Limited borrowed types to modded

- Pack validation rejects borrowedDimensionType on platforms that generate their own Iris dimension type, with an error naming the supported loaders
- Dimension type keys and registry contracts honor borrowedDimensionType only when the platform generation registry supports borrowed types, so Bukkit always derives its generated iris type
- The Bukkit runtime contract always expects the generated Iris type; hotload still refuses a borrowedDimensionType change

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

@@ -46,6 +46,11 @@ final class ModdedGenerationRegistry implements PlatformGenerationRegistry {
         );
     }
 
+    @Override
+    public boolean supportsBorrowedDimensionTypes() {
+        return true;
+    }
+
     @Override
     public Definition definition(String registryKey, String resourceKey) {
         return nativeRegistry.definition(registryKey, resourceKey);

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

@@ -80,6 +80,11 @@ public final class ModdedPlatform implements IrisPlatform {
         return loader.platformName();
     }
 
+    @Override
+    public boolean supportsBorrowedDimensionTypes() {
+        return true;
+    }
+
     @Override
     public String minecraftVersion() {
         return loader.minecraftVersion();

core/src/main/java/art/arcane/iris/generation/terrain/IrisDimension.java

@@ -1112,8 +1112,12 @@ public class IrisDimension extends IrisRegistrant {
         return borrowedDimensionType != null && !borrowedDimensionType.isBlank();
     }
 
+    public boolean borrowsDimensionType(PlatformGenerationRegistry registry) {
+        return hasBorrowedDimensionType() && registry.supportsBorrowedDimensionTypes();
+    }
+
     public String dimensionTypeResourceKey(PlatformGenerationRegistry registry, String packName) {
-        if (hasBorrowedDimensionType()) {
+        if (borrowsDimensionType(registry)) {
             return borrowedDimensionType.trim();
         }
         return registry.dimensionTypeResourceKey(packName, getLoadKey(), getDimensionTypeKey());

core/src/main/java/art/arcane/iris/generation/terrain/IrisDimensionRuntimeContract.java

@@ -46,9 +46,7 @@ public record IrisDimensionRuntimeContract(
         Objects.requireNonNull(dimension, "dimension");
         Objects.requireNonNull(namespace, "namespace");
         return new IrisDimensionRuntimeContract(
-                dimension.hasBorrowedDimensionType()
+                namespace + ":" + dimension.getDimensionTypeKey(),
-                        ? dimension.getBorrowedDimensionType().trim()
-                        : namespace + ":" + dimension.getDimensionTypeKey(),
                 dimension.getMinHeight(),
                 dimension.getMaxHeight() - dimension.getMinHeight(),
                 dimension.getLogicalHeight());
@@ -63,6 +61,7 @@ public record IrisDimensionRuntimeContract(
         IrisDimensionRuntimeContract actual = expected(active, namespace);
         IrisDimensionRuntimeContract proposed = expected(replacement, namespace);
         proposed.requireExact(runtimeName, actual);
+        requireBorrowedTypeUnchanged(runtimeName, active, replacement);
         requireGeneratedTypeCompatible(runtimeName, active, replacement);
     }
 
@@ -120,6 +119,25 @@ public record IrisDimensionRuntimeContract(
                 + ". Generation was refused before any chunk writes. Restart after installing the exact Iris dimension type; terrain clipping is not allowed.");
     }
 
+    private static void requireBorrowedTypeUnchanged(
+            String runtimeName,
+            IrisDimension active,
+            IrisDimension replacement
+    ) {
+        String activeBorrowed = borrowedType(active);
+        String replacementBorrowed = borrowedType(replacement);
+        if (activeBorrowed.equals(replacementBorrowed)) {
+            return;
+        }
+        throw new IrisDimensionContractException(runtimeName + " cannot hotload a changed borrowedDimensionType. The active pack borrows '"
+                + activeBorrowed + "' and the replacement borrows '" + replacementBorrowed
+                + "'. Close and reopen Studio after changing borrowedDimensionType.");
+    }
+
+    private static String borrowedType(IrisDimension dimension) {
+        return dimension.hasBorrowedDimensionType() ? dimension.getBorrowedDimensionType().trim() : "";
+    }
+
     private static void requireGeneratedTypeCompatible(
             String runtimeName,
             IrisDimension active,

core/src/main/java/art/arcane/iris/pack/PackDimensionValidator.java

@@ -19,6 +19,7 @@
 package art.arcane.iris.pack;
 
