1 use std::cell::RefCell; 2 3 4 main()5fn main() { 6 let m = RefCell::new(0); 7 let mut b = m.borrow_mut(); 8 let b1 = &mut *b; 9 let b2 = &mut *b; //~ ERROR cannot borrow 10 b1.use_mut(); 11 } 12 use_mut(&mut self)13trait Fake { fn use_mut(&mut self) { } fn use_ref(&self) { } } 14 impl<T> Fake for T { } 15