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();
+}