我是Swift后端的新手，我想我应该使用Vapor来快速启动和运行一个副业项目...我运行了vapor new WebServer --template=auth-template，现在我试图弄清楚return \.email之类的东西是什么意思。要了解更多上下文，我正在查看WebServer〉Sources〉App〉Models〉Users。swift：

import Authentication
import FluentSQLite
import Vapor

/// Allows users to be verified by basic / password auth middleware.
extension User: PasswordAuthenticatable {
    /// See `PasswordAuthenticatable`.
    static var usernameKey: WritableKeyPath<User, String> {
        return \.email
    }

// ...
}

下面是User类的定义：

/// A registered user, capable of owning todo items.
final class User: SQLiteModel {
    // {omit extra code} ...

    var email: String

    // {omit extra code} ...

    /// Creates a new `User`.
    init(id: Int? = nil, name: String, email: String, passwordHash: String) {
        // {omit extra code} ...
        self.email = email
        // {omit extra code} ...
    }
}

这个反斜线点符号是什么意思？