How to use guard outside of function?

Is it possible to use guard outside of a function?

The following throws an error that return or break needs to be used but that isn't possible in this case.

var var1 = String?()
guard let validVar = var1 else {
    print("not nil")
}

No its not possible. To instanciate variables with knowledge of other variables in the class, you can use lazy initialisation or getter.

var testString : String?
lazy var testString2 : String = {
     guard let t = self.testString else { return String()}
      return t
}()

if iam wrong feel free to correct me :)

guard is made for robustness of functions i think, and will do a break in the function if the conditions are wrong. So if you really need this variable it has to be meet the conditions. Like an if let but more clean :)

From your example: var testString = String?() is invalid. Instantiate an String will never be nil so no optional is requiert.

Edit: I wrote an example in my Playground.

import UIKit

var var1 : String?

var validVar : String = {
    guard let validVar = var1 else {
        print("not nil")
        return "NIL"
    }
    return validVar
}()

print("\(validVar)")