Add new compiler errors and flags when JVM compiles against JVM IR

From now on, the old JVM backend will report an error by default when compiling against class files produced by the JVM IR backend. This is needed because we're not yet sure that the ABI generated by JVM IR is fully correct and do not want to land in a 2-dimensional compatibility situation where we'll need to consider twice more scenarios when introducing any breaking change in the language. This is generally OK since the JVM IR backend is still going to be experimental in 1.4. However, for purposes of users which _do_ need to compile something with the old backend against JVM IR, we provide two new compiler flags: * -Xallow-jvm-ir-dependencies -- allows to suppress the error when compiling with the old backend against JVM IR. * -Xir-binary-with-stable-api -- allows to mark the generated binaries as stable, when compiling anything with JVM IR, so that dependent modules will compile even with the old backend automatically. In this case, the author usually does not care for the generated ABI, or s/he ensures that it's consistent with the one expected by the old compiler with some external tools. Internally, this is implemented by storing two new flags in kotlin.Metadata: one tells if the class file was compiled with the JVM IR, and another tells if the class file is stable (in case it's compiled with JVM IR). Implementation is similar to the diagnostic reported by the pre-release dependency checker.
2020-01-15 19:27:39 +01:00
parent f262f61096
commit 953b461c53
37 changed files with 177 additions and 20 deletions
@@ -264,6 +264,7 @@ class GenerationState private constructor(
    val disableOptimization = configuration.get(JVMConfigurationKeys.DISABLE_OPTIMIZATION, false)

    val metadataVersion = configuration.get(CommonConfigurationKeys.METADATA_VERSION) ?: JvmMetadataVersion.INSTANCE
+    val isIrWithStableAbi = configuration.getBoolean(JVMConfigurationKeys.IS_IR_WITH_STABLE_ABI)

    val globalSerializationBindings = JvmSerializationBindings()
    lateinit var irBasedMapAsmMethod: (FunctionDescriptor) -> Method
@@ -93,6 +93,20 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
    )
    var irCheckLocalNames: Boolean by FreezableVar(false)

+    @Argument(
+        value = "-Xallow-jvm-ir-dependencies",
+        description = "When not using the IR backend, do not report errors on those classes in dependencies, " +
+                "which were compiled by the IR backend"
+    )
+    var allowJvmIrDependencies: Boolean by FreezableVar(false)
+
+    @Argument(
+        value = "-Xir-binary-with-stable-abi",
+        description = "When using the IR backend, produce binaries which can be read by non-IR backend.\n" +
+                "The author is responsible for verifying that the resulting binaries do indeed have the correct ABI"
+    )
+    var isIrWithStableAbi: Boolean by FreezableVar(false)
+
    @Argument(value = "-Xmodule-path", valueDescription = "<path>", description = "Paths where to find Java 9+ modules")
    var javaModulePath: String? by NullableStringFreezableVar(null)

@@ -327,6 +341,7 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
        result[JvmAnalysisFlags.sanitizeParentheses] = sanitizeParentheses
        result[JvmAnalysisFlags.suppressMissingBuiltinsError] = suppressMissingBuiltinsError
        result[JvmAnalysisFlags.irCheckLocalNames] = irCheckLocalNames
+        result[AnalysisFlags.reportErrorsOnIrDependencies] = !useIR && !useFir && !allowJvmIrDependencies
        return result
    }

@@ -146,25 +146,19 @@ class AnalyzerWithCompilerReport(
            return diagnostic.severity == Severity.ERROR
        }

-        data class ReportDiagnosticsResult(val hasErrors: Boolean, val hasIncompatibleClassErrors: Boolean)
-
-        fun reportDiagnostics(unsortedDiagnostics: Diagnostics, reporter: DiagnosticMessageReporter): ReportDiagnosticsResult {
+        fun reportDiagnostics(unsortedDiagnostics: Diagnostics, reporter: DiagnosticMessageReporter): Boolean {
            var hasErrors = false
-            var hasIncompatibleClassErrors = false
            val diagnostics = sortedDiagnostics(unsortedDiagnostics.all())
            for (diagnostic in diagnostics) {
                hasErrors = hasErrors or reportDiagnostic(diagnostic, reporter)
-                hasIncompatibleClassErrors = hasIncompatibleClassErrors or
-                        (diagnostic.factory == Errors.INCOMPATIBLE_CLASS || diagnostic.factory == Errors.PRE_RELEASE_CLASS)
            }
-
-            return ReportDiagnosticsResult(hasErrors, hasIncompatibleClassErrors)
+            return hasErrors
        }

