#![allow(dead_code, deprecated, unused_variables, unused_mut)]
use std::cell::{RefCell, RefMut};
fn main() {
    let c = RefCell::new((5, 'b'));
    {
        let b1: RefMut<'_, (u32, char)> = c.borrow_mut();
        let mut b2: RefMut<'_, u32> = RefMut::map(b1, |t| &mut t.0);
        assert_eq!(* b2, 5);
        *b2 = 42;
    }
    assert_eq!(* c.borrow(), (42, 'b'));
}