DIP69 - Implement scope for escape proof references

[deadalnix via Digitalmars-d]

On Friday, 5 December 2014 at 21:32:53 UTC, Walter Bright wrote:
>>> 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.
>

The DIP say nothing about scoped rvalue having different behavior
than non scoped ones.

>> 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.
>

Through use of that view. I see it as follow:
   - When unconsumed owner goes out of scope, it can (must ?) be
free automatically.
   - scope uses do not consume.
   - When the compiler see a pair of alloc/free, it can promote on
stack.

That is a much more useful definition as it allow for stack
promotion after inlining:
class FooBuilder {
      Foo build() { return new Foo(); }
}

class Foo {}

void bar() {
      auto f = new FooBuilder().build();
      // Use f and do not consume...
}

This can be reduced in such a way no allocation happen at all.

>> 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?

For rvalues, yes. Not for lvalues.