        fun reportDiagnostics(diagnostics: Diagnostics, messageCollector: MessageCollector): Boolean {
-            val (hasErrors, hasIncompatibleClassErrors) = reportDiagnostics(diagnostics, DefaultDiagnosticReporter(messageCollector))
+            val hasErrors = reportDiagnostics(diagnostics, DefaultDiagnosticReporter(messageCollector))

-            if (hasIncompatibleClassErrors) {
+            if (diagnostics.any { it.factory == Errors.INCOMPATIBLE_CLASS || it.factory == Errors.PRE_RELEASE_CLASS }) {
                messageCollector.report(
                    ERROR,
                    "Incompatible classes were found in dependencies. " +
@@ -172,6 +166,14 @@ class AnalyzerWithCompilerReport(
                )
            }

+            if (diagnostics.any { it.factory == Errors.IR_COMPILED_CLASS }) {
+                messageCollector.report(
+                    ERROR,
+                    "Classes compiled by a new Kotlin compiler backend were found in dependencies. " +
+                            "Remove them from the classpath or use '-Xallow-jvm-ir-dependencies' to suppress errors"
+                )
+            }
+
            return hasErrors
        }

@@ -143,6 +143,7 @@ fun CompilerConfiguration.configureAdvancedJvmOptions(arguments: K2JVMCompilerAr
    put(JVMConfigurationKeys.PARAMETERS_METADATA, arguments.javaParameters)

    put(JVMConfigurationKeys.IR, arguments.useIR && !arguments.noUseIR)
+    put(JVMConfigurationKeys.IS_IR_WITH_STABLE_ABI, arguments.isIrWithStableAbi)
    put(JVMConfigurationKeys.DISABLE_CALL_ASSERTIONS, arguments.noCallAssertions)
    put(JVMConfigurationKeys.DISABLE_RECEIVER_ASSERTIONS, arguments.noReceiverAssertions)
    put(JVMConfigurationKeys.DISABLE_PARAM_ASSERTIONS, arguments.noParamAssertions)
@@ -116,4 +116,7 @@ public class JVMConfigurationKeys {

    public static final CompilerConfigurationKey<List<String>> KLIB_PATHS =
            CompilerConfigurationKey.create("Paths to .klib libraries");
+
+    public static final CompilerConfigurationKey<Boolean> IS_IR_WITH_STABLE_ABI =
+            CompilerConfigurationKey.create("Is IR with stable ABI");
 }
@@ -41,4 +41,7 @@ object AnalysisFlags {

    @JvmStatic
    val ideMode by AnalysisFlag.Delegates.Boolean
+
+    @JvmStatic
+    val reportErrorsOnIrDependencies by AnalysisFlag.Delegates.Boolean
 }
@@ -109,6 +109,7 @@ public interface Errors {
    DiagnosticFactory1<PsiElement, String> MISSING_IMPORTED_SCRIPT_PSI = DiagnosticFactory1.create(ERROR);
    DiagnosticFactory1<PsiElement, String> MISSING_SCRIPT_PROVIDED_PROPERTY_CLASS = DiagnosticFactory1.create(ERROR);
    DiagnosticFactory1<PsiElement, String> PRE_RELEASE_CLASS = DiagnosticFactory1.create(ERROR);
+    DiagnosticFactory1<PsiElement, String> IR_COMPILED_CLASS = DiagnosticFactory1.create(ERROR);
    DiagnosticFactory2<PsiElement, String, IncompatibleVersionErrorData<?>> INCOMPATIBLE_CLASS = DiagnosticFactory2.create(ERROR);

