Rule Request: Prevent using `default:` in a switch

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

Why should this rule be added?

When iterating over cases in a switch, all cases should be handled to prevent undefined/unexpected behaviour if a new case is added.

Provide several examples of what would and wouldn't trigger violations.

enum SomeEnum {
  case foo, bar, baz
}

let value: SomeEnum = .foo
...
switch value {
  case .foo:
    return "Foo"
  default:↓
    return ""
}
// OR
switch value {
  default:↓
    return ""
}

Exception would be @unknown default because new cases are specified as, well, unknown. Swift compiler will show a warning if new case is added, whereas in case of SomeEnum from the example above it will not.

switch CLLocationManager.authorizationStatus() {
case .authorizedAlways, .authorizedWhenInUse, .notDetermined:
  //authorized!
case .denied, .restricted:
  //denied!
@unknown default:
  fatalError("Unhandled case!")
}

Non-triggering examples:

enum SomeEnum {
  case one, two, `default`
}

let value: SomeEnum = .two
...
switch value {
  case .one:
    return "One"
  case .two:
    return "Two"
  case .default:
    return "Default"
}
//OR
if case .default = value {
  return "Default"
}

It would be nice, but not required, to also not trigger a warning/error on switches where not all cases can be covered, for example on numeric or String value.

let number: Int = 5
...
switch number {
  case 1...4:
    return "Number is less than 5"
  case 5:
    return "Number is 5"
  default:
    return "Number is greater than 5"
}

let text: String = "Welcome"
...
switch text {
  case "Welcome":
    return true
  case "Goodbye":
    return false
  default:
    return nil
}

Should the rule be configurable, if so what parameters should be configurable?

I don't see any configuration options.

Should the rule be opt-in or enabled by default? Why?

I would say opt-in if the rule would not be triggered on String or numeric switches, otherwise it would be good to have it enabled by default, as it prevents undefined behaviour if a new case to an enum is added.

No implementation file, entry point, or test is named in the issue. Review the requested enum-switch behavior and the numeric, String, @unknown default, and backticked-case examples first; the work is done when the rule's triggering, exceptions, configuration, and opt-in or default status are resolved and covered by tests.