pub struct Middleware {
    listener: TcpListener,
    queue: Vec<String>,
    servers: Vec<String>,
}

impl Middleware {
    pub fn run(&mut self) {
        for stream in self.listener.incoming() {
            let mut stream = stream.unwrap();
            let mut buffer = vec![0u8; 512];
            stream.read(&mut buffer).unwrap();
            self.queue.push(String::from_utf8(buffer).unwrap());
    }
}

impl Middleware {
    pub fn run(&mut self) {
        for stream in self.listener.incoming() {
            let mut stream = stream.unwrap();
            self.handle_client(stream);
        }
    }

    fn handle_client(&mut self, mut stream: TcpStream) {
        let mut buffer = vec![0u8; 512];
        stream.read(&mut buffer).unwrap();
        self.queue.push(String::from_utf8(buffer).unwrap());
    }
}

cannot borrow `*self` as mutable because `self.listener` is also borrowed as immutable
--> src/lib.rs:21:13
   |
19 |         for stream in self.listener.incoming() {
   |                       ------------- immutable borrow occurs here
20 |             let mut stream = stream.unwrap();
21 |             self.handle_client(stream);
   |             ^^^^ mutable borrow occurs here
22 |         }
   |         - immutable borrow ends here