Return a reference to a member iterator - rust

Question

I have a struct that has an iterator over a borrowed slice, and I want to create a method that returns an iterator that does some computation on the contained iterator
Basically this:
#[derive(Clone)]
struct Foo<'a> {
inner: ::std::iter::Cycle<::std::slice::Iter<'a, u8>>,
}
impl<'a> Foo<'a> {
pub fn new<T: AsRef<[u8]> + ?Sized>(vals: &'a T) -> Foo<'a> {
Self {
inner: vals.as_ref().iter().cycle(),
}
}
pub fn iter(&mut self) -> impl Iterator<Item = u8> + 'a {
self.inner.by_ref().map(Clone::clone) // simple case but this could be any computation
}
}
Which yields a cryptic error.
Here's my understanding: I need to bind the returned iterator's lifetime to the &mut self of iter() so that &mut self is only dropped when .collect() is called on the returned iterator.
BUT, changing to fn iter(&'a mut self) isn't good enough because &'a mut self now lives for the entire duration of the struct instance (meaning that calling collect() doesn't drop that reference). Which prevents something like this
#[test]
fn test(){
let mut foo = Foo::new("hello, there");
foo.iter().zip(0..4).collect::<Vec<_>>(); // call once, ok
foo.iter().zip(0..4).collect::<Vec<_>>(); // error because 2 &mut references exist at the same
}
rust playground link

Answer 1

The way I would do this is to make a struct just for iterating over those items. It makes things much easier.
...
pub fn iter(&mut self) -> ClonedFooIter<'a, '_> {
ClonedFooIter {
inner: &mut self.inner,
}
}
}
pub struct ClonedFooIter<'a, 'b> {
inner: &'b mut std::iter::Cycle<::std::slice::Iter<'a, u8>>,
}
impl<'a, 'b> Iterator for ClonedFooIter<'a, 'b> {
type Item = u8;
fn next(&mut self) -> Option<Self::Item> {
self.inner.next().cloned()
}
}
The idea is that the returned struct only lives till the mutable borrow of self lives.
Using a struct makes naming the iterator type easier too.