JVM IR: generate correct metadata for multi-file classes

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/JvmBackendContext.kt

@@ -65,6 +65,7 @@ class JvmBackendContext(
     internal val localDelegatedProperties = mutableMapOf<IrClass, List<IrLocalDelegatedPropertySymbol>>()
 
     internal val multifileFacadesToAdd = mutableMapOf<JvmClassName, MutableList<IrClass>>()
+    internal val multifileFacadeForPart = mutableMapOf<IrClass, JvmClassName>()
 
     override var inVerbosePhase: Boolean = false
 

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

@@ -22,6 +22,7 @@ import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
 import org.jetbrains.kotlin.ir.types.IrSimpleType
 import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.load.java.JvmAbi
+import org.jetbrains.kotlin.load.java.JvmAnnotationNames
 import org.jetbrains.kotlin.load.kotlin.header.KotlinClassHeader
 import org.jetbrains.kotlin.name.SpecialNames
 import org.jetbrains.kotlin.resolve.jvm.AsmTypes
@@ -144,13 +145,31 @@ open class ClassCodegen protected constructor(
 
                 serializerExtension.serializeJvmPackage(packageProto, type)
 
-                writeKotlinMetadata(visitor, state, KotlinClassHeader.Kind.FILE_FACADE, 0) {
-                    AsmUtil.writeAnnotationData(it, serializer, packageProto.build())
+                val facadeClassName = context.multifileFacadeForPart[irClass.attributeOwnerId]
+                val kind = if (facadeClassName != null) KotlinClassHeader.Kind.MULTIFILE_CLASS_PART else KotlinClassHeader.Kind.FILE_FACADE
+                writeKotlinMetadata(visitor, state, kind, 0) { av ->
+                    AsmUtil.writeAnnotationData(av, serializer, packageProto.build())
+
+                    if (facadeClassName != null) {
+                        av.visit(JvmAnnotationNames.METADATA_MULTIFILE_CLASS_NAME_FIELD_NAME, facadeClassName.internalName)
+                    }
+
                     // TODO: JvmPackageName
                 }
             }
             else -> {
-                writeSyntheticClassMetadata(visitor, state)
+                val entry = irClass.fileParent.fileEntry
+                if (entry is MultifileFacadeFileEntry) {
+                    val partInternalNames = entry.partFiles.mapNotNull { partFile ->
+                        val fileClass = partFile.declarations.singleOrNull { it.origin == IrDeclarationOrigin.FILE_CLASS } as IrClass?
+                        if (fileClass != null) typeMapper.mapClass(fileClass).internalName else null
+                    }
+                    MultifileClassCodegenImpl.writeMetadata(
+                        visitor, state, 0 /* TODO */, partInternalNames, type, irClass.fqNameWhenAvailable!!.parent()
+                    )
+                } else {
+                    writeSyntheticClassMetadata(visitor, state)
+                }
             }
         }
     }

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/GenerateMultifileFacades.kt

@@ -85,6 +85,8 @@ private fun generateMultifileFacades(module: ModuleDescriptor, context: JvmBacke
         file.declarations.add(facadeClass)
 
         for (partClass in partClasses) {
+            context.multifileFacadeForPart[partClass.attributeOwnerId as IrClass] = jvmClassName
+
             for (member in partClass.declarations) {
                 member.createMultifileDelegateIfNeeded(context, facadeClass)
             }