DIP69 - Implement scope for escape proof references

[deadalnix via Digitalmars-d]

On Thursday, 4 December 2014 at 09:25:11 UTC, Walter Bright wrote:
> http://wiki.dlang.org/DIP69
>
> Despite its length, this is a fairly simple proposal. It adds 
> the missing semantics for the 'scope' storage class in order to 
> make it possible to pass a reference to a function without it 
> being possible for it to escape.
>
> This, among other things, makes a ref counting type practical. 
> It also makes it more practical to use other storage allocation 
> schemes than garbage collection.
>
> It does not make scope into a type constructor, nor a general 
> type-annotation system.
>
> It does not provide an ownership system, though it would 
> complement one.

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.
  - 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).

Also, considering *e has infinite lifetime, (but not e[i] ???) 
what is the point of scope at all ? If all indirection goes to 
infinite lifetime (ie GC) then lifetime only apply to local 
variable and it should not be able to escape these, scope or not.

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.