Add FeatureFlag API

paper-api/src/main/java/io/papermc/paper/world/flag/FeatureDependant.java

@@ -0,0 +1,28 @@
+package io.papermc.paper.world.flag;
+
+import java.util.Set;
+import org.bukkit.FeatureFlag;
+import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.Unmodifiable;
+import org.jspecify.annotations.NullMarked;
+
+/**
+ * Implemented by types in built-in registries that are controlled by {@link FeatureFlag FeatureFlags}.
+ * Types in data-driven registries that are controlled by feature flags just will not exist in that registry.
+ * @apiNote When a type that currently implements this interface transitions to being data-drive, this
+ * interface will be removed from that type in the following major version.
+ */
+@NullMarked
+@ApiStatus.NonExtendable
+public interface FeatureDependant {
+
+    /**
+     * Gets the set of required feature flags for this
+     * to be enabled.
+     *
+     * @return the immutable set of feature flags
+     */
+    default @Unmodifiable Set<FeatureFlag> requiredFeatures() {
+        return FeatureFlagProvider.provider().requiredFeatures(this);
+    }
+}

paper-api/src/main/java/io/papermc/paper/world/flag/FeatureFlagProvider.java

@@ -0,0 +1,21 @@
+package io.papermc.paper.world.flag;
+
+import java.util.Optional;
+import java.util.ServiceLoader;
+import java.util.Set;
+import org.bukkit.FeatureFlag;
+import org.jetbrains.annotations.ApiStatus;
+import org.jspecify.annotations.NullMarked;
+
+@NullMarked
+@ApiStatus.Internal
+interface FeatureFlagProvider {
+
+    Optional<FeatureFlagProvider> PROVIDER = ServiceLoader.load(FeatureFlagProvider.class).findFirst();
+
+    static FeatureFlagProvider provider() {
+        return PROVIDER.orElseThrow();
+    }
+
+    Set<FeatureFlag> requiredFeatures(FeatureDependant dependant);
+}

paper-api/src/main/java/io/papermc/paper/world/flag/FeatureFlagSetHolder.java

@@ -0,0 +1,32 @@
+package io.papermc.paper.world.flag;
+
+import java.util.Set;
+import org.bukkit.FeatureFlag;
+import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.Unmodifiable;
+import org.jspecify.annotations.NullMarked;
+
+/**
+ * Implemented by types that hold {@link FeatureFlag FeatureFlags} like
+ * {@link org.bukkit.generator.WorldInfo} and {@link org.bukkit.RegionAccessor}.
+ */
+@NullMarked
+@ApiStatus.NonExtendable
+public interface FeatureFlagSetHolder {
+
+    /**
+     * Checks if this is enabled based on the loaded feature flags.
+     *
+     * @return true if enabled
+     */
+    default boolean isEnabled(final FeatureDependant featureDependant) {
+        return this.getFeatureFlags().containsAll(featureDependant.requiredFeatures());
+    }
+
+    /**
+     * Get all {@link FeatureFlag FeatureFlags} enabled in this world.
+     *
+     * @return all enabled {@link FeatureFlag FeatureFlags}
+     */
+    @Unmodifiable Set<FeatureFlag> getFeatureFlags();
+}

paper-api/src/main/java/org/bukkit/FeatureFlag.java

@@ -1,17 +1,24 @@
 package org.bukkit;
 
+// Paper start - overhaul FeatureFlag API
+import com.google.common.base.Preconditions;
+import java.util.List;
+import net.kyori.adventure.key.Key;
+import net.kyori.adventure.util.Index;
+import org.intellij.lang.annotations.Subst;
+// Paper end - overhaul FeatureFlag API
 import org.jetbrains.annotations.ApiStatus;
 
 /**
- * This represents a Feature Flag for a World.
- * <br>
- * Flags which are unavailable in the current version will be null and/or
- * removed.
+ * This represents a Feature Flag for a {@link io.papermc.paper.world.flag.FeatureFlagSetHolder}.
  */
-@ApiStatus.Experimental
 public interface FeatureFlag extends Keyed {
 
-    public static final FeatureFlag VANILLA = Bukkit.getUnsafe().getFeatureFlag(NamespacedKey.minecraft("vanilla"));
+    // Paper start - overhaul FeatureFlag API
+    /**
+     * The {@code vanilla} feature flag.
+     */
+    FeatureFlag VANILLA = create("vanilla");
 
     /**
      * <strong>AVAILABLE BETWEEN VERSIONS:</strong> 1.19.3 - 1.21.1
@@ -19,18 +26,19 @@ public interface FeatureFlag extends Keyed {
      * @deprecated not available since 1.21.2
      */
     @Deprecated(since = "1.21.2")
-    public static final FeatureFlag BUNDLE = Bukkit.getUnsafe().getFeatureFlag(NamespacedKey.minecraft("bundle"));
+    FeatureFlag BUNDLE = deprecated("bundle");
 
