okay so, the un-elided version of foo_no is

  fn foo_no<'a>(v: &'a [i32]) -> std::iter::Cloned<std::slice::Iter<'a, i32>> {

I am not sure why the impl Trait version doesn't elide properly; I wrote it without the lifetime first and the compiler told me I need it, so I wrote it out, which is often what happens to me in these situations. I assume, since the elision rules were written years before impl Trait became a thing, that there's some sort of reason why it doesn't elide, but honestly, I'm not sure.