message: permits things to become *more* durable without a "change"
  This will be important if we want to automatically upgrade the
  durability of inputs.
diff: @@ -219,9 +219,11 @@ where
  // old value.
  if let Some(old_memo) = &panic_guard.memo {
  if let Some(old_value) = &old_memo.value {
  - // Careful: the "constant-ness" must also not have
  - // changed, see the test `constant_to_non_constant`.
  - if old_memo.durability == result.durability
  + // Careful: if the value became less durable than it
  + // used to be, that is a "breaking change" that our
  + // consumers must be aware of. Becoming *more* durable
  + // is not. See the test `constant_to_non_constant`.
  + if result.durability >= old_memo.durability
  && MP::memoized_value_eq(&old_value, &result.value)
  {
  debug!(
  