warning: Non-sendable type 'MyResult' returned by implicitly async call
let result = await actor.getData() // Returns non-Sendable type

Fixes:

1. Make the return type Sendable
2. Return Sendable projections (IDs, copies of data)
3. Keep processing within the actor's isolation

Actor Isolation Warnings

"Main actor-isolated property accessed from non-isolated context"

// Warning: Main actor-isolated property 'title' cannot be referenced from a non-isolated context
func updateTitle() {
    viewModel.title = "New" // viewModel is @MainActor
}

Fixes:

1. Mark the calling function @MainActor
2. Use await MainActor.run { } for one-off access
3. Reconsider if the property truly needs @MainActor isolation

Suppression Strategies

When to Suppress vs. Fix

Fix when:

• The warning identifies a real data race risk
• The fix is straightforward (add Sendable, adjust isolation)
• The code is new or actively maintained

Suppress when:

• Protocol/inheritance requires the signature
• Third-party code forces the pattern
• Migration is in progress (with tracked ticket)

Suppression Annotations

// Suppress Sendable warnings for legacy imports
@preconcurrency import LegacyFramework

// Suppress for a single declaration
nonisolated(unsafe) var legacyCallback: (() -> Void)?

// Type-level suppression (use sparingly)
struct LegacyWrapper: @unchecked Sendable {
    // Document why this is safe
    private let lock = NSLock()
    private var value: Int
}

Documentation Requirements

When using suppression annotations, document:

1. Why the suppression is needed
2. What invariant makes it safe
3. When it can be removed (link to migration ticket)

/// Thread-safe: Internal lock protects all mutations.
/// TODO: Remove @unchecked when migrated to actor (JIRA-1234)
final class ThreadSafeCache: @unchecked Sendable {
    private let lock = NSLock()
    private var storage: [String: Data] = [:]
}