Incomplete types question

[NoUseForAName via Digitalmars-d-learn]

On Tuesday, 8 July 2014 at 09:08:30 UTC, bearophile wrote:
> NoUseForAName:
>
>> Thanks.. but this is awkward. I expected there to be keywords 
>> to declare type declarations public or private.
>
> The keywords are "public" and "private".

So I could just put..

public struct S;

private struct S
{
     int i;
}

.. in the .d file and that would work? I mean I expected 
something like this but then why would Ali post such a complex 
solution?

Also side question. What are these .di files? D propaganda radio 
told me that D does not need header files because it has a real 
module system (like e.g. Pascal or Go) yet in the solution Ali 
posted these .di files seem to be.. pretty much like C header 
files.