Is there a trait supplying `iter()`?

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No, there is no trait that provides iter().

However, IntoIterator is implemented on references to some containers. For example, Vec<T>, &Vec<T> and &mut Vec<T> are three separate types that implement IntoIterator, and you’ll notice that they all map to different iterators. In fact, Vec::iter() and Vec::iter_mut() are just convenience methods equivalent to &Vec::into_iter() and &mut Vec::into_iter() respectively.

fn foo(_x: std::slice::Iter<i32>) {}

fn main() {
    let v = vec![1, 2, 3];
    foo(v.iter());
    foo((&v).into_iter()); // iter() exists because this is awkward
}

If you want to write a function that is generic over containers that can be converted into an iterator that iterates over references, you can do so like this:

fn foo<'a, I: IntoIterator<Item=&'a i32>>(_x: I) {}

fn main() {
    let v = vec![1, 2, 3];
    foo(&v);
}

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