RCArray is unsafe

[Walter Bright via Digitalmars-d]

On 3/1/2015 7:44 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm at gmx.net>" wrote:
> A weakness of the same kind affects DIP25, too. The core of the problem is
> borrowing (ref return as in DIP25), combined with manual (albeit hidden) memory
> management. An example to illustrate:
>
>      struct T {
>          void doSomething();
>      }
>      struct S {
>          RCArray!T array;
>      }
>      void main() {
>          auto s = S(RCArray!T([T()])); // s.array's refcount is now 1
>          foo(s, s.array[0]);           // pass by ref
>      }
>      void foo(ref S s, ref T T) {
>          s.array = RCArray!T([]);      // drop the old s.array
>          t.doSomething();              // oops, t is gone
>      }

The trouble seems to happen when there are two references to the same object 
passed to a function. I.e. there can be only one "borrowed" ref at a time.

I'm thinking this could be statically disallowed in @safe code.