 import art.arcane.iris.generation.terrain.IrisDimensionType;
+import art.arcane.iris.spi.IrisPlatforms;
 import art.arcane.iris.structure.object.IrisObjectScale;
 import art.arcane.iris.world.IrisWorldBoundary;
 import art.arcane.volmlib.util.json.JSONArray;
@@ -60,7 +61,7 @@ final class PackDimensionValidator {
 
             validateImportedStructurePolicy(dimensionKey, dimJson, blockingErrors, warnings);
             validateDimensionHeights(packFolder, dimensionKey, dimJson, blockingErrors);
-            validateBorrowedDimensionType(dimensionKey, dimJson, blockingErrors);
+            validateBorrowedDimensionType(dimensionKey, dimJson, borrowedDimensionTypesSupported(), blockingErrors);
             validateSamplingStep(dimensionKey, dimJson, "terrainSamplingStep", 1, blockingErrors);
             validateSamplingStep(dimensionKey, dimJson, "caveDensitySamplingStep", 1, blockingErrors);
             validateSamplingStep(dimensionKey, dimJson, "biomeBoundsSamplingStep", 4, blockingErrors);
@@ -128,16 +129,29 @@ final class PackDimensionValidator {
         }
     }
 
-    static void validateBorrowedDimensionType(String dimensionKey, JSONObject dimension, List<String> errors) {
+    static void validateBorrowedDimensionType(String dimensionKey, JSONObject dimension, boolean borrowingSupported,
+                                              List<String> errors) {
         if (!dimension.has(BORROWED_DIMENSION_TYPE) || dimension.isNull(BORROWED_DIMENSION_TYPE)) {
             return;
         }
         Object value = dimension.opt(BORROWED_DIMENSION_TYPE);
-        if (value instanceof String key && (key.isEmpty() || RESOURCE_KEY.matcher(key).matches())) {
+        if (value instanceof String key && key.isEmpty()) {
             return;
         }
-        errors.add("Dimension '" + dimensionKey + "' " + BORROWED_DIMENSION_TYPE
+        if (!(value instanceof String key) || !RESOURCE_KEY.matcher(key).matches()) {
-                + " must be a namespaced dimension type key such as aether:the_aether, but was '" + value + "'.");
+            errors.add("Dimension '" + dimensionKey + "' " + BORROWED_DIMENSION_TYPE
+                    + " must be a namespaced dimension type key such as aether:the_aether, but was '" + value + "'.");
+            return;
+        }
+        if (!borrowingSupported) {
+            errors.add("Dimension '" + dimensionKey + "' sets " + BORROWED_DIMENSION_TYPE + " to '" + key
+                    + "', which only Fabric, Forge, and NeoForge servers support. This server generates its own Iris dimension type, so remove "
+                    + BORROWED_DIMENSION_TYPE + " to use this pack here.");
+        }
+    }
+
+    private static boolean borrowedDimensionTypesSupported() {
+        return !IrisPlatforms.isBound() || IrisPlatforms.get().supportsBorrowedDimensionTypes();
     }
 
     static void validateObjectScaleFactor(String dimensionKey, JSONObject dimension, List<String> errors) {

core/src/main/java/art/arcane/iris/world/history/GenerationRegistryContractFactory.java

@@ -175,7 +175,7 @@ public final class GenerationRegistryContractFactory {
         List<IrisBiome> reachableBiomes = new ArrayList<>(requiredDimension.getReachableBiomes(() -> requiredData));
         for (IrisDimension registryDimension : registryDimensions(requiredData, requiredDimension)) {
             String dimensionKey = requireText(registryDimension.getLoadKey(), "Dimension load key");
-            if (!registryDimension.hasBorrowedDimensionType()) {
+            if (!registryDimension.borrowsDimensionType(requiredGenerationRegistry)) {
                 addGeneratedDimensionType(
                         definitions,
                         generatedSources,

core/src/test/java/art/arcane/iris/generation/terrain/IrisDimensionRuntimeContractTest.java

@@ -139,16 +139,15 @@ public class IrisDimensionRuntimeContractTest {
     }
 
     @Test
-    public void expectedContractUsesTheBorrowedTypeKey() {
+    public void expectedContractIsTheGeneratedIrisTypeEvenWhenThePackBorrowsOne() {
         IrisDimension dimension = borrowedDimension("aether:the_aether", 0, 256);
 
-        assertEquals("aether:the_aether", IrisDimensionRuntimeContract.expected(dimension, "iris").typeKey());
+        assertEquals("iris:skylands", IrisDimensionRuntimeContract.expected(dimension, "iris").typeKey());
     }
 
