How to list all classes conforming to protocol in Swift?

How to list all classes implementing a given protocol in Swift?

Say we have an example:

protocol Animal {
    func speak()
}

class Cat:Animal {
    func speak() {
        print("meow")
    }
}

class Dog: Animal {
    func speak() {
        print("Av Av!")
    }
}

class Horse: Animal {
    func speak() {
        print("Hurrrr")
    }
}

Here is my current (not compilable) approach:

func getClassesImplementingProtocol(p: Protocol) -> [AnyClass] {
    let classes = objc_getClassList()
    var ret = [AnyClass]()

    for cls in classes {
        if class_conformsToProtocol(cls, p) {
            ret.append(cls)
        }
    }
    return ret
}

func objc_getClassList() -> [AnyClass] {
    let expectedClassCount = objc_getClassList(nil, 0)
    let allClasses = UnsafeMutablePointer<AnyClass?>.alloc(Int(expectedClassCount))
    let autoreleasingAllClasses = AutoreleasingUnsafeMutablePointer<AnyClass?>(allClasses)
    let actualClassCount:Int32 = objc_getClassList(autoreleasingAllClasses, expectedClassCount)

    var classes = [AnyClass]()
    for i in 0 ..< actualClassCount {
        if let currentClass: AnyClass = allClasses[Int(i)] {
            classes.append(currentClass)
        }
    }

    allClasses.dealloc(Int(expectedClassCount))

    return classes
}

But when calling either

getClassesImplementingProtocol(Animal.Protocol) or

getClassesImplementingProtocol(Animal) or

getClassesImplementingProtocol(Animal.self)

results in Xcode error: cannot convert value of type (Animal.Protocol).Type to expected argument type 'Protocol'.

Did anyone manage get this working?