Keep native structures out of static object cells

Vanilla structure and feature placement now runs through the bounded
worldgen access with a protected-position predicate, so a jigsaw piece
or an imported feature can read a static object's blocks but never
overwrite them, and never schedules a tick or block entity there.
Dimensions without static objects keep the plain level as before.

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/ImportedFeatureStage.java

@@ -7,6 +7,7 @@ import art.arcane.iris.engine.object.IrisBiomeCustom;
 import art.arcane.iris.engine.object.IrisDecorationStep;
 import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.engine.object.IrisImportedFeatureControl;
+import art.arcane.iris.engine.object.IrisStaticObjectLayer;
 import art.arcane.iris.spi.IrisLogging;
 import it.unimi.dsi.fastutil.ints.IntArraySet;
 import it.unimi.dsi.fastutil.ints.IntSet;
@@ -26,6 +27,7 @@ import net.minecraft.world.level.chunk.ChunkAccess;
 import net.minecraft.world.level.chunk.LevelChunkSection;
 import net.minecraft.world.level.chunk.ChunkGenerator;
 import net.minecraft.world.level.levelgen.RandomSupport;
+import net.minecraft.world.level.levelgen.Heightmap;
 import net.minecraft.world.level.levelgen.WorldgenRandom;
 import net.minecraft.world.level.levelgen.XoroshiroRandomSource;
 import net.minecraft.world.level.levelgen.placement.PlacedFeature;
@@ -287,13 +289,20 @@ final class ImportedFeatureStage {
         WorldgenRandom random = new WorldgenRandom(new XoroshiroRandomSource(RandomSupport.generateUniqueSeed()));
         long decorationSeed = random.setDecorationSeed(level.getSeed(), origin.getX(), origin.getZ());
         Set<Holder<Biome>> chunkBiomes = chunkBiomes(level, sectionPos, table);
+        IrisStaticObjectLayer staticObjects = engine.getDimension().getStaticObjectLayer(engine.getData());
+        int staticMinY = engine.getMinHeight();
+        WorldGenLevel placementLevel = staticObjects.isEmpty() ? level : NativeStructureWorldgenAccess.create(
+                level, centerPos,
+                (x, z) -> level.getHeight(Heightmap.Types.WORLD_SURFACE_WG, x, z),
+                (x, z) -> level.getHeight(Heightmap.Types.OCEAN_FLOOR_WG, x, z),
+                position -> staticObjects.contains(position.getX(), position.getY() - staticMinY, position.getZ()));
 
         try {
             for (int stepIndex = 0; stepIndex < steps.size(); stepIndex++) {
                 if (!table.control().shouldGenerateStep(IrisDecorationStep.byOrdinal(stepIndex))) {
                     continue;
                 }
-                placeStep(level, table, steps.get(stepIndex), featureRegistry, chunkBiomes, owner,
+                placeStep(placementLevel, table, steps.get(stepIndex), featureRegistry, chunkBiomes, owner,
                         random, decorationSeed, origin, stepIndex);
             }
         } catch (Throwable error) {

