D safety! New Feature?

[Mark J Twain via Digitalmars-d]

On Tuesday, 2 August 2016 at 21:48:58 UTC, Mark Twain wrote:
> [...]

Another useful feature I forgot to mention is that we can cast 
between Immutable and Muttable types.

class ImmutableTest
{
     private MutableArray!int data;
     public ImmutableArray!int Data;

     this()
     {
         // Creates a view to data, but Data cannot modify data.
         Data.View(data);
     }

     public Add(int x)
     {
         data.Add(x);
     }
}

Data can then be exposed without worrying about it getting 
trampled on. Sense a MutableArray contains all the functionality 
of an ImmutableArray, there should be no problems with the 
interpretation.