JVM IR: write inherited multifile parts flag to kotlin.Metadata

This flag is unused at the moment, but might be used one day to support
proper incremental compilation for multifile classes.

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/codegen/FirBlackBoxCodegenTestGenerated.java

@@ -24010,6 +24010,12 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/multifileClasses/kt16077.kt");
         }
 
+        @Test
+        @TestMetadata("metadataFlag.kt")
+        public void testMetadataFlag() throws Exception {
+            runTest("compiler/testData/codegen/box/multifileClasses/metadataFlag.kt");
+        }
+
         @Test
         @TestMetadata("multifileClassPartsInitialization.kt")
         public void testMultifileClassPartsInitialization() throws Exception {

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/ClassCodegen.kt

@@ -10,8 +10,12 @@ import org.jetbrains.kotlin.backend.jvm.JvmLoweredDeclarationOrigin
 import org.jetbrains.kotlin.backend.jvm.lower.MultifileFacadeFileEntry
 import org.jetbrains.kotlin.backend.jvm.lower.buildAssertionsDisabledField
 import org.jetbrains.kotlin.backend.jvm.lower.hasAssertionsDisabledField
-import org.jetbrains.kotlin.codegen.*
+import org.jetbrains.kotlin.codegen.DescriptorAsmUtil
+import org.jetbrains.kotlin.codegen.VersionIndependentOpcodes
+import org.jetbrains.kotlin.codegen.addRecordComponent
 import org.jetbrains.kotlin.codegen.inline.*
+import org.jetbrains.kotlin.codegen.writeKotlinMetadata
+import org.jetbrains.kotlin.config.JvmAnalysisFlags
 import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.DescriptorVisibilities
@@ -194,9 +198,6 @@ class ClassCodegen private constructor(
         )
 
     private fun generateKotlinMetadataAnnotation() {
-        // TODO: if `-Xmultifile-parts-inherit` is enabled, write the corresponding flag for parts and facades to [Metadata.extraInt].
-        val extraFlags = context.backendExtension.generateMetadataExtraFlags(state.abiStability)
-
         val facadeClassName = context.multifileFacadeForPart[irClass.attributeOwnerId]
         val metadata = irClass.metadata
         val entry = irClass.fileParent.fileEntry
@@ -208,6 +209,14 @@ class ClassCodegen private constructor(
             entry is MultifileFacadeFileEntry -> KotlinClassHeader.Kind.MULTIFILE_CLASS
             else -> KotlinClassHeader.Kind.SYNTHETIC_CLASS
         }
+
+        val isMultifileClassOrPart = kind == KotlinClassHeader.Kind.MULTIFILE_CLASS || kind == KotlinClassHeader.Kind.MULTIFILE_CLASS_PART
+
+        var extraFlags = context.backendExtension.generateMetadataExtraFlags(state.abiStability)
+        if (isMultifileClassOrPart && state.languageVersionSettings.getFlag(JvmAnalysisFlags.inheritMultifileParts)) {
+            extraFlags = extraFlags or JvmAnnotationNames.METADATA_MULTIFILE_PARTS_INHERIT_FLAG
+        }
+
         writeKotlinMetadata(visitor, state, kind, extraFlags) {
             if (metadata != null) {
                 metadataSerializer.serialize(metadata)?.let { (proto, stringTable) ->
@@ -229,9 +238,7 @@ class ClassCodegen private constructor(
             }
 
             if (irClass in context.classNameOverride) {
-                val isFileClass = kind == KotlinClassHeader.Kind.MULTIFILE_CLASS ||
+                val isFileClass = isMultifileClassOrPart || kind == KotlinClassHeader.Kind.FILE_FACADE
-                        kind == KotlinClassHeader.Kind.MULTIFILE_CLASS_PART ||
-                        kind == KotlinClassHeader.Kind.FILE_FACADE
                 assert(isFileClass) { "JvmPackageName is not supported for classes: ${irClass.render()}" }
                 it.visit(JvmAnnotationNames.METADATA_PACKAGE_NAME_FIELD_NAME, irClass.fqNameWhenAvailable!!.parent().asString())
             }

compiler/testData/codegen/box/multifileClasses/metadataFlag.kt

@@ -0,0 +1,50 @@
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+// FIR reports unresolved reference on `extraInt`.
+// IGNORE_BACKEND_FIR: JVM_IR
+
+// MODULE: optimized
+// !INHERIT_MULTIFILE_PARTS
+// FILE: optimized.kt
+
+@file:JvmMultifileClass
+@file:JvmName("Optimized")
+package optimized
+
+fun test() {}
+
+class RandomClass
+
+// MODULE: main(optimized)
+// FILE: default.kt
+
+@file:JvmMultifileClass
+@file:JvmName("Default")
+package default
+
+fun test() {}
+
+// FILE: box.kt
+
+fun isFlagSet(className: String): Boolean {
+    val extraInt = Class.forName(className).getAnnotation(Metadata::class.java).extraInt
+    return (extraInt and (1 shl 0)) != 0
+}
+
+fun box(): String {
+    if (isFlagSet("default.Default"))
+        return "Fail: inherited multifile parts flag should NOT be set by default for the facade"
+    if (isFlagSet("default.Default__DefaultKt"))
+        return "Fail: inherited multifile parts flag should NOT be set by default for the part"
+
+    if (!isFlagSet("optimized.Optimized"))
+        return "Fail: inherited multifile parts flag SHOULD be set by default for the facade"
+    if (!isFlagSet("optimized.Optimized__OptimizedKt"))
+        return "Fail: inherited multifile parts flag SHOULD be set by default for the part"
+
+    if (isFlagSet("optimized.RandomClass"))
+        return "Fail: inherited multifile parts flag should NOT be set by default for some random class if -Xmultifile-part-inherit is enabled"
+
+    return "OK"
+}

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/codegen/BlackBoxCodegenTestGenerated.java

@@ -24010,6 +24010,12 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/multifileClasses/kt16077.kt");
         }
 
+        @Test
+        @TestMetadata("metadataFlag.kt")
+        public void testMetadataFlag() throws Exception {
+            runTest("compiler/testData/codegen/box/multifileClasses/metadataFlag.kt");
+        }
+
         @Test
         @TestMetadata("multifileClassPartsInitialization.kt")
         public void testMultifileClassPartsInitialization() throws Exception {

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/codegen/IrBlackBoxCodegenTestGenerated.java

@@ -24010,6 +24010,12 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/multifileClasses/kt16077.kt");
         }
 
+        @Test
+        @TestMetadata("metadataFlag.kt")
+        public void testMetadataFlag() throws Exception {
+            runTest("compiler/testData/codegen/box/multifileClasses/metadataFlag.kt");
+        }
+
         @Test
         @TestMetadata("multifileClassPartsInitialization.kt")
         public void testMultifileClassPartsInitialization() throws Exception {

compiler/tests-gen/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -20367,6 +20367,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/multifileClasses/kt16077.kt");
         }
 
+        @TestMetadata("metadataFlag.kt")
+        public void testMetadataFlag() throws Exception {
+            runTest("compiler/testData/codegen/box/multifileClasses/metadataFlag.kt");
+        }
+
         @TestMetadata("multifileClassPartsInitialization.kt")
         public void testMultifileClassPartsInitialization() throws Exception {
             runTest("compiler/testData/codegen/box/multifileClasses/multifileClassPartsInitialization.kt");