    //Elements with "INVISIBLE_REFERENCE" error are marked as unresolved, unlike elements with "INVISIBLE_MEMBER" error
@@ -384,6 +384,7 @@ public class DefaultErrorMessages {
        MAP.put(MISSING_IMPORTED_SCRIPT_PSI, "Imported script file ''{0}'' is not loaded. Check your script imports", TO_STRING);
        MAP.put(MISSING_SCRIPT_PROVIDED_PROPERTY_CLASS, "Cannot access script provided property class ''{0}''. Check your module classpath for missing or conflicting dependencies", TO_STRING);
        MAP.put(PRE_RELEASE_CLASS, "{0} is compiled by a pre-release version of Kotlin and cannot be loaded by this version of the compiler", TO_STRING);
+        MAP.put(IR_COMPILED_CLASS, "{0} is compiled by a new Kotlin compiler backend and cannot be loaded by the old compiler", TO_STRING);
        MAP.put(INCOMPATIBLE_CLASS,
                "{0} was compiled with an incompatible version of Kotlin. {1}",
                TO_STRING,
@@ -17,6 +17,8 @@ class CompilerDeserializationConfiguration(languageVersionSettings: LanguageVers
    override val reportErrorsOnPreReleaseDependencies =
        !skipMetadataVersionCheck && !languageVersionSettings.isPreRelease() && !KotlinCompilerVersion.isPreRelease()

+    override val reportErrorsOnIrDependencies = languageVersionSettings.getFlag(AnalysisFlags.reportErrorsOnIrDependencies)
+
    override val typeAliasesAllowed = languageVersionSettings.supportsFeature(LanguageFeature.TypeAliases)

    override val isJvmPackageNameSupported = languageVersionSettings.supportsFeature(LanguageFeature.JvmPackageName)
@@ -58,6 +58,9 @@ object MissingDependencyClassChecker : CallChecker {
            if (source.isPreReleaseInvisible) {
                return PRE_RELEASE_CLASS.on(reportOn, source.presentableString)
            }
+            if (source.isInvisibleIrDependency) {
+                return IR_COMPILED_CLASS.on(reportOn, source.presentableString)
+            }
        }

        return null
@@ -216,10 +216,16 @@ open class ClassCodegen protected constructor(
            state.bindingTrace.record(CodegenBinding.DELEGATED_PROPERTIES_WITH_METADATA, type, localDelegatedProperties.map { it.descriptor })
        }

+        // TODO: if `-Xmultifile-parts-inherit` is enabled, write the corresponding flag for parts and facades to [Metadata.extraInt].
+        var extraFlags = JvmAnnotationNames.METADATA_JVM_IR_FLAG
+        if (state.isIrWithStableAbi) {
+            extraFlags += JvmAnnotationNames.METADATA_JVM_IR_STABLE_ABI_FLAG
+        }
+
        when (val metadata = irClass.metadata) {
            is MetadataSource.Class -> {
                val classProto = serializer!!.classProto(metadata.descriptor).build()
-                writeKotlinMetadata(visitor, state, KotlinClassHeader.Kind.CLASS, 0) {
+                writeKotlinMetadata(visitor, state, KotlinClassHeader.Kind.CLASS, extraFlags) {
                    AsmUtil.writeAnnotationData(it, serializer, classProto)
                }

@@ -235,7 +241,7 @@ open class ClassCodegen protected constructor(

                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 ->
+                writeKotlinMetadata(visitor, state, kind, extraFlags) { av ->
                    AsmUtil.writeAnnotationData(av, serializer, packageProto.build())

