Naturally, @alexcrichton already asked my question, so don't feel the need to repeat your answer. I am not sure if I'm satisfied yet ;) -- have to read through and try to understand just where unsafe code is being used. Seems like it's being used at least partially to avoid use of `Option<>` in the vector due to fear of size bloat and to avoid code indexing, and because the algorithm already knows what parts of the vector are initialized. But as I said I don't fully understand what's going on.