inser.rs

use std::fs::File;
use std::io::prelude::*;

fn main() {
    let text = {
        let mut f = File::open("test.txt").unwrap();
        let mut buf = String::new();
        f.read_to_string(&mut buf).unwrap();
        buf
    };
    let text_to_insert_before = "Insert before me please!";
    let text_to_insert = "I am an inserted line.\n";
    let index = text.find(text_to_insert_before).expect("Could not find replacement guide.");
    let mut new_text = String::with_capacity(text.len() + text_to_insert.len());
    new_text.push_str(&text[..index]);
    new_text.push_str(text_to_insert);
    new_text.push_str(&text[index..]);
    let mut f = File::create("test.txt").unwrap();
    f.write_all(new_text.as_bytes()).unwrap();
}

test.txt

I contain some text.
Insert before me please!
Thanks!