DIP 36: Rvalue References

[Zach the Mystic]

On Wednesday, 10 April 2013 at 05:21:40 UTC, Manu wrote:
> struct X { int *p; }
> int* global;
>
> void func(scope X x) // note: is not ref
> {
>   global = x.p; // <- error! scope prevents any part of x 
> escaping, even
> though it's a copy.
> }
>
> This way, scope has meaning with or without ref.

I think this is a subtle point, but I don't think it's correct. I 
think there's a difference between 'x.p' above, and what '&x.p' 
or '&x' would give. I think 'scope' should guarantee that you 
won't assign the *address* of the passed-in parameter:

void func(scope X x, ref X y) {
   static X* xx = &x; // Error
   static int** xpp = &x.p; // Error
   xx = &y; // unsafe, but pass
   xpp = &y.p; // same
}

I don't think it should guarantee that you won't copy something 
that is *already* a pointer. Copying the pointer may be 
dangerous, but I don't think 'scope' will be very useful if it's 
supposed to track the internal pointers of aggregate types like 
that. I think somebody else has to make sure that pointer is 
safe. I could be wrong.