It's fork of the rst_lex to clean source code up from debugging stuff. Let's look at workable example:

Code Overview

use std::env;
use std::fs;
use std::path::Path;
use std::io::{Read, Write};

// extern crate for_build_rs
#[cfg(feature = "cleanup")]
use for_build_rs::lex::{stream_cleanup, leave_file_mark};
#[cfg(feature = "cleanup")]
use for_build_rs::faav::cleanup_dbg_attr;

#[cfg(feature = "cleanup")]
pub fn cleanup() {
    let mut attr = cleanup_dbg_attr::new();
    attr._1st_token = "dbg!".to_string();
    attr.end_token = ";".to_string();
    
    let src = "./src/dev_lex.rs";
    let dst = "./src/stable_lex.rs";
    let mut dev_lex = String::new();
    
    let mut src_file = match fs::File::open(src) {
        Ok(f) => f,
        Err(e) => {
            panic!("cleanup for rst_lex failed to open file {src} w/ err {:?}", e)
        }
    };
    
    let _ = match src_file.read_to_string(&mut dev_lex) {
        Ok(_) => {},
        Err(e) => {
            eprintln!("cleanup() in rst_lex build.rs failed to read from {src} to dev_lex due to {:?}", e);
            return;
        }
    };
    
    dev_lex = stream_cleanup(&dev_lex, &attr._1st_token, 0, &attr.end_token);
    dev_lex = stream_cleanup(&dev_lex, &"dbg_stuff".to_string(), 0, &"dbg_stuff".to_string());
    
    let mut dst_file = match fs::File::create(dst) {
        Ok(f) => f,
        Err(e) => {
            panic!("cleanup for rst_lex failed to write file {dst} w/ err {:?}", e)
        }
    };
    
    dev_lex = dev_lex.replace(";;", "");
    leave_file_mark("/tmp/cleanup", &dev_lex);
    dst_file.write_all(dev_lex.as_bytes()).unwrap();
}

fn main() {
    #[cfg(feature = "cleanup")]
    cleanup();
}

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