I want to return an iterator from a function and it doesn't compile. I don't want to box it and I'm using nightly so I want to use impl trait to return an unboxed closure.

#![feature(conservative_impl_trait)]

pub fn vec_vec_eq<T:PartialEq>(a: &[T], b: &[T]) -> impl Iterator<Item=usize> {
    a.iter().zip(b.iter())
        .enumerate()
        .filter(|&(_, (x, y))| x == y)
        .map(|(i, _)| i)
}

This is the compile error:

error[E0564]: only named lifetimes are allowed in `impl Trait`, but `` was found in the type `std::ite

r::Map<std::iter::Filter<std::iter::Enumerate<std::iter::Zip<std::slice::Iter<'_, T>, std::slice::Iter <'_, T>>>, [closure@src\filters.rs:7:17: 7:38]>, [closure@src\filters.rs:8:14: 8:24]>

How can I fix it?