Recognizing a Closure in Swift Code

I am working through a tutorial on URLSession and URLRequest and ran into an interesting block of code... my first instinct was that this was a closure but I'm not 100% sure... if it is, I havent seen one like this until now. I've been familraiizng my self with closure since they are used a lot in networking code especially things like completion handlers...

Here is the code below... basically a URL is being constructed from a baseURLString decalred earlier and a relative path to a resource... Where I thought this was a closure was on line 36 where there is a pair of parantheses... Is this a closure syntaxt or some other special Swift syntax?

let url: URL = {
         
         /**
          * A string that contains the relative path. In other words, this string
          * would be appended to the url made from the baseURLString to access a specific resource.
          *
          * For example, for the case .getPublicGists(), the relative path as described by GitHub is "gists/public". The resulting url
          * would be a combination of the baseURLString and this relative path: "https://api.github.com/gists/public".
          */
         let relativePath: String?
         
         
         switch self {
            
         case .getPublicGists():
            relativePath = "gists/public"
            
            
         }
        
         
         var url = URL(string: GistRouter2.baseURLString)!
         
         if let relativePath = relativePath {
            
            url = url.appendingPathComponent(relativePath)
            
         }
         
         return url 
      
      



   }()