Swift 6.4: What's New in Concurrency
Article Summary
Antoine van der Lee breaks down Swift 6.4's concurrency updates so you don't have to parse through Evolution proposals yourself. These aren't just theoretical improvements: they solve real friction points in async/await adoption.
Swift 6.4 shipped at WWDC 2026 with several concurrency enhancements that didn't make it into session talks. Van der Lee, who tracks Swift Evolution proposals weekly for 30K+ developers, highlights the changes that matter most for teams migrating to strict concurrency.
Key Takeaways
- Async defer now works: cleanup code can finally await inside defer blocks
- Task cancellation shields let critical cleanup run even when tasks are cancelled
- New compiler warnings catch silently ignored errors in throwing Task initializers
- weak let references solve Sendable conformance issues for delegate patterns
- ~Sendable explicitly marks audited types as intentionally non-Sendable
Swift 6.4 removes key friction points in concurrency adoption, from async cleanup to better error visibility in unstructured tasks.
About This Article
Keeping up with Swift Evolution proposals is overwhelming for iOS developers. There are over 30,000 of them trying to stay informed about language changes, but most don't have time to read through the actual proposals themselves.
Antoine van der Lee built SwiftLee Weekly to solve this. It's a free email newsletter that goes out every Tuesday with a 5-minute read. Each issue breaks down accepted proposals into practical code examples and actionable takeaways for Swift and iOS developers.
The newsletter now has 30,000+ weekly subscribers. Developers can learn what changed in Swift 6.4 and understand the reasoning behind each feature without having to dig through proposals on their own.