Retag(mut_ref); // tag gets changed to `Uniq(2)`
// stack: [Uniq(0), Shr, Uniq(2)]; not frozen
// And at the same time, a fresh shared reference to its outside!
// This counts as a read access through `rc`, so we have to pop until
// at least a `Shr` is at the top of the stack.
Retag(mut_ref); // tag gets changed to `Uniq(2)`
// stack: [Uniq(0), Shr, Uniq(2)]; not frozen
// And at the same time, a fresh shared reference to its outside!
// This counts as a read access through `rc`, so we have to pop until
// at least a `Shr` is at the top of the stack.