title: Swift: What does backslash dot "\." mean?
body: <p>I'm new to backend Swift and thought I'd use Vapor to get up-and-running on a side project fast...
  I ran <code>vapor new WebServer --template=auth-template</code>, and now I'm trying to figure out what something like <code>return \.email</code> means.
  For more context, I'm looking in WebServer > Sources > App > Models > Users.swift:</p>
  
  <pre class="lang-swift prettyprint-override"><code>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&lt;User, String&gt; {
          return \.email
      }
  
  // ...
  }
  </code></pre>
  
  <p>And here is the definition of the User class:</p>
  
  <pre class="lang-swift prettyprint-override"><code>/// 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} ...
      }
  }
  </code></pre>
  
  <p>What does this backslash-dot notation mean?</p>