#![allow(dead_code, deprecated, unused_variables, unused_mut)]
use std::cell::RefCell;
fn main() {
    let c = RefCell::new(5);
    {
        let m = c.borrow_mut();
        assert!(c.try_borrow().is_err());
    }
    {
        let m = c.borrow();
        assert!(c.try_borrow().is_ok());
    }
}