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/IrisChunkGenerator.java

@@ -13,6 +13,7 @@ import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.engine.object.IrisDimensionCarvingResolver;
 import art.arcane.iris.engine.object.IrisMaterialPalette;
 import art.arcane.iris.engine.object.IrisNativeStructureDecision;
+import art.arcane.iris.engine.object.IrisStaticObjectLayer;
 import art.arcane.iris.nativegen.NativeStructureGenerationException;
 import art.arcane.iris.nativegen.NativeStructureStartInjector;
 import art.arcane.iris.nativegen.NativeStructureReferenceEnvelope;
@@ -873,9 +874,14 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
                     "heightmap priming", nativeStructureBatchContext(placementGroups),
                     chunkPos.x(), chunkPos.z(), error);
         }
+        IrisStaticObjectLayer staticObjects = engine.getDimension().getStaticObjectLayer(engine.getData());
+        int staticMinY = engine.getMinHeight();
+        WorldGenLevel boundedWorld = staticObjects.isEmpty() ? world : NativeStructureWorldgenAccess.create(
+                world, chunkPos, worldgenSurfaceHeight(), worldgenFloorHeight(),
+                position -> staticObjects.contains(position.getX(), position.getY() - staticMinY, position.getZ()));
         try {
             NativeStructureVegetationClearer.clearIntersectingVegetation(
-                    world, chunk, area, vegetationTargets);
+                    boundedWorld, chunk, area, vegetationTargets);
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "vegetation cleanup", nativeStructureBatchContext(placementGroups),
@@ -884,7 +890,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
         NativeStructureSurfaceFitter.VacuumFoundationPlan vacuumFoundationPlan;
         try {
             vacuumFoundationPlan = NativeStructureSurfaceFitter.prepareSurfaceStructures(
-                    world, area, terrainTargets,
+                    boundedWorld, area, terrainTargets,
                     (x, z) -> engine.getHeight(x, z, true) + engine.getMinHeight());
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
@@ -893,7 +899,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
         }
         try {
             NativeStructurePostProcessor.prepareTerrain(
-                    world, area, terrainTargets, this::resolvePaletteBlock);
+                    boundedWorld, area, terrainTargets, this::resolvePaletteBlock);
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "terrain preparation", nativeStructureBatchContext(placementGroups),
@@ -903,7 +909,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
             random.setFeatureSeed(decoSeed, group.featureIndex(), group.step());
             try {
                 for (NativePlacement placement : group.placements()) {
-                    placeVanillaStructure(world, structureManager, random, area, chunkPos,
+                    placeVanillaStructure(boundedWorld, structureManager, random, area, chunkPos,
                             group.structureId(), placement.start(), placement.decision());
                 }
             } catch (Throwable error) {
@@ -913,7 +919,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
         }
         try {
             NativeStructureSurfaceFitter.repairVacuumFoundations(
-                    world, area, vacuumFoundationPlan);
+                    boundedWorld, area, vacuumFoundationPlan);
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "foundation repair", nativeStructureBatchContext(placementGroups),
@@ -938,8 +944,8 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
     private void placeVanillaStructure(WorldGenLevel world, StructureManager structureManager, WorldgenRandom random,
                                        BoundingBox area, ChunkPos chunkPos, String structureId, StructureStart start,
                                        IrisNativeStructureDecision decision) {
-        WorldGenLevel boundedWorld = NativeStructureWorldgenAccess.create(
-                world, chunkPos, worldgenSurfaceHeight(), worldgenFloorHeight());
+        WorldGenLevel boundedWorld = world instanceof NativeStructureWorldgenAccess ? world : NativeStructureWorldgenAccess.create(
+                world, chunkPos, worldgenSurfaceHeight(), worldgenFloorHeight(), position -> false);
         world.setCurrentlyGenerating(() -> "Iris native structure " + structureId);
         try {
             NativeStructurePostProcessor.place(boundedWorld, structureManager, this, random, area, chunkPos,

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/NativeStructureWorldgenAccess.java

@@ -64,6 +64,7 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
     private final ChunkPos generationCenter;
     private final IntBinaryOperator surfaceFirstFreeY;
     private final IntBinaryOperator floorFirstFreeY;
+    private final Predicate<BlockPos> protectedPosition;
     private final Holder<Biome> fallbackBiome;
     private final BiomeManager biomeManager;
     private final AABB generationBounds;
@@ -80,6 +81,8 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
                 boundary.surfaceFirstFreeY(), "Native structure world access requires a surface resolver");
         this.floorFirstFreeY = Objects.requireNonNull(
                 boundary.floorFirstFreeY(), "Native structure world access requires an ocean-floor resolver");
+        this.protectedPosition = Objects.requireNonNull(
+                boundary.protectedPosition(), "World access requires a protected-position predicate");
         BlockPos biomeSample = generationCenter.getMiddleBlockPosition(delegate.getSeaLevel());
         this.fallbackBiome = delegate.getBiome(biomeSample);
         this.biomeManager = delegate.getBiomeManager().withDifferentSource(this);
@@ -99,9 +102,10 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
 