                    if (facadeClassName != null) {
@@ -250,7 +256,7 @@ open class ClassCodegen protected constructor(
            is MetadataSource.Function -> {
                val fakeDescriptor = createFreeFakeLambdaDescriptor(metadata.descriptor)
                val functionProto = serializer!!.functionProto(fakeDescriptor)?.build()
-                writeKotlinMetadata(visitor, state, KotlinClassHeader.Kind.SYNTHETIC_CLASS, 0) {
+                writeKotlinMetadata(visitor, state, KotlinClassHeader.Kind.SYNTHETIC_CLASS, extraFlags) {
                    if (functionProto != null) {
                        AsmUtil.writeAnnotationData(it, serializer, functionProto)
                    }
@@ -264,7 +270,7 @@ open class ClassCodegen protected constructor(
                        if (fileClass != null) typeMapper.mapClass(fileClass).internalName else null
                    }
                    MultifileClassCodegenImpl.writeMetadata(
-                        visitor, state, 0 /* TODO */, partInternalNames, type, irClass.fqNameWhenAvailable!!.parent()
+                        visitor, state, extraFlags, partInternalNames, type, irClass.fqNameWhenAvailable!!.parent()
                    )
                } else {
                    writeSyntheticClassMetadata(visitor, state)
@@ -2,6 +2,7 @@ Usage: kotlinc-jvm <options> <source files>
 where advanced options include:
  -Xadd-modules=<module[,]>  Root modules to resolve in addition to the initial modules,
                             or all modules on the module path if <module> is ALL-MODULE-PATH
+  -Xallow-jvm-ir-dependencies When not using the IR backend, do not report errors on those classes in dependencies, which were compiled by the IR backend
  -Xallow-no-source-files    Allow no source files
  -Xassertions={always-enable|always-disable|jvm|legacy}
                             Assert calls behaviour
@@ -22,6 +23,8 @@ where advanced options include:
  -Xfriend-paths=<path>      Paths to output directories for friend modules (whose internals should be visible)
  -Xmultifile-parts-inherit  Compile multifile classes as a hierarchy of parts and facade
  -Xir-check-local-names     Check that names of local classes and anonymous objects are the same in the IR backend as in the old backend
+  -Xir-binary-with-stable-abi When using the IR backend, produce binaries which can be read by non-IR backend.
+                             The author is responsible for verifying that the resulting binaries do indeed have the correct ABI
  -Xmodule-path=<path>       Paths where to find Java 9+ modules
  -Xjava-package-prefix      Package prefix for Java files
  -Xjava-source-roots=<path> Paths to directories with Java source files
@@ -0,0 +1,5 @@
+package lib
+
+class Box(val value: String)
+
+inline fun <T> get(block: () -> T): T = block()
@@ -0,0 +1 @@
+OK
@@ -0,0 +1,5 @@
+import lib.*
+
+fun main() {
+    get { Box("OK").value }
+}
@@ -0,0 +1,5 @@
+package lib
+
+class Box(val value: String)
+
+inline fun <T> get(block: () -> T): T = block()
@@ -0,0 +1 @@
+OK
@@ -0,0 +1,5 @@
+import lib.*
+
+fun main() {
+    get { Box("OK").value }
+}
@@ -0,0 +1,5 @@
+package lib
+
+class Box(val value: String)
+
+inline fun <T> get(block: () -> T): T = block()
@@ -0,0 +1,8 @@
+error: classes compiled by a new Kotlin compiler backend were found in dependencies. Remove them from the classpath or use '-Xallow-jvm-ir-dependencies' to suppress errors
+compiler/testData/compileKotlinAgainstCustomBinaries/oldAgainstJvmIr/source.kt:4:5: error: class 'lib.AKt' is compiled by a new Kotlin compiler backend and cannot be loaded by the old compiler
+    get { Box("OK").value }
+    ^
+compiler/testData/compileKotlinAgainstCustomBinaries/oldAgainstJvmIr/source.kt:4:11: error: class 'lib.Box' is compiled by a new Kotlin compiler backend and cannot be loaded by the old compiler
+    get { Box("OK").value }
+          ^
+COMPILATION_ERROR
@@ -0,0 +1,5 @@
+import lib.*
+
+fun main() {
+    get { Box("OK").value }
+}
@@ -0,0 +1,5 @@
+package lib
+
+class Box(val value: String)
+
+inline fun <T> get(block: () -> T): T = block()
@@ -0,0 +1 @@
+OK
@@ -0,0 +1,5 @@
+import lib.*
+
+fun main() {
+    get { Box("OK").value }
+}
@@ -0,0 +1,5 @@
+package lib
+
+class Box(val value: String)
+
+inline fun <T> get(block: () -> T): T = block()
@@ -0,0 +1 @@
+OK
@@ -0,0 +1,5 @@
+import lib.*
+
+fun main() {
+    get { Box("OK").value }
+}
@@ -629,6 +629,31 @@ class CompileKotlinAgainstCustomBinariesTest : AbstractKotlinCompilerIntegration
        classLoader.loadClass("SourceKt").getDeclaredMethod("main").invoke(null)
    }

