B. Compilation Error.

rust_bytes_ref is a mutable reference to the String rust_bytes. This is the first mutable reference to the String type.

Let’s look at the signature of push_str method:

#![allow(unused)]
fn main() {
pub fn push_str(&mut self, string: &str) {
    self.vec.extend_from_slice(string.as_bytes())
} 
}

This method needs a mutable reference. So, the invocation of this method results in another mutable reference to the same string

Rust does not allow multiple mutable references to a type in the same scope.

Solutions:

Option 1:

Remove the mutable reference rust_bytes_ref.

Option 2:

Restructure the code and create a separate method that takes the mutable reference and pushes into the string.

fn main() {
    let mut rust_bytes = String::from("Rust Bytes");
    push(&mut rust_bytes);

    let rust_bytes_ref = &mut rust_bytes;
    println!("{}", rust_bytes_ref);
}

fn push(str: &mut String) {
    str.push_str(" is great.");
}