     static NativeStructureWorldgenAccess create(WorldGenLevel delegate, ChunkPos generationCenter,
                                                 IntBinaryOperator surfaceFirstFreeY,
-                                                IntBinaryOperator floorFirstFreeY) {
+                                                IntBinaryOperator floorFirstFreeY,
+                                                Predicate<BlockPos> protectedPosition) {
         return new NativeStructureWorldgenAccess(delegate, new Boundary(
-                generationCenter, surfaceFirstFreeY, floorFirstFreeY));
+                generationCenter, surfaceFirstFreeY, floorFirstFreeY, protectedPosition));
     }
 
     @Override
@@ -359,13 +363,13 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
 
     @Override
     public BlockEntity getBlockEntity(BlockPos position) {
-        return isReadable(position) ? delegate.getBlockEntity(position) : null;
+        return isWritable(position) ? delegate.getBlockEntity(position) : null;
     }
 
     @Override
     public <T extends BlockEntity> Optional<T> getBlockEntity(
             BlockPos position, BlockEntityType<T> type) {
-        return isReadable(position) ? delegate.getBlockEntity(position, type) : Optional.empty();
+        return isWritable(position) ? delegate.getBlockEntity(position, type) : Optional.empty();
     }
 
     @Override
@@ -453,7 +457,7 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
     }
 
     private boolean isWritable(BlockPos position) {
-        return isReadable(position);
+        return isReadable(position) && !protectedPosition.test(position);
     }
 
     private boolean isWritableNeighbourhood(BlockPos position) {
@@ -544,7 +548,8 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
 
     private record Boundary(ChunkPos generationCenter,
                             IntBinaryOperator surfaceFirstFreeY,
-                            IntBinaryOperator floorFirstFreeY) {
+                            IntBinaryOperator floorFirstFreeY,
+                            Predicate<BlockPos> protectedPosition) {
     }
 
     private static final class BoundedTickAccess<T> implements LevelTickAccess<T> {

adapters/bukkit/nms/v26_2_R1/src/test/java/art/arcane/iris/core/nms/v26_2_R1/NativeStructureWorldgenAccessTest.java

@@ -135,6 +135,34 @@ public class NativeStructureWorldgenAccessTest {
         assertEquals(1, recording.blockTicks.count());
     }
 
+    @Test
+    public void protectedCellsRemainReadableButRejectWritesTilesAndTicks() {
+        RecordingDelegate recording = new RecordingDelegate();
+        BlockPos position = generationCenter().getMiddleBlockPosition(70);
+        NativeStructureWorldgenAccess access = NativeStructureWorldgenAccess.create(
+                recording.world(), generationCenter(),
+                (x, z) -> SURFACE_FIRST_FREE_Y,
+                (x, z) -> FLOOR_FIRST_FREE_Y, position::equals);
+
+        assertSame(Blocks.DIRT, access.getBlockState(position).getBlock());
+        assertFalse(access.ensureCanWrite(position));
+        assertFalse(access.setBlock(position, Blocks.STONE.defaultBlockState(), 2));
+        assertFalse(access.removeBlock(position, false));
+        assertFalse(access.destroyBlock(position, false));
+        assertNull(access.getBlockEntity(position));
+        assertTrue(access.getBlockEntity(position, null).isEmpty());
+        access.getBlockTicks().schedule(new ScheduledTick<>(Blocks.DIRT, position, 1L, 0L));
+        access.updateNeighborsAt(position.east(), Blocks.DIRT);
+        access.neighborShapeChanged(
+                Direction.EAST, position, position.east(), Blocks.DIRT.defaultBlockState(), 2, 512);
+
+        assertEquals(1, recording.terrainReads);
+        assertEquals(0, recording.mutations);
+        assertEquals(0, recording.blockTicks.count());
+        assertTrue(access.setBlock(position.above(), Blocks.STONE.defaultBlockState(), 2));
+        assertEquals(1, recording.mutations);
+    }
+
     @Test
     public void statuslessChunkReadsPreserveWorldgenDelegateSemantics() {
         RecordingDelegate recording = new RecordingDelegate();
@@ -183,7 +211,7 @@ public class NativeStructureWorldgenAccessTest {
         return NativeStructureWorldgenAccess.create(
                 recording.world(), generationCenter(),
                 (x, z) -> SURFACE_FIRST_FREE_Y,
-                (x, z) -> FLOOR_FIRST_FREE_Y);
+                (x, z) -> FLOOR_FIRST_FREE_Y, position -> false);
     }
 