     /**
      * <strong>AVAILABLE BETWEEN VERSIONS:</strong> 1.19 - 1.19.4
      *
      * @deprecated not available since 1.20
+     * The {@code trade_rebalance} feature flag.
      */
-    @Deprecated(since = "1.20.2")
-    public static final FeatureFlag UPDATE_1_20 = Bukkit.getUnsafe().getFeatureFlag(NamespacedKey.minecraft("update_1_20"));
+    @Deprecated(since = "1.20")
+    FeatureFlag UPDATE_1_20 = deprecated("update_1_20");
 
     @ApiStatus.Experimental // Paper - add missing annotation
-    public static final FeatureFlag TRADE_REBALANCE = Bukkit.getUnsafe().getFeatureFlag(NamespacedKey.minecraft("trade_rebalance"));
+    FeatureFlag TRADE_REBALANCE = create("trade_rebalance");
 
     /**
      * <strong>AVAILABLE BETWEEN VERSIONS:</strong> 1.20.5 - 1.20.6
@@ -38,7 +46,7 @@ public interface FeatureFlag extends Keyed {
      * @deprecated not available since 1.21
      */
     @Deprecated(since = "1.21")
-    public static final FeatureFlag UPDATE_121 = Bukkit.getUnsafe().getFeatureFlag(NamespacedKey.minecraft("update_1_21"));
+    FeatureFlag UPDATE_121 = deprecated("update_1_21");
 
     /**
      * <strong>AVAILABLE BETWEEN VERSIONS:</strong> 1.21.2 - 1.21.3
@@ -46,12 +54,28 @@ public interface FeatureFlag extends Keyed {
      * @deprecated not available since 1.21.4
      */
     @Deprecated(since = "1.21.4")
-    public static final FeatureFlag WINTER_DROP = Bukkit.getUnsafe().getFeatureFlag(NamespacedKey.minecraft("winter_drop"));
+    FeatureFlag WINTER_DROP = deprecated("winter_drop");
 
     @ApiStatus.Experimental // Paper - add missing annotation
-    public static final FeatureFlag REDSTONE_EXPERIMENTS = Bukkit.getUnsafe().getFeatureFlag(NamespacedKey.minecraft("redstone_experiments"));
+    FeatureFlag REDSTONE_EXPERIMENTS = create("redstone_experiments");
 
     @ApiStatus.Experimental // Paper - add missing annotation
-    public static final FeatureFlag MINECART_IMPROVEMENTS = Bukkit.getUnsafe().getFeatureFlag(NamespacedKey.minecraft("minecart_improvements"));
+    FeatureFlag MINECART_IMPROVEMENTS = create("minecart_improvements");
+
+    /**
+     * An index of all feature flags.
+     */
+    Index<Key, FeatureFlag> ALL_FLAGS = Index.create(FeatureFlag::key, List.copyOf(FeatureFlagImpl.ALL_FLAGS));
+
+    private static FeatureFlag create(@Subst("vanilla") final String name) {
+        final FeatureFlag flag = new FeatureFlagImpl(NamespacedKey.minecraft(name));
+        Preconditions.checkState(FeatureFlagImpl.ALL_FLAGS.add(flag), "Tried to add duplicate feature flag: " + name);
+        return flag;
+    }
+
+    private static FeatureFlag deprecated(@Subst("vanilla") final String name) {
+        return new FeatureFlagImpl.Deprecated(NamespacedKey.minecraft(name));
+    }
+    // Paper end - overhaul FeatureFlag API
 
 }

paper-api/src/main/java/org/bukkit/FeatureFlagImpl.java

@@ -0,0 +1,27 @@
+package org.bukkit;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.jetbrains.annotations.ApiStatus;
+import org.jspecify.annotations.NullMarked;
+
+@ApiStatus.Internal
+@NullMarked
+record FeatureFlagImpl(NamespacedKey key) implements FeatureFlag {
+
+    static final Set<FeatureFlag> ALL_FLAGS = new HashSet<>();
+
+    @Override
+    public NamespacedKey getKey() {
+        return this.key;
+    }
+
+    @ApiStatus.Internal
+    record Deprecated(NamespacedKey key) implements FeatureFlag {
+
+        @Override
+        public NamespacedKey getKey() {
+            return this.key;
+        }
+    }
+}

paper-api/src/main/java/org/bukkit/RegionAccessor.java

@@ -18,7 +18,7 @@ import org.jetbrains.annotations.Nullable;
  * A RegionAccessor gives access to getting, modifying and spawning {@link Biome}, {@link BlockState} and {@link Entity},
  * as well as generating some basic structures.
  */
-public interface RegionAccessor extends Keyed { // Paper
+public interface RegionAccessor extends Keyed, io.papermc.paper.world.flag.FeatureFlagSetHolder { // Paper - feature flag API
 
     /**
      * Gets the {@link Biome} at the given {@link Location}.

paper-api/src/main/java/org/bukkit/UnsafeValues.java

@@ -111,8 +111,7 @@ public interface UnsafeValues {
     @Deprecated(since = "1.21.3", forRemoval = true)
     String getTranslationKey(Attribute attribute);
 
-    @Nullable
-    FeatureFlag getFeatureFlag(@NotNull NamespacedKey key);
+    // Paper - replace with better system
 
