Actually, some of those features are still buggy when operating to Obj-C land :)

      //Warning if you don't use let :-) 
      let array = NSMutableArray(capacity: 50)
      array.addObject(notification)

I am aware that the let essentially means I can't reassign the variable, and does not have anything to do with the mutability of the class. Since it's a class and not a struct.

However, what I am saying is that Apple needs to somehow add the keywords to the container classes imported from Cocoa, this is manually I guess.

I wonder if adding a mutating keyword on the declaration would help

    public mutable func addObject(anObject: AnyObject) mutable

That way that should throw a warning on a let usage. But that keyword only exists on Swift. They would have to go through all of them like they did with nullable keyword.

If you're wondering why I am using CFArray -> performance. Structs are pretty slow since you'd have to copy it from the dictionary to mutate and then insert into the dictionary again.

Though, those numbers were with Swift 1. Will try with Swift 2 again, but given the nature of immutability, I suspect it will still be slower.

Now that I think about it. I might make my own generic structs that uses NSArray/NSDictionary internally. Though I wonder if you can pass structs by reference instead of copy, that would be more useful.