Gecko implements most of its IDNA processing on its own, but it currently uses ICU4C's uidna_labelToUnicode for UTS 46 processing.

Code that wraps ComposingNormalizer::try_new_uts46_without_ignored_and_disallowed_unstable and augments it to provide the functionality of ICU4C's uidna_labelToUnicode needs to go somewhere: Since it needs some data for ignored and disallowed that is in sync with ICU4X data, it would make sense to start a new ICU4X component for it. (We already reserve the name.)