@ptrCast between real pointers and 0-sized pointers should be a compile error

[thejoshwolfe]

This came up in #960.

test "*usize to *void" {
    var i = usize(0);
    var v = @ptrCast(*void, &i);
    v.* = {};
}

This should be a compile error. A *void has 0 size (just like void itself) so this cast makes no sense.

@Hejsil was there a usecase you were running into in #960 where you wanted to do this cast?

[Hejsil]

@thejoshwolfe I think I was implementing some vtable/interface stuff. Was trying to use *void as I would use void * in C, and hit this. So no, there wasn't really a usecase.

[tadeokondrak]

Casting from normal pointers to pointers to zero-length arrays seems to be reasonable code, which happens in std.heap.GeneralPurposeAllocator.

[SpexGuy]

In #6706 we've decided to remove 0-sized pointers, so this is no longer relevant.