     /**
      * Do not use, method will get removed, and the plugin won't run

paper-api/src/main/java/org/bukkit/block/BlockType.java

@@ -129,7 +129,7 @@ import org.jetbrains.annotations.Nullable;
  * changes may occur. Do not use this API in plugins.
  */
 @ApiStatus.Internal
-public interface BlockType extends Keyed, Translatable, net.kyori.adventure.translation.Translatable { // Paper - add translatable
+public interface BlockType extends Keyed, Translatable, net.kyori.adventure.translation.Translatable, io.papermc.paper.world.flag.FeatureDependant { // Paper - add translatable & feature flag API
 
     /**
      * Typed represents a subtype of {@link BlockType}s that have a known block
@@ -3604,7 +3604,9 @@ public interface BlockType extends Keyed, Translatable, net.kyori.adventure.tran
      *
      * @param world the world to check
      * @return true if this BlockType can be used in this World.
+     * @deprecated Use {@link io.papermc.paper.world.flag.FeatureFlagSetHolder#isEnabled(io.papermc.paper.world.flag.FeatureDependant)}
      */
+    @Deprecated(forRemoval = true, since = "1.21.1") // Paper
     boolean isEnabledByFeature(@NotNull World world);
 
     /**

paper-api/src/main/java/org/bukkit/entity/EntityType.java

@@ -43,7 +43,7 @@ import org.jetbrains.annotations.Contract;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
-public enum EntityType implements Keyed, Translatable, net.kyori.adventure.translation.Translatable { // Paper - translatable
+public enum EntityType implements Keyed, Translatable, net.kyori.adventure.translation.Translatable, io.papermc.paper.world.flag.FeatureDependant { // Paper - translatable
 
     // These strings MUST match the strings in nms.EntityTypes and are case sensitive.
     /**

paper-api/src/main/java/org/bukkit/generator/WorldInfo.java

@@ -7,7 +7,7 @@ import org.jetbrains.annotations.NotNull;
 /**
  * Holds various information of a World
  */
-public interface WorldInfo {
+public interface WorldInfo extends io.papermc.paper.world.flag.FeatureFlagSetHolder { // Paper - feature flag API
 
     /**
      * Gets the unique name of this world

paper-api/src/main/java/org/bukkit/inventory/ItemType.java

@@ -47,7 +47,7 @@ import org.jetbrains.annotations.Nullable;
  * changes may occur. Do not use this API in plugins.
  */
 @ApiStatus.Experimental // Paper - already required for registry builders
-public interface ItemType extends Keyed, Translatable, net.kyori.adventure.translation.Translatable { // Paper - add Translatable
+public interface ItemType extends Keyed, Translatable, net.kyori.adventure.translation.Translatable, io.papermc.paper.world.flag.FeatureDependant { // Paper - add Translatable & feature flag API
 
     /**
      * Typed represents a subtype of {@link ItemType}s that have a known item meta type
@@ -2412,7 +2412,9 @@ public interface ItemType extends Keyed, Translatable, net.kyori.adventure.trans
      *
      * @param world the world to check
      * @return true if this ItemType can be used in this World.
+     * @deprecated use {@link io.papermc.paper.world.flag.FeatureFlagSetHolder#isEnabled(io.papermc.paper.world.flag.FeatureDependant)}
      */
+    @Deprecated(forRemoval = true, since = "1.21.1") // Paper
     boolean isEnabledByFeature(@NotNull World world);
 
     /**

paper-api/src/main/java/org/bukkit/inventory/MenuType.java

@@ -22,7 +22,7 @@ import org.jetbrains.annotations.NotNull;
  * created and viewed by the player.
  */
 @ApiStatus.Experimental
-public interface MenuType extends Keyed {
+public interface MenuType extends Keyed, io.papermc.paper.world.flag.FeatureDependant { // Paper - make FeatureDependant
 
     /**
      * A MenuType which represents a chest with 1 row.

paper-api/src/main/java/org/bukkit/potion/PotionEffectType.java

@@ -17,7 +17,7 @@ import org.jetbrains.annotations.Nullable;
 /**
  * Represents a type of potion and its effect on an entity.
  */
-public abstract class PotionEffectType implements Keyed, Translatable, net.kyori.adventure.translation.Translatable { // Paper - implement Translatable
+public abstract class PotionEffectType implements Keyed, Translatable, net.kyori.adventure.translation.Translatable, io.papermc.paper.world.flag.FeatureDependant { // Paper - implement Translatable & feature flag API
     private static final BiMap<Integer, PotionEffectType> ID_MAP = HashBiMap.create();
 
     /**

paper-api/src/main/java/org/bukkit/potion/PotionType.java

@@ -14,7 +14,7 @@ import org.jetbrains.annotations.Nullable;
  * This enum reflects and matches each potion state that can be obtained from
  * the Creative mode inventory
  */
-public enum PotionType implements Keyed {
+public enum PotionType implements Keyed, io.papermc.paper.world.flag.FeatureDependant { // Paper - feature flag API
     WATER("water"),
     MUNDANE("mundane"),
     THICK("thick"),