     @Test
     public void borrowedTypeAcceptsAPackRangeInsideTheBorrowedRange() {
-        IrisDimensionRuntimeContract expected = IrisDimensionRuntimeContract.expected(
+        IrisDimensionRuntimeContract expected = borrowedContract("aether:the_aether", 0, 192);
-                borrowedDimension("aether:the_aether", 0, 192), "iris");
 
         expected.requireBorrowedFit("Modded level 'aether:the_aether'",
                 new IrisDimensionRuntimeContract("aether:the_aether", 0, 256, 256));
@@ -158,8 +157,7 @@ public class IrisDimensionRuntimeContractTest {
 
     @Test
     public void borrowedTypeRejectsAPackTopAboveTheBorrowedTop() {
-        IrisDimensionRuntimeContract expected = IrisDimensionRuntimeContract.expected(
+        IrisDimensionRuntimeContract expected = borrowedContract("aether:the_aether", 0, 320);
-                borrowedDimension("aether:the_aether", 0, 320), "iris");
 
         assertBorrowedFitRejected(expected, new IrisDimensionRuntimeContract("aether:the_aether", 0, 256, 256),
                 "0..320", "0..256");
@@ -167,8 +165,7 @@ public class IrisDimensionRuntimeContractTest {
 
     @Test
     public void borrowedTypeRejectsAPackMinimumThatDiffersFromTheBorrowedMinimum() {
-        IrisDimensionRuntimeContract expected = IrisDimensionRuntimeContract.expected(
+        IrisDimensionRuntimeContract expected = borrowedContract("aether:the_aether", 16, 256);
-                borrowedDimension("aether:the_aether", 16, 256), "iris");
 
         assertBorrowedFitRejected(expected, new IrisDimensionRuntimeContract("aether:the_aether", 0, 256, 256),
                 "16..256", "0..256");
@@ -176,8 +173,7 @@ public class IrisDimensionRuntimeContractTest {
 
     @Test
     public void borrowedTypeRejectsALevelUsingAnotherType() {
-        IrisDimensionRuntimeContract expected = IrisDimensionRuntimeContract.expected(
+        IrisDimensionRuntimeContract expected = borrowedContract("aether:the_aether", 0, 256);
-                borrowedDimension("aether:the_aether", 0, 256), "iris");
 
         assertBorrowedFitRejected(expected, new IrisDimensionRuntimeContract("minecraft:overworld", 0, 256, 256),
                 "aether:the_aether", "minecraft:overworld");
@@ -192,6 +188,10 @@ public class IrisDimensionRuntimeContractTest {
         assertHotloadRejected(active, replacement, "aether:the_aether");
     }
 
+    private static IrisDimensionRuntimeContract borrowedContract(String typeKey, int minHeight, int maxHeight) {
+        return new IrisDimensionRuntimeContract(typeKey, minHeight, maxHeight - minHeight, maxHeight - minHeight);
+    }
+
     private IrisDimension borrowedDimension(String typeKey, int minHeight, int maxHeight) {
         IrisDimension dimension = new IrisDimension();
         dimension.setLoadKey("skylands");

core/src/test/java/art/arcane/iris/pack/PackValidatorBorrowedDimensionTypeTest.java

@@ -46,9 +46,23 @@ public class PackValidatorBorrowedDimensionTypeTest {
                 validate("{\"borrowedDimensionType\":42}"));
     }
 
+    @Test
+    public void rejectsBorrowedTypesOnPlatformsThatGenerateTheirOwnType() {
+        assertTrue(validate("{\"borrowedDimensionType\":\"\"}", false).isEmpty());
+        List<String> errors = validate("{\"borrowedDimensionType\":\"aether:the_aether\"}", false);
+
+        assertEquals(1, errors.size());
+        assertTrue(errors.getFirst(), errors.getFirst().contains("borrowedDimensionType"));
+        assertTrue(errors.getFirst(), errors.getFirst().contains("Fabric, Forge, and NeoForge"));
+    }
+
     private static List<String> validate(String json) {
+        return validate(json, true);
+    }
+
+    private static List<String> validate(String json, boolean borrowingSupported) {
         List<String> errors = new ArrayList<>();
-        PackDimensionValidator.validateBorrowedDimensionType("main", new JSONObject(json), errors);
+        PackDimensionValidator.validateBorrowedDimensionType("main", new JSONObject(json), borrowingSupported, errors);
         return errors;
     }
 }

core/src/test/java/art/arcane/iris/world/history/GenerationRegistryContractFactoryTest.java

