How do I create a char[] from a const(char)[]?

[Dan]

I have a const(char)[] string being passed in as a parameter (can't change that) and I want to create a duplicate that is not const.

I tried all manner of things that would make sense to dispose of the constness of it.  It's looking though that I will have to actually loop through and cast every freaking character?

void myfunc(const(char)[] sin){
  char[] sout = cast(char[]) sin; // nope
  char[] sout = cast(char[]) sin.dup; // nope
  char[] sout = (cast(char[]) sin).dup; // nope

What's the trick?