Helping with __mutable (which will be renamed to __metadata)

[Timon Gehr]

On 15.04.19 14:32, Steven Schveighoffer wrote:
> On 4/15/19 7:17 AM, Timon Gehr wrote:
>> On 14.04.19 22:55, Suleyman wrote:
>>> On Sunday, 14 April 2019 at 00:38:16 UTC, Timon Gehr wrote:
>>>> which implies that there are wrong ways to use `__mutable` (hence 
>>>> unsafe)
>>>
>>> can you elaborate?
>>>
>>
>> struct S{
>>      private __metadata x;
>> }
>>
>> void foo(immutable ref S s)pure{
>>      s.x += 1;
>> }
>>
>> void main(){
>>      immutable S s;
>>      foo(s); // there is no reason for this call to happen
>>      assert(s.x==1); // can't rely on this, it might also be 0
>> }
>>
>> struct S{
>>      private __mutable x;
>> }
>>
>> int foo(immutable ref S s)pure{
>>      s.x += 1;
>>      return s.x;
>> }
>>
>> void main(){
>>      immutable S s;
>>      int a=foo(s);
>>      int b=foo(s); // could just reuse the previous result
>>      assert(a!=b); // can't rely on this, might be the same
>> }
> 
> Note that this is exactly the use case of reference counting. Which is 
> the main draw of having a mutable portion of an immutable. I would 
> hazard to guess that if the above has to be the semantics, this is DOA.
> 
> -Steve

No, this is clearly not the use case of reference counting. Where do you 
see references that are being counted?
Why should the fact that the data structure is reference counted block 
optimizations such as eliding reference copies?