The point is that there isn't any integer that looks like a pointer — odd numbers are immediate, even are addresses — so Ruby's GC is spared from having to figure it out. It avoids the problem as a side effect of a performance optimization in the language implementation, so the GC doesn't have to be smarter. The compromise is in losing a bit of integer precision. But now that I think twice about it, I suppose I might kind of be splitting hairs.
Well, I'm not sure what you call it, but if you can't even have an integer that looks like a pointer, then you effectively don't have a conservative collector. The important distinction is whether an object can be kept alive through a false reference, and it sounds like it definitively can't in this case.
