Nice! Is this going into the standard library?

I've looked at the output of both inner loops, and it seems I put too much faith in std::valarray's expression templates. Changing the distance function to

    int distance(std::valarray<int> const& x, std::valarray<int> const& y) {
        return inner_product(begin(x), end(x), begin(y), 0, 
        std::plus<int>(), [](int x, int y){ return (x-y)*(x-y); });
    }

results in the exact same inner loop as Rust. The remaining speed difference comes from the file loading code, which admittedly is crap (way too many memory allocations). So for all intents and purposes, I admit defeat.