Add FeatureFlag API

paper-server/src/main/java/io/papermc/paper/world/flag/PaperFeatureDependent.java

@@ -0,0 +1,17 @@
+package io.papermc.paper.world.flag;
+
+import java.util.Set;
+import net.minecraft.world.flag.FeatureElement;
+import org.bukkit.FeatureFlag;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.jetbrains.annotations.Unmodifiable;
+
+public interface PaperFeatureDependent extends FeatureDependant {
+
+    <M extends FeatureElement> M getHandle();
+
+    @Override
+    default @Unmodifiable @NonNull Set<FeatureFlag> requiredFeatures() {
+        return PaperFeatureFlagProviderImpl.fromNms(this.getHandle().requiredFeatures());
+    }
+}

paper-server/src/main/java/io/papermc/paper/world/flag/PaperFeatureFlagProviderImpl.java

@@ -0,0 +1,54 @@
+package io.papermc.paper.world.flag;
+
+import com.google.common.collect.BiMap;
+import com.google.common.collect.ImmutableBiMap;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Set;
+import net.minecraft.world.flag.FeatureElement;
+import net.minecraft.world.flag.FeatureFlagSet;
+import net.minecraft.world.flag.FeatureFlags;
+import org.bukkit.FeatureFlag;
+import org.bukkit.craftbukkit.entity.CraftEntityType;
+import org.bukkit.craftbukkit.entity.CraftEntityTypes;
+import org.bukkit.craftbukkit.potion.CraftPotionType;
+import org.bukkit.craftbukkit.util.CraftMagicNumbers;
+import org.bukkit.entity.EntityType;
+import org.bukkit.potion.PotionType;
+
+public class PaperFeatureFlagProviderImpl implements FeatureFlagProvider {
+
+    public static final BiMap<FeatureFlag, net.minecraft.world.flag.FeatureFlag> FLAGS = ImmutableBiMap.of(
+        FeatureFlag.VANILLA, FeatureFlags.VANILLA,
+        FeatureFlag.TRADE_REBALANCE, FeatureFlags.TRADE_REBALANCE,
+        FeatureFlag.MINECART_IMPROVEMENTS, FeatureFlags.MINECART_IMPROVEMENTS,
+        FeatureFlag.REDSTONE_EXPERIMENTS, FeatureFlags.REDSTONE_EXPERIMENTS
+    );
+
+    @Override
+    public Set<FeatureFlag> requiredFeatures(final FeatureDependant dependant) {
+        final FeatureFlagSet requiredFeatures = getFeatureElement(dependant).requiredFeatures();
+        return fromNms(requiredFeatures);
+    }
+
+    public static Set<FeatureFlag> fromNms(final FeatureFlagSet flagSet) {
+        final Set<FeatureFlag> flags = new HashSet<>();
+        for (final net.minecraft.world.flag.FeatureFlag nmsFlag : FeatureFlags.REGISTRY.names.values()) {
+            if (flagSet.contains(nmsFlag)) {
+                flags.add(FLAGS.inverse().get(nmsFlag));
+            }
+        }
+        return Collections.unmodifiableSet(flags);
+    }
+
+    static FeatureElement getFeatureElement(final FeatureDependant dependant) {
+        if (dependant instanceof final EntityType entityType) {
+            // TODO remove when EntityType is server-backed
+            return CraftEntityType.bukkitToMinecraft(entityType);
+        } else if (dependant instanceof final PotionType potionType) {
+            return CraftPotionType.bukkitToMinecraft(potionType);
+        } else {
+            throw new IllegalArgumentException(dependant + " is not a valid feature dependant");
+        }
+    }
+}