     private static ChunkPos generationCenter() {

adapters/bukkit/nms/v26_2_R1/src/test/java/art/arcane/iris/nativegen/NativeStructurePostProcessorEncaseTest.java

@@ -226,7 +226,7 @@ public class NativeStructurePostProcessorEncaseTest {
         String source = Files.readString(Path.of(System.getProperty("iris.nmsChunkGeneratorSource"))).replace("\r\n", "\n");
         int prepareTerrain = source.indexOf("NativeStructurePostProcessor.prepareTerrain(");
         int placementLoop = source.indexOf("for (NativePlacementGroup group : placementGroups)", prepareTerrain);
-        int placement = source.indexOf("placeVanillaStructure(world, structureManager, random", placementLoop);
+        int placement = source.indexOf("placeVanillaStructure(boundedWorld, structureManager, random", placementLoop);
 
         assertTrue(prepareTerrain >= 0);
         assertTrue(placementLoop > prepareTerrain);

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedImportedFeatureStage.java

@@ -25,6 +25,7 @@ import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisBiomeCustom;
 import art.arcane.iris.engine.object.IrisDecorationStep;
 import art.arcane.iris.engine.object.IrisImportedFeatureControl;
+import art.arcane.iris.engine.object.IrisStaticObjectLayer;
 import art.arcane.iris.util.project.context.IrisContext;
 import it.unimi.dsi.fastutil.ints.IntArraySet;
 import it.unimi.dsi.fastutil.ints.IntSet;
@@ -43,6 +44,7 @@ import net.minecraft.world.level.biome.FeatureSorter;
 import net.minecraft.world.level.chunk.ChunkAccess;
 import net.minecraft.world.level.chunk.LevelChunkSection;
 import net.minecraft.world.level.levelgen.RandomSupport;
+import net.minecraft.world.level.levelgen.Heightmap;
 import net.minecraft.world.level.levelgen.WorldgenRandom;
 import net.minecraft.world.level.levelgen.XoroshiroRandomSource;
 import net.minecraft.world.level.levelgen.placement.PlacedFeature;
@@ -321,13 +323,20 @@ final class ModdedImportedFeatureStage {
         WorldgenRandom random = new WorldgenRandom(new XoroshiroRandomSource(RandomSupport.generateUniqueSeed()));
         long decorationSeed = random.setDecorationSeed(level.getSeed(), origin.getX(), origin.getZ());
         Set<Holder<Biome>> chunkBiomes = chunkBiomes(level, sectionPos, table);
+        IrisStaticObjectLayer staticObjects = engine.getDimension().getStaticObjectLayer(engine.getData());
+        int staticMinY = engine.getMinHeight();
+        WorldGenLevel placementLevel = staticObjects.isEmpty() ? level : ModdedNativeStructureWorldgenAccess.create(
+                level, centerPos,
+                (x, z) -> level.getHeight(Heightmap.Types.WORLD_SURFACE_WG, x, z),
+                (x, z) -> level.getHeight(Heightmap.Types.OCEAN_FLOOR_WG, x, z),
+                position -> staticObjects.contains(position.getX(), position.getY() - staticMinY, position.getZ()));
 
