C++'s standard library has valarray, which is supposed to avoid aliasing effects, and is allowed to use expression templates and other tricks. Something like:

    float dot(std::valarray<float> const& x)
    {
        return (x*x).sum();
        // Alternative: inner_product(begin(x), end(x), begin(x), 0.0f);
    }

should, in theory, achieve the same kind of performance as the C version (but none of this is guaranteed, sadly). A smart implementation would actually end up calling BLAS routines, which is what libraries like Eigen do.

Which reminds me: does Rust have infrastructure that would allow something like expression templates to exist?

I believe you can use Rust traits to the same effect, although I haven't tried so you might hit stuff.