I talk a little about why in my "fearless SIMD" blog post (also introducing a prototype that wraps the unsafety but at the moment is very limited in exactly what SIMD operations are exposed). https://raphlinus.github.io/rust/simd/2018/10/19/fearless-si...

https://users.rust-lang.org/t/how-to-zip-two-slices-efficien... explores eliding the bounds checks even without an assert! (though by using the smaller length as the bounds, which might not be the desired behavior)