Closure use of non-escaping parameter may allow it to escape

This is due to a change in the default behaviour for parameters of function type. Prior to Swift 3 (specifically the build that ships with Xcode 8 beta 6), they would default to being escaping – you would have to mark them @noescape in order to prevent them from being stored or captured, which guarantees they won’t outlive the duration of the function call.

However, now @noescape is the default for function-typed parameters. If you want to store or capture such functions, you now need to mark them @escaping:

protocol DataServiceType {
  func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void)
  func cachedData(location: String) -> Data?
}

func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void) {
  // ...
}

See the Swift Evolution proposal for more info about this change.