How to define a =copy hook for a distinct cstring type?

[Isofruit]

For interacting with the GTK library I call some functions that hand me a 
c-string (`char*`) that my code owns going forward. I want to have those 
strings in a custom distinct cstring type and define a =copy hook for those 
strings for use with nim's ARC/ORC system.

My minimal example looks likes this:
    
    
    type OwnedGtkString* = distinct cstring
    
    proc `=copy`*(dest: var OwnedGtkString, source: OwnedGtkString) =
      let areSameObject = pointer(source) == pointer(dest)
      if areSameObject:
        return
      
      `=destroy`(dest)
      wasMoved(dest)
      
      dest = source
    
    let x: OwnedGtkString = "lala".OwnedGtkString
    let y = x
    echo x.pointer.repr
    echo y.pointer.repr
    
    
    Run

Naturally this is naive implementation. This doesn't work because `dest = 
source` triggers the =copy hook again, making the entire thing recursive.

Now what else am I supposed to use here? copyMem would be an option if I could 
know how long the string is. Given that =copy is supposed to work for a whole 
breadth of different cstrings that seems non-feasible.

I also can't do simply assignments like this as that triggers the copy proc 
again.

So how is it supposed to be possible to define a copy-hook for a distinct 
cstring type?