preparing for const, final, and invariant

[Walter Bright]

Regan Heath wrote:
> Why not make this implicit 'in' parameter 'scope const final' avoiding the need to explicity say 'in' everywhere?

It's a good idea, but then there needs to be some way to pass a mutable 
reference. Such as:

class C { int x; }
void foo(C c)
{
     c.x = 3;
}

That doesn't work if 'const' is the default. Using out or inout doesn't 
work either, as those have slightly different semantics (an extra level 
of indirection).