error[E0499]: cannot borrow `y` as mutable more than once at a time
  --> $DIR/issue-18783.rs:7:21
   |
LL |     c.push(Box::new(|| y = 0));
   |                     -- - first borrow occurs due to use of `y` in closure
   |                     |
   |                     first mutable borrow occurs here
LL |     c.push(Box::new(|| y = 0));
   |                     ^^ - second borrow occurs due to use of `y` in closure
   |                     |
   |                     second mutable borrow occurs here
LL |
LL | }
   | - first borrow might be used here, when `c` is dropped and runs the destructor for type `std::cell::RefCell<std::vec::Vec<std::boxed::Box<dyn std::ops::FnMut()>>>`

error[E0499]: cannot borrow `y` as mutable more than once at a time
  --> $DIR/issue-18783.rs:16:29
   |
LL |     Push::push(&c, Box::new(|| y = 0));
   |                             -- - first borrow occurs due to use of `y` in closure
   |                             |
   |                             first mutable borrow occurs here
LL |     Push::push(&c, Box::new(|| y = 0));
   |                             ^^ - second borrow occurs due to use of `y` in closure
   |                             |
   |                             second mutable borrow occurs here
LL |
LL | }
   | - first borrow might be used here, when `c` is dropped and runs the destructor for type `std::cell::RefCell<std::vec::Vec<std::boxed::Box<dyn std::ops::FnMut()>>>`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0499`.