Helping with __mutable (which will be renamed to __metadata)

[Steven Schveighoffer]

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