Modifying immutable structs without var in Swift

immutable.swift

protocol ImmutableCopy {
    associatedtype InitializerTuple
    var lastInitParams: AnyInitializerParams<InitializerTuple> { get }
    func copy(from: (InitializerTuple) -> Self, applying: (inout InitializerTuple) -> Void) -> Self
}

extension ImmutableCopy {
    func copy(from: (InitializerTuple) -> Self, applying: (inout InitializerTuple) -> Void) -> Self {
        var values = lastInitParams.values
        applying(&values)
        return from(values)
    }
}

// This type exists so types that inherit ImmutableCopy can be Equatable/Codable/Hashable and so on
// TODO: Make this type all of that
struct AnyInitializerParams<Tuple> {
    let values: Tuple

    init(_ values: Tuple) {
        self.values = values
    }
}

struct MyStruct: ImmutableCopy {

    //
    typealias InitializerTuple = (a: Int, b: String)
    let lastInitParams: AnyInitializerParams<InitializerTuple>
    //

    let a: Int
    let b: String

    init(a: Int, b: String) {
        self.a = a
        self.b = b
        self.lastInitParams = AnyInitializerParams((a, b))
    }
}

/////////////

let a = MyStruct(a: 1, b: "")

let b = a.copy(from: MyStruct.init(a:b:)) { values in
    values.b = "2"
}

dump(a) // MyStruct(a: 1, b: "")
dump(b) // MyStruct(a: 1, b: "2")

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