/// # In this case, borrowing x mutably is not possible, because x is not mut. But at the same time, borrowing x immutably would make the assignment illegal, because a & &mut reference might not be unique, so it cannot safely be used to modify a value. So a unique immutable borrow is used: it borrows x immutably, but like a mutable borrow, it must be unique.
/// In the above example, uncommenting the declaration of y will produce an error because it would violate the uniqueness of the closure's borrow of x; the declaration of z is valid because the closure's lifetime has expired at the end of the block, releasing the borrow.#
#![allow(unused)]
fn main() {
let mut b = false;
let x = &mut b;
{
let mut c = || { *x = true; };
// The following line is an error:
// let y = &x;
c();
}
let z = &x;
}