Implement legacy custom biome

Author: lynxplay

paper-server/src/main/java/org/bukkit/craftbukkit/block/CraftBiome.java

@@ -3,10 +3,14 @@
 import io.papermc.paper.util.OldEnumHolderable;
 import net.minecraft.core.Holder;
 import net.minecraft.core.registries.Registries;
+import org.bukkit.NamespacedKey;
 import org.bukkit.block.Biome;
 import org.bukkit.craftbukkit.CraftRegistry;
+import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.NotNull;
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
+import java.util.Objects;
 
 @NullMarked
 public class CraftBiome extends OldEnumHolderable<Biome, net.minecraft.world.level.biome.Biome> implements Biome {
@@ -39,4 +43,58 @@ public static Biome minecraftHolderToBukkit(Holder<net.minecraft.world.level.bio
     public CraftBiome(final Holder<net.minecraft.world.level.biome.Biome> holder) {
         super(holder, count++);
     }
+
+    /**
+     * Implementation for the deprecated, API only, CUSTOM biome.
+     * As per {@link #bukkitToMinecraft(Biome)} and {@link #bukkitToMinecraftHolder(Biome)} it cannot be
+     * converted into an internal biome and only serves backwards compatibility reasons.
+     */
+    @Deprecated(forRemoval = true, since = "1.21.5")
+    @ApiStatus.ScheduledForRemoval(inVersion = "1.22")
+    public static class LegacyCustomBiomeImpl implements Biome {
+
+        private static final NamespacedKey LEGACY_CUSTOM_KEY = new NamespacedKey("minecraft", "custom");
+        private final int ordinal;
+
+        public LegacyCustomBiomeImpl() {
+            this.ordinal = count++;
+        }
+
+        @Override
+        public @NotNull NamespacedKey getKey() {
+            return LEGACY_CUSTOM_KEY;
+        }
+
+        @Override
+        public int compareTo(@NotNull final Biome other) {
+            return this.ordinal - other.ordinal();
+        }
+
+        @Override
+        public @NotNull String name() {
+            return "CUSTOM";
+        }
+
+        @Override
+        public int ordinal() {
+            return this.ordinal;
+        }
+
+        @Override
+        public boolean equals(final Object object) {
+            if (object == null || getClass() != object.getClass()) return false;
+            final LegacyCustomBiomeImpl that = (LegacyCustomBiomeImpl) object;
+            return ordinal == that.ordinal;
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hashCode(ordinal);
+        }
+
+        @Override
+        public String toString() {
+            return "CUSTOM";
+        }
+    }
 }

paper-server/src/main/java/org/bukkit/craftbukkit/util/CraftMagicNumbers.java

@@ -483,7 +483,7 @@ public <B extends Keyed> B get(RegistryKey<B> registry, NamespacedKey namespaced
     @Override
     public Biome getCustomBiome() {
         if (this.customBiome == null) {
-            this.customBiome = new CraftBiome(NamespacedKey.minecraft("custom"), null);
+            this.customBiome = new CraftBiome.LegacyCustomBiomeImpl();
         }
 
         return this.customBiome;