Make CLASS default class kind in binary metadata

Don't write the kind to the class file if it's CLASS to save some bytes

compiler/backend/src/org/jetbrains/kotlin/codegen/ImplementationBodyCodegen.java

@@ -239,13 +239,20 @@ public class ImplementationBodyCodegen extends ClassBodyCodegen {
             kind = KotlinClass.Kind.ANONYMOUS_OBJECT;
         }
         else if (isTopLevelOrInnerClass(descriptor)) {
-            kind = KotlinClass.Kind.CLASS;
+            // Default value is Kind.CLASS
+            kind = null;
         }
         else {
             // LOCAL_CLASS is also written to inner classes of local classes
             kind = KotlinClass.Kind.LOCAL_CLASS;
         }
 
+        // Temporarily write class kind anyway because old compiler may not expect its absence
+        // TODO: remove after M13
+        if (kind == null) {
+            kind = KotlinClass.Kind.CLASS;
+        }
+
         DescriptorSerializer serializer =
                 DescriptorSerializer.create(descriptor, new JvmSerializerExtension(v.getSerializationBindings(), typeMapper));
 
@@ -257,11 +264,14 @@ public class ImplementationBodyCodegen extends ClassBodyCodegen {
 
         AnnotationVisitor av = v.getVisitor().visitAnnotation(asmDescByFqNameWithoutInnerClasses(JvmAnnotationNames.KOTLIN_CLASS), true);
         av.visit(JvmAnnotationNames.ABI_VERSION_FIELD_NAME, JvmAbi.VERSION);
-        av.visitEnum(
+        //noinspection ConstantConditions
-                JvmAnnotationNames.KIND_FIELD_NAME,
+        if (kind != null) {
-                Type.getObjectType(KotlinClass.KIND_INTERNAL_NAME).getDescriptor(),
+            av.visitEnum(
-                kind.toString()
+                    JvmAnnotationNames.KIND_FIELD_NAME,
-        );
+                    Type.getObjectType(KotlinClass.KIND_INTERNAL_NAME).getDescriptor(),
+                    kind.toString()
+            );
+        }
         AnnotationVisitor array = av.visitArray(JvmAnnotationNames.DATA_FIELD_NAME);
         for (String string : BitEncoding.encodeBytes(SerializationUtil.serializeClassData(data))) {
             array.visit(null, string);

core/descriptor.loader.java/src/org/jetbrains/kotlin/load/kotlin/header/ReadKotlinClassHeaderAnnotationVisitor.java

@@ -68,6 +68,11 @@ public class ReadKotlinClassHeaderAnnotationVisitor implements AnnotationVisitor
             return null;
         }
 
+        if (headerKind == CLASS && classKind == null) {
+            // Default class kind is Kind.CLASS
+            classKind = KotlinClass.Kind.CLASS;
+        }
+
         if (!AbiVersionUtil.isAbiVersionCompatible(version)) {
             return new KotlinClassHeader(headerKind, version, null, classKind, syntheticClassKind);
         }

core/runtime.jvm/src/kotlin/jvm/internal/KotlinClass.java

@@ -23,7 +23,7 @@ import java.lang.annotation.RetentionPolicy;
 public @interface KotlinClass {
     int abiVersion();
 
-    Kind kind();
+    Kind kind() default Kind.CLASS;
 
     String[] data();
 
@@ -31,7 +31,7 @@ public @interface KotlinClass {
         CLASS,
 
         /**
-         * A class has this kind if and only if its first non-class container is not a package.
+         * A class has kind LOCAL_CLASS if and only if it's not an anonymous object and its first non-class container is not a package.
          */
         LOCAL_CLASS,