RCArray is unsafe

[via Digitalmars-d]

On Sunday, 1 March 2015 at 19:22:06 UTC, Walter Bright wrote:
> 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.

Yes, it's a classical aliasing problem. Of course, if the 
references can't possible alias because of their types, it's ok. 
However, for non-pure functions, it's always (?) unsafe, because 
they have access to all kinds of global variables. Too bad we 
don't have pure by default :-(