Xcode 16 Beta 3: "Cannot expand accessors on variable declared with 'let'"

Yeah, this is a change introduced in beta 3. Every SwiftData attribute is converted (expanded) to a computed property with accessors via SwiftMacro, and since beta 3, the conversion requires the attribute be var.

I think you can go ahead to apply the change to your SwiftData attributes. Doing that shouldn't have any negative impact.

Best,
——
Ziqiao Chen
 Worldwide Developer Relations.

Thank you for confirming the change. Is this going to be a documented limitation of SwiftData?

It is useful to be able to express the immutability of particular model attributes. I can understand if SwiftData may now be unable to handle that immutability constraint, but that change seems like it should be a documented property of SwiftData and not just a compiler error.

If it isn't a SwiftData design decision but a limitation of macros I would encourage the restoration of the previously shipping behavior.

Disallowing let for an attribute that is converted to a computed property is a Swift language feature, and so the change is intentional.

I agree though that there is a need to express immutability for a SwiftData model attribute, and so will encourage you to voice your opinion in your feedback report for the team to consider.

Best,
——
Ziqiao Chen
 Worldwide Developer Relations.

I am also encountering this and surprisingly found nothing else about the issue. I also filed feedback: FB14443662. It really doesn't make sense to prevent immutability on properties that don't need mutation. A unique identifier for example, should not be a 'var'.

Late to the game but for what its worth: FB15133265 - Swift Data / Swift 6: Create mechanism for object identifier that is 'immutable' that satisfies 'expand accessors on variable declared with let'