One example I know of is that when trying to calculate a dot product, you can improve the generated code and elide the bounds check only by a very particular syntax:

    // Must slice to equal lengths, and then bounds checks are eliminated!
    let len = cmp::min(xs.len(), ys.len());
    let mut xs = &xs[..len];
    let mut ys = &ys[..len];
    let mut s = 0.;
    for i in 0..xs.len() {
        s += xs[i] * ys[i];
    }

https://github.com/rustsim/nalgebra/blob/b1b18d17ee64d3fd28b...