Existential Conatiner

ExistentialContainer.swift

struct ExistentialContainer {
    var valueBuffer: (Int, Int, Int)
    var vwt: UnsafePointer<ValueWitness>
    var pwt: UnsafePointer<ProtocolWitness>
}

protocol Fooable {
    func foo()
}

struct Concrete: Fooable {
    let x = 99
    let y = 2
    func foo() {
        print("foo in concrete")
    }
}

// In main.swift

var concrete: Fooable = Concrete()
withUnsafePointer(to: &concrete) { pointer in
    pointer.withMemoryRebound(to: ExistentialContainer.self, capacity: 1) { wpointer in
        print(wpointer.pointee.valueBuffer.0) // 99
        print(wpointer.pointee.valueBuffer.1) // 2
    }
}