         try {
             for (int stepIndex = 0; stepIndex < steps.size(); stepIndex++) {
                 if (!table.control().shouldGenerateStep(IrisDecorationStep.byOrdinal(stepIndex))) {
                     continue;
                 }
-                placeStep(level, table, steps.get(stepIndex), featureRegistry, chunkBiomes, owner,
+                placeStep(placementLevel, table, steps.get(stepIndex), featureRegistry, chunkBiomes, owner,
                         random, decorationSeed, origin, stepIndex);
             }
         } catch (Throwable error) {

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedNativeStructureStage.java

@@ -25,6 +25,7 @@ import art.arcane.iris.engine.framework.NativeStructureOwnershipRecord;
 import art.arcane.iris.engine.framework.NativeStructureStartPlan;
 import art.arcane.iris.engine.object.IrisMaterialPalette;
 import art.arcane.iris.engine.object.IrisNativeStructureDecision;
+import art.arcane.iris.engine.object.IrisStaticObjectLayer;
 import art.arcane.iris.nativegen.NativeStructureGenerationException;
 import art.arcane.iris.nativegen.NativeStructureLocatePersistence;
 import art.arcane.iris.nativegen.NativeStructureLocateResults;
@@ -365,9 +366,14 @@ final class ModdedNativeStructureStage {
                     "heightmap priming", nativeStructureBatchContext(placementGroups),
                     chunkPos.x(), chunkPos.z(), error);
         }
+        IrisStaticObjectLayer staticObjects = current.getDimension().getStaticObjectLayer(current.getData());
+        int staticMinY = current.getMinHeight();
+        WorldGenLevel boundedWorld = staticObjects.isEmpty() ? world : ModdedNativeStructureWorldgenAccess.create(
+                world, chunkPos, worldgenSurfaceHeight(current, world.getMinY()), worldgenFloorHeight(current, world.getMinY()),
+                position -> staticObjects.contains(position.getX(), position.getY() - staticMinY, position.getZ()));
         try {
             NativeStructureVegetationClearer.clearIntersectingVegetation(
-                    world, chunk, area, vegetationTargets);
+                    boundedWorld, chunk, area, vegetationTargets);
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "vegetation cleanup", nativeStructureBatchContext(placementGroups),
@@ -376,7 +382,7 @@ final class ModdedNativeStructureStage {
         NativeStructureSurfaceFitter.VacuumFoundationPlan vacuumFoundationPlan;
         try {
             vacuumFoundationPlan = NativeStructureSurfaceFitter.prepareSurfaceStructures(
-                    world, area, terrainTargets,
+                    boundedWorld, area, terrainTargets,
                     (x, z) -> current.getHeight(x, z, true) + current.getMinHeight());
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
@@ -385,7 +391,7 @@ final class ModdedNativeStructureStage {
         }
         try {
             NativeStructurePostProcessor.prepareTerrain(
-                    world, area, terrainTargets, this::resolvePaletteBlock);
+                    boundedWorld, area, terrainTargets, this::resolvePaletteBlock);
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "terrain preparation", nativeStructureBatchContext(placementGroups),
@@ -395,7 +401,7 @@ final class ModdedNativeStructureStage {
             random.setFeatureSeed(decorationSeed, group.featureIndex(), group.step());
             try {
                 for (NativePlacement placement : group.placements()) {
-                    placeVanillaStructure(world, structureManager, random, area, chunkPos,
+                    placeVanillaStructure(boundedWorld, structureManager, random, area, chunkPos,
                             group.structureId(), placement.start(), placement.decision());
                 }
             } catch (Throwable error) {
@@ -405,7 +411,7 @@ final class ModdedNativeStructureStage {
         }
         try {
             NativeStructureSurfaceFitter.repairVacuumFoundations(
-                    world, area, vacuumFoundationPlan);
+                    boundedWorld, area, vacuumFoundationPlan);
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "foundation repair", nativeStructureBatchContext(placementGroups),
@@ -436,11 +442,8 @@ final class ModdedNativeStructureStage {
                                        String structureId, StructureStart start,
                                        IrisNativeStructureDecision decision) {
         Engine current = generator.engine();
-        int runtimeMinY = world.getMinY();
-        WorldGenLevel boundedWorld = ModdedNativeStructureWorldgenAccess.create(
-                world, chunkPos,
-                worldgenSurfaceHeight(current, runtimeMinY),
-                worldgenFloorHeight(current, runtimeMinY));
+        WorldGenLevel boundedWorld = world instanceof ModdedNativeStructureWorldgenAccess ? world : ModdedNativeStructureWorldgenAccess.create(
+                world, chunkPos, worldgenSurfaceHeight(current, world.getMinY()), worldgenFloorHeight(current, world.getMinY()), position -> false);
         world.setCurrentlyGenerating(() -> "Iris native structure " + structureId);
         try {
             NativeStructurePostProcessor.place(

