如何在方法参数中使用Raw类型的协议方法？

protocol Measurement {
     mutating func convert(#toUnit: String)
}

enum MassUnit : String {
    case Milligram = "mg"
}

enum VolumeUnit : String {
     case Milliliter = "ml"
}

struct Mass : Measurement {
     mutating func convert(#toUnit: MassUnit)
     // Build error: Does not adhere to 'Measurement'
}

struct Volume : Measurement {
     mutating func convert(#toUnit: VolumeUnit)
     // Build error: Does not adhere to 'Measurement'
}

func +<T: Measurement> (left:T, right:T) -> Measurement {
    let newRightValue  = right.convert(toUnit: left.unit)
    return T(quantity: left.quantity + newRightValue.quantity , unit: left.unit)
}
protocol Measurement {
     mutating func conve