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/smithay/src/wayland/presentation
    
    the code of the file until where you have to start completion: //! Utilities for handling the `wp_presentation` protocol
//!
//! ## How to use it
//!
//! ### Initialization
//!
//! To initialize this implementation, create [`PresentationState`], store it in your `State` struct 
    
    fn merge_into(mut self, into: &mut Self, _dh: &DisplayHandle) {
        // discard unprocessed callbacks as defined by the spec
        // ...the user did not see the content update because it was superseded...
        for callback in std::mem::replace(&mut into.callbacks, std::mem::take(&mut self.callbacks)) {
            callback.discarded();
        }
    }