error[E0502]: cannot borrow `heap` as immutable because it is also borrowed as mutable
  --> $DIR/issue-47646.rs:22:30
   |
LL |     let borrow = heap.peek_mut();
   |                  ---- mutable borrow occurs here
...
LL |             println!("{:?}", heap); //~ ERROR cannot borrow `heap` as immutable
   |                              ^^^^ immutable borrow occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0502`.