如何在扩展中使用Swift返回Int中的变异函数？

Why doesn't this code work?

为什么这段代码不起作用?

protocol ExampleProtocol {
    var simpleDescription: String { get }
    mutating func adjust()
}
extension Int: ExampleProtocol {
    var simpleDescription: String {
    return "The number \(self)"
    }
    mutating func adjust() {
        self += 42
    }
}
var x:Int = 7
let y:Int = x.adjust()
pr