Keeping a mutable reference to a struct with immutable members

[pineapple via Digitalmars-d-learn]

On Sunday, 29 May 2016 at 18:52:36 UTC, pineapple wrote:
> What's the best way to handle something like this?

Well I did get something to work but it's ugly and I refuse to 
believe there isn't a better way to handle this.

Where `Range` is an alias to a struct with an immutable member, 
and `this.source` is the attribute that I need to be able to 
re-assign to a locally scoped return value:

     import core.stdc.stdlib : malloc, free;

     if(this.source) free(source);
     ubyte* newptr = cast(ubyte*) malloc(Range.sizeof);
     assert(newptr !is null, "Failed to allocate memory.");

     Range saved = this.original.save;
     ubyte* savedptr = cast(ubyte*) &saved;
     for(size_t i; i < Range.sizeof; i++){
         newptr[i] = savedptr[i];
     }
     this.source = cast(Range*) newptr;