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.

So why is the lifetime of the return value linked to the lifetime of the function argument? Couldn't the return value live longer than the function argument in that example?

In both cases, the iterators are over the slice, so their lifetimes need to be tied together. The iterator can't live longer than the slice, or it'd be a use after free. Does that make sense?

Yes. Thanks for the explanation.

The Cloned iterator clones the elements of the underlying iterator, but only on-demand, so obviously it can't do that once the argument isn't live anymore.