+    fun testJvmIrAgainstJvmIr() {
+        val library = compileLibrary("library", additionalOptions = listOf("-Xuse-ir"))
+        compileKotlin("source.kt", tmpdir, listOf(library), additionalOptions = listOf("-Xuse-ir"))
+    }
+
+    fun testJvmIrAgainstOld() {
+        val library = compileLibrary("library")
+        compileKotlin("source.kt", tmpdir, listOf(library), additionalOptions = listOf("-Xuse-ir"))
+    }
+
+    fun testOldAgainstJvmIr() {
+        val library = compileLibrary("library", additionalOptions = listOf("-Xuse-ir"))
+        compileKotlin("source.kt", tmpdir, listOf(library))
+    }
+
+    fun testOldAgainstJvmIrWithStableAbi() {
+        val library = compileLibrary("library", additionalOptions = listOf("-Xuse-ir", "-Xir-binary-with-stable-abi"))
+        compileKotlin("source.kt", tmpdir, listOf(library))
+    }
+
+    fun testOldAgainstJvmIrWithAllowIrDependencies() {
+        val library = compileLibrary("library", additionalOptions = listOf("-Xuse-ir"))
+        compileKotlin("source.kt", tmpdir, listOf(library), additionalOptions = listOf("-Xallow-jvm-ir-dependencies"))
+    }
+
    companion object {
        // compiler before 1.1.4 version  did not include suspension marks into bytecode.
        private fun stripSuspensionMarksToImitateLegacyCompiler(bytes: ByteArray): Pair<ByteArray, Int> {
@@ -39,6 +39,8 @@ public final class JvmAnnotationNames {
    public static final int METADATA_PRE_RELEASE_FLAG = 1 << 1;
    public static final int METADATA_SCRIPT_FLAG = 1 << 2;
    public static final int METADATA_STRICT_VERSION_SEMANTICS_FLAG = 1 << 3;
+    public static final int METADATA_JVM_IR_FLAG = 1 << 4;
+    public static final int METADATA_JVM_IR_STABLE_ABI_FLAG = 1 << 5;

    public static final Name DEFAULT_ANNOTATION_MEMBER_NAME = Name.identifier("value");

@@ -52,7 +52,9 @@ class DeserializedDescriptorResolver {
        val (nameResolver, classProto) = parseProto(kotlinClass) {
            JvmProtoBufUtil.readClassDataFrom(data, strings)
        } ?: return null
-        val source = KotlinJvmBinarySourceElement(kotlinClass, kotlinClass.incompatibility, kotlinClass.isPreReleaseInvisible)
+        val source = KotlinJvmBinarySourceElement(
+            kotlinClass, kotlinClass.incompatibility, kotlinClass.isPreReleaseInvisible, kotlinClass.isInvisibleJvmIrDependency
+        )
        return ClassData(nameResolver, classProto, kotlinClass.classHeader.metadataVersion, source)
    }

@@ -63,7 +65,8 @@ class DeserializedDescriptorResolver {
            JvmProtoBufUtil.readPackageDataFrom(data, strings)
        } ?: return null
        val source = JvmPackagePartSource(
-            kotlinClass, packageProto, nameResolver, kotlinClass.incompatibility, kotlinClass.isPreReleaseInvisible
+            kotlinClass, packageProto, nameResolver, kotlinClass.incompatibility, kotlinClass.isPreReleaseInvisible,
+            kotlinClass.isInvisibleJvmIrDependency
        )
        return DeserializedPackageMemberScope(
            descriptor, packageProto, nameResolver, kotlinClass.classHeader.metadataVersion, source, components
@@ -94,6 +97,9 @@ class DeserializedDescriptorResolver {
        get() = !components.configuration.skipMetadataVersionCheck &&
                classHeader.isPreRelease && classHeader.metadataVersion == KOTLIN_1_3_M1_METADATA_VERSION

