error[E0501]: cannot borrow `x` as immutable because previous closure requires unique access
  --> $DIR/yield-while-ref-reborrowed.rs:36:20
   |
LL |     let mut b = || {
   |                 -- coroutine construction occurs here
LL |         let a = &mut *x;
   |                      -- first borrow occurs due to use of `x` in coroutine
...
LL |     println!("{}", x);
   |                    ^ second borrow occurs here
LL |     Pin::new(&mut b).resume(());
   |              ------ first borrow later used here
   |
   = note: this error originates in the macro `$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 1 previous error

For more information about this error, try `rustc --explain E0501`.