Unable to edit Rc <RefCell>'s T

use std::borough::BorrowMut;
usestd::cell::RefCell;
use std::rc::Rc;

# [derive (Debug)]
US>structure Test {
    num —i32,
}

US>fnmain(){
    leta=Rc::new(RefCell::new(Test {num:100});
    let a1 = a.clone();
    a.borough_mut().num=100;
    a1.borough_mut().num = 100;
    println!("a:{:?}", a);
    println!("a1:{:?}", a1);
}

 error [E0609]: no field `num` on type`&mut Rc<RefCell<Test>>`
  -->src/main.rs:13:20
   |
13 | a.borough_mut().num = 100;
   |                    ^^^unknown field