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedNativeStructureWorldgenAccess.java

@@ -63,6 +63,7 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
     private final ChunkPos generationCenter;
     private final IntBinaryOperator surfaceFirstFreeY;
     private final IntBinaryOperator floorFirstFreeY;
+    private final Predicate<BlockPos> protectedPosition;
     private final Holder<Biome> fallbackBiome;
     private final BiomeManager biomeManager;
     private final AABB generationBounds;
@@ -79,6 +80,8 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
                 boundary.surfaceFirstFreeY(), "Native structure world access requires a surface resolver");
         this.floorFirstFreeY = Objects.requireNonNull(
                 boundary.floorFirstFreeY(), "Native structure world access requires an ocean-floor resolver");
+        this.protectedPosition = Objects.requireNonNull(
+                boundary.protectedPosition(), "World access requires a protected-position predicate");
         BlockPos biomeSample = generationCenter.getMiddleBlockPosition(delegate.getSeaLevel());
         this.fallbackBiome = delegate.getBiome(biomeSample);
         this.biomeManager = delegate.getBiomeManager().withDifferentSource(this);
@@ -98,9 +101,10 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
 
     static ModdedNativeStructureWorldgenAccess create(WorldGenLevel delegate, ChunkPos generationCenter,
                                                 IntBinaryOperator surfaceFirstFreeY,
-                                                IntBinaryOperator floorFirstFreeY) {
+                                                IntBinaryOperator floorFirstFreeY,
+                                                Predicate<BlockPos> protectedPosition) {
         return new ModdedNativeStructureWorldgenAccess(delegate, new Boundary(
-                generationCenter, surfaceFirstFreeY, floorFirstFreeY));
+                generationCenter, surfaceFirstFreeY, floorFirstFreeY, protectedPosition));
     }
 
     @Override
@@ -350,13 +354,13 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
 
     @Override
     public BlockEntity getBlockEntity(BlockPos position) {
-        return isReadable(position) ? delegate.getBlockEntity(position) : null;
+        return isWritable(position) ? delegate.getBlockEntity(position) : null;
     }
 
     @Override
     public <T extends BlockEntity> Optional<T> getBlockEntity(
             BlockPos position, BlockEntityType<T> type) {
-        return isReadable(position) ? delegate.getBlockEntity(position, type) : Optional.empty();
+        return isWritable(position) ? delegate.getBlockEntity(position, type) : Optional.empty();
     }
 
     @Override
@@ -430,7 +434,7 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
     }
 
     private boolean isWritable(BlockPos position) {
-        return isReadable(position);
+        return isReadable(position) && !protectedPosition.test(position);
     }
 
     private boolean isWritableNeighbourhood(BlockPos position) {
@@ -521,7 +525,8 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
 
     private record Boundary(ChunkPos generationCenter,
                             IntBinaryOperator surfaceFirstFreeY,
-                            IntBinaryOperator floorFirstFreeY) {
+                            IntBinaryOperator floorFirstFreeY,
+                            Predicate<BlockPos> protectedPosition) {
     }
 
     private static final class BoundedTickAccess<T> implements LevelTickAccess<T> {

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedNativeStructureWorldgenAccessTest.java

