Pitch #2: Protocol-based Actor Isolation

[wowbagger (冀卓疌)]

I think this proposal will have some impact on the "stored properties in extensions" idea that had been discussed many times in the forums. The latest (short) discussion was from 4 months ago:

Organizing stored properties in extensions

A very common pattern for organizing methods is using extensions within the module a type is declared in, in order to group methods semantically or for some other reason. These methods behave as if they are declared within the initial declaration itself and UIKit also uses this system of organization in its (generated) Swift interfaces. However, we can't do this today with properties because all stored properties need to be declared within the initial declaration. Thoughts on relaxing this cons…

For example, if String conforms to ActorSendable, then it would be an error to extend it with a stored reference-type property:

extension String {
    let newStoredProperty: NSMutableString = "a mutable string"
}

"Stored properties in extensions" is not a Swift feature, and definitely outside of the scope of this proposal, but I think it might be beneficial to discuss about this proposal's impact on it.