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: slab.rs
    path of file: ./repos/wasmer/lib/wai-bindgen-wasmer/src
    
    the code of the file until where you have to start completion: use std::fmt;
use std::mem;

pub struct Slab<T> {
    storage: Vec<Entry<T>>,
    next: usiz
    
    pub fn insert(&mut self, item: T) -> u32 {
        if self.next == self.storage.len() {
            self.storage.push(Entry::Empty {
                next: self.next + 1,
            });
        }
        let ret = self.next as u32;
        let entry = Entry::Full(item);
        self.next = match mem::replace(&mut self.storage[self.next], entry) {
            Entry::Empty { next } => next,
            _ => unreachable!(),
        };
        ret
    }