Does rust have a trait for a collection with `iter()` method or a resettable iterator?

Question

It is useful for functions to accept a dyn IntoIterator<blah> object, which allows you to use into_iter(), but that requires taking ownership of the collection.

It seems like Rust lacks a trait for the iter() method. Does something like the itertools crate provide a replacement?

Is there an trait for a resettable/restartable iterator?

Is there some alternative for a function that:

1. Wants to iterate over a collection several times without cloning it.
2. Does not want to take ownership of anything.

I guess I could take Iterator<Item=T>+Clone, but that's a bit ugly (I'd have to use the iterator before using it the first time).

---

I should add my actual goal: I would like to make a bunch of functions that can take both &[T] or &IndexSet<T> (from indexmap crate) as arguments.

Answer 1

that requires taking ownership of the collection

No it does not, if the items can be references:

fn main() {
    let a = vec![1, 2, 3];
    do_it_twice_by_ref(&a);
    dbg!(a);
}

fn do_it_twice_by_ref<'a>(it: impl IntoIterator<Item = &'a u8> + Copy) {
    for i in it {
        dbg!(i);
    }
    for i in it {
        dbg!(i);
    }
}

This works because usually there is also a impl IntoIterator for &Collection (see here for IndexSet) which is implemented using the iter method. And Copy comes for free on shared references.