Head Const

[Timon Gehr via Digitalmars-d]

On 16.02.2016 01:14, H. S. Teoh via Digitalmars-d wrote:
> Just out of curiosity, since we already have Rebindable in Phobos and
> HeadConst is being proposed, what are the disadvantages / shortcomings
> of a library solution that would justify adding yet another feature to
> the language?

struct S{
     int[] a;
     void foo()headconst{
         a[0]=1; // ok
         // arr.length=2 // error
     }
}

void main(){
     headconst(S) s([0,2,3]);
     s.foo();
     assert(s.a==[1,2,3]);
}

How to do this in the library?