AnyHolderSet and NotHolderSet cannot be datagenned

[KnightMiner]

Minecraft Version: 1.21

NeoForge Version: 21.4.51-beta

Exception:

Caused by: java.lang.ClassCastException: class net.minecraft.core.RegistrySetBuilder$UniversalLookup cannot be cast to class net.minecraft.core.HolderLookup$RegistryLookup (net.minecraft.core.RegistrySetBuilder$UniversalLookup and net.minecraft.core.HolderLookup$RegistryLookup are in module [email protected] of loader 'TRANSFORMER' @989da1)

Description of issue:

Attempting to use either AnyHolderSet or NotHolderSet in datagen leads to a crash. The reason for this is both require a HolderLookup.RegistryLookup to construct, and the Neo provided method to obtain a RegistryLookup does an unchecked cast from a HolderGetter to a HolderLookup.RegistryLookup which fails during datagen (I've not verified if that cast ever works at runtime, but I assume it must otherwise it wouldn't be there).

One commonly attempted fix is simply passing in null to the constructor since we don't need the RegistryLookup information during datagen, but this leads to a crash in Mojang's canSerializeIn which attempts to use the lookup during datagen.

Workarounds:

• The most general workaround is to construct a dummy RegistryLookup by either wrapping the HolderLookup or throwing in most methods, an example is here: https://github.com/SlimeKnights/TinkersConstruct/blob/e335dfb1c81b7698fe534b1b74ddf4ed85692bed/src/main/java/slimeknights/tconstruct/world/data/WorldgenProvider.java#L372
• A simpler workaround is extending NotHolderSet/AnyHolderSet for a datagennable version by making canSerializeIn always return true, thus making the constructor able to accept null during datagen, example here: https://github.com/SlimeKnights/TinkersConstruct/blob/85478dc6122864c6ac8d9001cc025882bb506714/src/main/java/slimeknights/tconstruct/world/data/WorldgenProvider.java#L363

Related Issues:

• Original Forge issue, closed without a fix: NotHolderSet cannot be serialized durring datagen MinecraftForge/MinecraftForge#9634
• PR fixing the Forge issue, uses an approach different from either of my workarounds that is worth considering: make NotHolderSet serialisable during datagen #9634  MinecraftForge/MinecraftForge#9663