Error "cannot borrow as mutable because it is also borrowed as immutable" in Rust: how to fix?

Hello! I just started learning Rust and encountered a problem that I can't understand. I have a simple program that takes a string and tries to first get its length, then change the first character: ```rust fn main() { let mut text = String::from("hello"); let first = &text[0..1]; // immutable reference text.push_str(" world"); // attempt to mutable borrow println!("First char: {}", first); } ``` The compiler gives an error: ``` error[E0502]: cannot borrow `text` as mutable because it is also borrowed as immutable ``` I understand that Rust has borrowing rules: either one mutable reference or any number of immutable references. But why does my case violate this rule? After all, I first take an immutable reference to part of the string, and then