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: adapter.rs
    path of file: ./repos/trustfall/trustfall_wasm/src
    
    the code of the file until where you have to start completion: use std::{cell::RefCell, collections::BTreeMap, rc::Rc, sync::Arc};

use gloo_utils::format::JsValueSerdeExt;
use js_sys::try_iter;
use trustfall_core::{
    interpreter::{
        Adapter, AsVertex, ContextIterator, ContextOutcomeIterator, DataCont
    
    fn next(&mut self) -> Option<Self::Item> {
        let iter_next =
            self.inner.next().expect("unexpected value returned from JS iterator next()");

        if iter_next.done() {
            assert!(self.registry.borrow().is_empty());
            None
        } else {
            let next_item = self.next_item;
            let value = iter_next.value();

            let next_element: ReturnedContextIdAndValue =
                value.into_serde().expect("not a legal iterator element");
            assert_eq!(next_element.local_id, next_item);

            self.next_item = self.next_item.wrapping_add(1);

            let ctx = self.registry.borrow_mut().remove(&next_item).expect("id not found");

            Some((ctx, next_element.value.into()))
        }
    }