Crossing the Border: Sendable vs. Non-Sendable Types

In Swift 6, every time you move data between actors or into a background Task, you are crossing an Isolation Boundary. The compiler uses the Sendable protocol to ensure that this crossing doesn’t introduce data races.

The Hook: The Surprise Warning

You’ve just refactored a class to be an actor, and suddenly your entire networking layer is throwing “Non-sendable type cannot be passed” warnings. The temptation is to use @unchecked Sendable, but that is often a sign of architectural debt rather than a solution.

The “Why”: thread-safety by Categorization

Sendable is a marker protocol. It tells the compiler that a type can be safely shared across threads because it is either:

1. Immutable: Like a struct with only let properties.
2. Internally Synchronized: Like an actor.
3. Disconnected: Like a unique reference that the compiler can prove isn’t used anywhere else (Region-based Isolation).

The Implementation: Fixing the Leak

Instead of forcing a class to be Sendable, senior engineers look to Regional Isolation (SE-0414) to pass non-sendable objects safely.

class InternalBuffer { // Non-Sendable class
    var data: [UInt8] = []
}

func processBuffer() async {
    let buffer = InternalBuffer()
    buffer.data = [1, 2, 3]
    
    // In Swift 6, this is valid if 'buffer' is not used 
    // again in the caller's region.
    await performRemoteAction(with: buffer) 
}

func performRemoteAction(with buffer: InternalBuffer) async {
    // buffer is now isolated to this task's region
}

By keeping the lifetime of non-sendable objects short and localized, you can avoid the complexity of making every single object in your system thread-safe.

The Verdict: Value Types First

• Pros: Guarantees no data races; forces cleaner architecture.
• Cons: Can be frustrating to refactor legacy class-heavy codebases.
• When to use: Always. Avoid @unchecked Sendable unless you are wrapping a legacy C-library with proven thread-safety.

Internal Connectivity

• Foundation: Beyond Sendable: Region-based Isolation
• Next Step: Profiling Actor Hop Overhead in Instruments

External Resources

• Swift Evolution SE-0414: Region-based Isolation
• Apple: Sendable Documentation