Asynchronous cleanup

Async calls in defer

Swift 6.4 allows a defer block to call asynchronous functions, making it easier to guarantee async cleanup when leaving a scope.

Before Swift 6.4

In Swift 6.3 and earlier, using await inside a defer block produced a compiler error:

func performOperation() async {
    defer {
        await flush() // Error: 'async' call cannot occur in a defer body
    }

    // Perform work...
}

Swift 6.4

An async function can now suspend while running its deferred cleanup:

func performOperation() async {
    defer {
        await flush()
    }

    // Perform work...
}

The deferred block still runs when its surrounding scope exits, but it can now await asynchronous cleanup before the function returns.

Reference

• What’s new in Swift - WWDC26