@@ -135,6 +135,34 @@ public class ModdedNativeStructureWorldgenAccessTest {
         assertEquals(1, recording.blockTicks.count());
     }
 
+    @Test
+    public void protectedCellsRemainReadableButRejectWritesTilesAndTicks() {
+        RecordingDelegate recording = new RecordingDelegate();
+        BlockPos position = generationCenter().getMiddleBlockPosition(70);
+        ModdedNativeStructureWorldgenAccess access = ModdedNativeStructureWorldgenAccess.create(
+                recording.world(), generationCenter(),
+                (x, z) -> SURFACE_FIRST_FREE_Y,
+                (x, z) -> FLOOR_FIRST_FREE_Y, position::equals);
+
+        assertSame(Blocks.DIRT, access.getBlockState(position).getBlock());
+        assertFalse(access.ensureCanWrite(position));
+        assertFalse(access.setBlock(position, Blocks.STONE.defaultBlockState(), 2));
+        assertFalse(access.removeBlock(position, false));
+        assertFalse(access.destroyBlock(position, false));
+        assertNull(access.getBlockEntity(position));
+        assertTrue(access.getBlockEntity(position, null).isEmpty());
+        access.getBlockTicks().schedule(new ScheduledTick<>(Blocks.DIRT, position, 1L, 0L));
+        access.updateNeighborsAt(position.east(), Blocks.DIRT);
+        access.neighborShapeChanged(
+                Direction.EAST, position, position.east(), Blocks.DIRT.defaultBlockState(), 2, 512);
+
+        assertEquals(1, recording.terrainReads);
+        assertEquals(0, recording.mutations);
+        assertEquals(0, recording.blockTicks.count());
+        assertTrue(access.setBlock(position.above(), Blocks.STONE.defaultBlockState(), 2));
+        assertEquals(1, recording.mutations);
+    }
+
     @Test
     public void statuslessChunkReadsPreserveWorldgenDelegateSemantics() {
         RecordingDelegate recording = new RecordingDelegate();
@@ -183,7 +211,7 @@ public class ModdedNativeStructureWorldgenAccessTest {
         return ModdedNativeStructureWorldgenAccess.create(
                 recording.world(), generationCenter(),
                 (x, z) -> SURFACE_FIRST_FREE_Y,
-                (x, z) -> FLOOR_FIRST_FREE_Y);
+                (x, z) -> FLOOR_FIRST_FREE_Y, position -> false);
     }
 
     private static ChunkPos generationCenter() {

adapters/modded-common/src/test/java/art/arcane/iris/modded/NativeStructureFailureContractTest.java

@@ -103,12 +103,12 @@ public class NativeStructureFailureContractTest {
     @Test
     public void nativeStructurePostProcessingUsesBoundedWorldgenAccess() throws IOException {
         String source = moddedSource("ModdedNativeStructureStage.java");
-        int placementStart = source.indexOf("private void placeVanillaStructure");
+        int placementStart = source.indexOf("void placeVanillaStructures");
         int placementEnd = source.indexOf("private List<List<Structure>> structuresByStep", placementStart);
         String placement = source.substring(placementStart, placementEnd);
 
         assertTrue(placement.contains("ModdedNativeStructureWorldgenAccess.create("));
-        assertTrue(placement.contains("NativeStructurePostProcessor.place(\n                    boundedWorld"));
+        assertTrue(placement.contains("placeVanillaStructure(boundedWorld, structureManager"));
         assertFalse(placement.contains("NativeStructurePostProcessor.place(world,"));
         assertTrue(placement.contains("world.setCurrentlyGenerating(null);"));
     }