@@ -167,7 +167,7 @@ public class GenerationRegistryContractFactoryTest {
         PlatformRegistries registries = mock(PlatformRegistries.class);
         when(registries.blockTypeKeys()).thenReturn(new KList<>());
         when(registries.entityKeys()).thenReturn(new KList<>());
-        FakeGenerationRegistry generationRegistry = new FakeGenerationRegistry("test-runtime");
+        FakeGenerationRegistry generationRegistry = new BorrowingGenerationRegistry("test-runtime");
 
         GenerationRegistryContract contract = GenerationRegistryContractFactory.create(
                 data,
@@ -187,6 +187,38 @@ public class GenerationRegistryContractFactoryTest {
         assertEquals("aether:the_aether", dimension.dimensionTypeResourceKey(generationRegistry, "borrowed-pack"));
     }
 
+    @Test
+    public void platformWithoutBorrowedTypesKeepsTheGeneratedDimensionType() throws Exception {
+        File pack = temporary.newFolder("borrowed-pack-generated");
+        IrisData data = mock(IrisData.class);
+        when(data.getDataFolder()).thenReturn(pack);
+        when(data.getContentGate()).thenReturn(mock(ContentGate.class));
+        IrisDimension dimension = spy(new IrisDimension().setBorrowedDimensionType("aether:the_aether"));
+        dimension.setLoadKey("skylands");
+        stubDimensions(data, dimension);
+        doReturn(new KList<>()).when(dimension).getReachableBiomes(any());
+        doReturn(new KList<>()).when(dimension).getAllRegions(any());
+        PlatformRegistries registries = mock(PlatformRegistries.class);
+        when(registries.blockTypeKeys()).thenReturn(new KList<>());
+        when(registries.entityKeys()).thenReturn(new KList<>());
+        FakeGenerationRegistry generationRegistry = new FakeGenerationRegistry("test-runtime");
+
+        GenerationRegistryContract contract = GenerationRegistryContractFactory.create(
+                data,
+                dimension,
+                GenerationPackFingerprint.compute(pack.toPath(), GenerationPackFingerprint.CURRENT_VERSION),
+                DataVersion.getLatest().get(),
+                registries,
+                generationRegistry
+        );
+
+        assertFalse(contract.definitions().containsKey(key(
+                GenerationRegistryContractFactory.DIMENSION_TYPE_REGISTRY, "aether:the_aether")));
+        assertTrue(contract.definitions().containsKey(key(
+                GenerationRegistryContractFactory.DIMENSION_TYPE_REGISTRY, "test:skylands_type")));
+        assertEquals("test:skylands_type", dimension.dimensionTypeResourceKey(generationRegistry, "borrowed-pack-generated"));
+    }
+
     @Test
     public void duplicateLogicalBiomeIdsKeepExactDefinitionsAndChooseStableLegacyAlias() throws Exception {
         File pack = temporary.newFolder("duplicate-biomes");
@@ -956,6 +988,17 @@ public class GenerationRegistryContractFactoryTest {
         }
     }
 
+    private static final class BorrowingGenerationRegistry extends FakeGenerationRegistry {
+        private BorrowingGenerationRegistry(String runtimeIdentity) {
+            super(runtimeIdentity);
+        }
+
+        @Override
+        public boolean supportsBorrowedDimensionTypes() {
+            return true;
+        }
+    }
+
     private static class FakeGenerationRegistry implements PlatformGenerationRegistry {
         private final String runtimeIdentity;
         private final Map<GenerationRegistryContract.PhysicalResourceKey, Definition> definitions =

spi/src/main/java/art/arcane/iris/spi/IrisPlatform.java

@@ -76,6 +76,10 @@ public interface IrisPlatform {
         return false;
     }
 
+    default boolean supportsBorrowedDimensionTypes() {
+        return false;
+    }
+
     /**
      * Root folder Iris owns for packs, settings and generated data. Created if missing. Never null.
      */

spi/src/main/java/art/arcane/iris/spi/PlatformGenerationRegistry.java

@@ -25,6 +25,10 @@ public interface PlatformGenerationRegistry extends NativeGenerationRegistry {
 
     String dimensionTypeResourceKey(String packName, String dimensionKey, String dimensionTypeKey);
 
+    default boolean supportsBorrowedDimensionTypes() {
+        return false;
+    }
+
     static String contentAddressedCustomBiomeResourceKey(String identitySha256) {
         String requiredFingerprint = Objects.requireNonNull(identitySha256, "identitySha256");
         if (!SHA_256_PATTERN.matcher(requiredFingerprint).matches()) {