swift 语言中 let where 的意思

let vegetable = "red pepper"
var vegetableComment: String?;
switch vegetable {
    case "celery":
     vegetableComment = "Add some raisins and make ants on a log"
    case "cucumber", "watercress":
     vegetableComment = "That would make a good tea sandwich."
case let x where x.hasSuffix("pepper"):
     vegetableComment = "Is it a spicy \(x)?"
default:
     vegetableComment = "Everything tastes good in soup"
}

里面有一条语句

let x where x.hasSuffix("pepper")

他的意思是判断常量vegetable是否以pepper结尾。where 是 swift 语言的条件判断关键字。