DIP69 - Implement scope for escape proof references

[Walter Bright via Digitalmars-d]

On 12/4/2014 6:56 PM, deadalnix wrote:
> On Friday, 5 December 2014 at 00:32:32 UTC, Walter Bright wrote:
>> On 12/4/2014 3:04 PM, deadalnix wrote:
>>> So as mentioned, there are various problem with this DIP :
>>>  - rvalue are defined as having a scope that goes to the end of the statement.
>>> That mean they can never be assigned to anything as per spec.
>>
>> I don't believe this is correct. Rvalues can be assigned, just like:
>>
>>    __gshared int x;
>>    { int i; x = i; }
>>
>> i's scope ends at the } but it can still be assigned to x.
>>
>
> It work even better when i has indirections.

I understand what you're driving at, but only a scoped rvalue would not be copyable.


>>>  - It add more special casing with & (as if it wasn't enough of a mess with
>>> @property, optional () and the fact the function aren't first class). For
>>> instance *e has infinite lifetime when &(*e) is lifetime(e).
>>
>> That's right. I know you're worried about that, but I still don't see it as an
>> actual problem. (The optimizer makes use of this special case all the time.)
> Yes, this is the job of the optimizer to do this kind of stunt.
> Not the semantic analysis.

I don't see any other way, nor do I see the practical problem.


>> I originally had scope only apply to ref, but that made having scoped classes
>> impossible.
>>
>
> Promoting scoped class on stack is an ownership problem, and out
> of scope (!). It make sense to allow it as an optimization.
>
> Problem is, lifetime goes to infinite after indirection, so I'm
> not sure what the guarantee is.

The guarantee is there will be no references to the class instance after the 
scoped class goes out of scope.


>>> During discussion, I proposed to differentiate lifetime calculation between
>>> lvalues and rvalues (which are inherently different beasts with different
>>> lifetime) and carry (or not) the scope flag with each expression.
>>
>> I'm not sure how that would be different from the DIP as it stands now.
>
> I cause everything reached through the view to be scope and
> obliviate the need for things like &(*e) having special meaning.

Are you suggesting transitive scope?