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: inmemory.rs
    path of file: ./repos/LibAFL/libafl/src/corpus
    
    the code of the file until where you have to start completion: //! In-memory corpus, keeps all test cases in memory at all times

use alloc::vec::Vec;
use core::cell::RefCe
    
    pub fn next(&self, idx: CorpusId) -> Option<CorpusId> {
        if let Some(item) = self.map.get(&idx) {
            item.next
        } else {
            None
        }
    }