Reassigning aggregate arguments breaks garbage collection

What steps will reproduce the problem?

  import crack.io cout;

  class Obj {
      String val;
      oper init(String val0) : val = val0 {}
      oper del() {
          cout `$val deleted\n`;
      }
  }

  void f(Obj o) {
      o = Obj('internal');
  }

  Obj j = {'external'};
  f(j);
  cout `done\n`;

What is the expected output? 
internal
done
external

What do you see instead?
external
done

Please use labels and text to provide additional information.

For performance reasons, arguments don't bind/release on function enter/leave.  
However, when they are reassigned they do.  In the example above, the external 
reference is incorrectly decremented and the internal reference is leaked.

The only solutions I can think of are:
1) treat arguments like other variables and do bind/release for their scope
2) abandon "single pass" and convert arguments to true "owning" variables if we 
see an assignment.
3) make it illegal to assign argument types that have a "release"

option 1 would be a huge performance hit, option 2 changes a fundamental 
assumption that I am not willing to abandon, so it seems like option 3 is the 
only viable solution for crack 1.0.

We should add this to the list of decisions to revisit if we ever abandon 
single-pass.

Original issue reported on code.google.com by mind...@gmail.com on 22 Jan 2011 at 6:13

Dechenjm / crack-language

Reassigning aggregate arguments breaks garbage collection #65