Demonstration of casting to multiple trait objects

as_trait.rs

use std::thread;
use std::sync::Arc;
use std::sync::RwLock;

struct Wrap{one:usize,zero:usize}

trait WriteIt {
    fn write1(&mut self,w:usize);
    fn read1(&self) -> usize;
}
impl WriteIt for Wrap {
    fn write1(&mut self,w:usize) { self.zero = w; }
    fn read1(&self) -> usize { self.zero }
}
impl<B:WriteIt+?Sized> WriteIt for Arc<RwLock<B>> {
    fn write1(&mut self,w:usize) { self.write().unwrap().write1(w) }
    fn read1(&self) -> usize { self.read().unwrap().read1() }
}
trait ReadIt {
    fn read2(&self) -> usize;
}
impl ReadIt for Wrap {
    fn read2(&self) -> usize { self.zero }
}
impl<A:ReadIt+?Sized> ReadIt for Arc<RwLock<A>> {
    fn read2(&self) -> usize { self.read().unwrap().read2() }
}
    
fn main() {
    let a = Arc::new(RwLock::new(Wrap{zero:56,one:1}));
    let mut b = a.clone() as Arc<RwLock<WriteIt+Send+Sync>>;
    let c = a.clone() as Arc<RwLock<ReadIt+Send+Sync>>;
    // let c = b.clone() as Arc<RwLock<ReadIt+Send+Sync>>; // error, non-primitive cast
    
    println!("a: {}, b: {}, c: {}",a.read1(),b.read1(),c.read2());
    // a: 5, b: 5, c: 5
    
    *a.write().unwrap() = Wrap{zero:9,one:7};
    // *b.write().unwrap() = 9; // error, can't assign to trait object

    println!("a: {}, b: {}, c: {}",a.read1(),b.read1(),c.read2());
    // a: 9, b: 9, c: 9

    thread::spawn(move||{
        println!("b: {}, c: {}",b.read1(),c.read2());
        // b: 9, c: 9
        
        b.write1(8);
    
        println!("b: {}, c: {}",b.read1(),c.read2());
        // b: 8, c: 8
        
        // let _bb = b.write().unwrap();
        // println!("c: {}",c.read2());
        // panic! already mutably borrowed
        
    });
    thread::sleep(std::time::Duration::from_millis(50));
    
    println!("a: {}",a.read1());
    // a: 8
    
    let a0 = a.read().unwrap().zero;
    let a1 = a.read().unwrap().one;
    println!("a.zero: {}, a.one: {}",a0,a1)
    
}