error[E0502]: cannot borrow `heap` as immutable because it is also borrowed as mutable
  --> $DIR/issue-47646.rs:9:30
   |
LL |     let borrow = heap.peek_mut();
   |                  --------------- mutable borrow occurs here
LL |
LL |     match (borrow, ()) {
   |           ------------ a temporary with access to the mutable borrow is created here ...
LL |         (Some(_), ()) => {
LL |             println!("{:?}", heap);
   |                              ^^^^ immutable borrow occurs here
...
LL |     };
   |      - ... and the mutable borrow might be used here, when that temporary is dropped and runs the destructor for type `(Option<PeekMut<'_, i32>>, ())`
   |
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

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