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: mod.rs
    path of file: ./repos/cacao/src/defaults
    
    the code of the file until where you have to start completion: //! Wraps `NSUserDefaults`, providing an interface to fetch and store small amounts of data.
//!
//! In general, this tries to take an approach popularized by `ser
    
    pub fn insert<K: AsRef<str>>(&mut self, key: K, value: Value) {
        let key = NSString::new(key.as_ref());
        let value: id = &mut *value.into_id();

        unsafe {
            let _: () = msg_send![&*self.0, setObject: value, forKey: &*key];
        }
    }