scope as template struct

[Christian Kamm]

With the features of D2 you can get pretty close to implementing scope
classes as a template struct:

private final class PlacementNewClass(T) : T
{
  // forward constructors here!

  new(uint size, byte[__traits(classInstanceSize, PlacementNewClass)] mem) {
    return &mem;
  }
  delete(void* addr) {}
}

struct StackClass(T)
{
  // forward constructors here!
  void construct() { 
    new(memory) PlacementNewClass!(T);
  }

  ~this() {
    T tmp = cast(T)this;
    delete tmp;
  }

  // should be opImplicitCast
  T opCast() { return cast(T)cast(void*)&memory; }
  T opDot() { return cast(T)cast(void*)&memory; }

  byte[__traits(classInstanceSize, PlacementNewClass!(T))] memory;
}

Instances of StackClass!(C) behave pretty much like scope C, except they are
not limited to function local variables.

Unless there's a major difference to 'scope' I missed, I think that with the
addition of a few generic features it should be possible to move it into a
library entirely. In particular, D would need opImplicitCast as well as a
way of accessing and forwarding constructors to make this work nicely.