You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: structs.rs
    path of file: ./repos/jsonpath-rs/src
    
    the code of the file until where you have to start completion: use filter;
use serde_json::Value;
use std::iter::Enumerate;
use std::slice::Iter;

#[derive(Debug, Clone, PartialEq)]
pub enum Criterion {
    // $
    Root,

    // @
    Element,

    // .name
    NamedChild(String),

    // ?(path)
    Filter(Vec<Criterion>),

    // path
    SubExpression(Vec<Criterion>),

    // .*
    AnyChild,

    // [123]
    IndexedChild(usize),

    // [10:20]
    Slice(::std::ops::Range<usize>),

    // [:7]
    SliceTo(::std::ops::R
    
    pub fn next(&mut self) -> Option<(Self, Step<'a>)> {
        match self.iter {
            Some(ref mut item_iter) => match *item_iter {
                ItemIter::Array(ref mut iter) => match iter.next() {
                    Some((index, val)) => {
                        let sub_item = Item::new(val);
                        let step = Step::Index(index);
                        Some((sub_item, step))
                    }
                    None => None,
                },
                ItemIter::Object(ref mut iter) => match iter.next() {
                    Some((key, val)) => {
                        let sub_item = Item::new(val);
                        let step = Step::Key(key);
                        Some((sub_item, step))
                    }
                    None => None,
                },
            },
            None => None,
        }
    }