Sending references over a channel

Even references and pointers can be sent over a channel, like in this example:

// see code from Chapter 9/code/channel_box.rs 
use std::thread; 
use std::sync::mpsc; 
 
trait Message : Send { 
    fn print(&self); 
} 
 
struct Msg1 { 
    value: i32 
} 
 
impl Message for Msg1 { 
    fn print(&self) { 
        println!("value: {:?}", self.value); 
    } 
} 
 
fn main() { 
    let (tx, rx) = mpsc::channel::<Box<Message>>(); 
     
    let handle = thread::spawn(move|| { 
        let msg = rx.recv().unwrap(); 
        msg.print(); 
    }); 
     
    let msg = Box::new(Msg1{ value:1 }); 
    tx.send(msg).unwrap(); 
     
    handle.join().ok(); 
} 

This prints out the following:

value: 1

Exercise:
Make a program, shared_channel.rs, that lets any number of threads share a channel to send in a value, and one receiver that collects all the values.
(Hint: use clone() to give each thread access to the sending endpoint tx. See the example code in Chapter 9/exercises/shared_channel.rs.)