+    private val KotlinJvmBinaryClass.isInvisibleJvmIrDependency: Boolean
+        get() = components.configuration.reportErrorsOnIrDependencies && classHeader.isUnstableJvmIrBinary
+
    private fun readData(kotlinClass: KotlinJvmBinaryClass, expectedKinds: Set<KotlinClassHeader.Kind>): Array<String>? {
        val header = kotlinClass.classHeader
        return (header.data ?: header.incompatibleData)?.takeIf { header.kind in expectedKinds }
@@ -36,6 +36,7 @@ class JvmPackagePartSource(
    nameResolver: NameResolver,
    override val incompatibility: IncompatibleVersionErrorData<JvmMetadataVersion>? = null,
    override val isPreReleaseInvisible: Boolean = false,
+    override val isInvisibleIrDependency: Boolean = false,
    val knownJvmBinaryClass: KotlinJvmBinaryClass? = null
 ) : DeserializedContainerSource {
    constructor(
@@ -43,7 +44,8 @@ class JvmPackagePartSource(
        packageProto: ProtoBuf.Package,
        nameResolver: NameResolver,
        incompatibility: IncompatibleVersionErrorData<JvmMetadataVersion>? = null,
-        isPreReleaseInvisible: Boolean = false
+        isPreReleaseInvisible: Boolean = false,
+        isInvisibleIrDependency: Boolean = false
    ) : this(
        JvmClassName.byClassId(kotlinClass.classId),
        kotlinClass.classHeader.multifileClassName?.let {
@@ -53,6 +55,7 @@ class JvmPackagePartSource(
        nameResolver,
        incompatibility,
        isPreReleaseInvisible,
+        isInvisibleIrDependency,
        kotlinClass
    )

@@ -24,7 +24,8 @@ import org.jetbrains.kotlin.serialization.deserialization.descriptors.Deserializ
 class KotlinJvmBinarySourceElement(
    val binaryClass: KotlinJvmBinaryClass,
    override val incompatibility: IncompatibleVersionErrorData<JvmMetadataVersion>? = null,
-    override val isPreReleaseInvisible: Boolean = false
+    override val isPreReleaseInvisible: Boolean = false,
+    override val isInvisibleIrDependency: Boolean = false
 ) : DeserializedContainerSource {
    override val presentableString: String
        get() = "Class '${binaryClass.classId.asSingleFqName().asString()}'"
@@ -71,6 +71,10 @@ class KotlinClassHeader(
                DELEGATING
        } else null

+    val isUnstableJvmIrBinary: Boolean
+        get() = (extraInt and JvmAnnotationNames.METADATA_JVM_IR_FLAG) != 0 &&
+                (extraInt and JvmAnnotationNames.METADATA_JVM_IR_STABLE_ABI_FLAG == 0)
+
    val isPreRelease: Boolean
        get() = (extraInt and JvmAnnotationNames.METADATA_PRE_RELEASE_FLAG) != 0

@@ -12,6 +12,9 @@ interface DeserializationConfiguration {
    val reportErrorsOnPreReleaseDependencies: Boolean
        get() = false

+    val reportErrorsOnIrDependencies: Boolean
+        get() = false
+
    val typeAliasesAllowed: Boolean
        get() = true

@@ -52,6 +52,10 @@ interface DeserializedContainerSource : SourceElement {
    // True iff this is container is "invisible" because it's loaded from a pre-release class and this compiler is a release
    val isPreReleaseInvisible: Boolean

+    // True iff this container was compiled by the new IR backend, this compiler is not using the IR backend right now,
+    // and no additional flags to override this behavior were specified.
+    val isInvisibleIrDependency: Boolean
+
    // This string should only be used in error messages
    val presentableString: String
 }
@@ -94,6 +94,9 @@ class KotlinJavascriptPackageFragment(
        override val isPreReleaseInvisible: Boolean =
            configuration.reportErrorsOnPreReleaseDependencies && (header.flags and 1) != 0

+        override val isInvisibleIrDependency: Boolean
+            get() = false
+
        override val presentableString: String
            get() = "Package '$fqName'"
    }
@@ -42,7 +42,7 @@ public annotation class Metadata(
    @get:JvmName("d1")
    val data1: Array<String> = [],
    /**
-     * An addition to [d1]: array of strings which occur in metadata, written in plain text so that strings already present
+     * An addition to [data1]: array of strings which occur in metadata, written in plain text so that strings already present
     * in the constant pool are reused. These strings may be then indexed in the metadata by an integer index in this array.
     */
    @get:JvmName("d2")
@@ -67,7 +67,10 @@ public annotation class Metadata(
     * * 1 - this class file is compiled by a pre-release version of Kotlin and is not visible to release versions.
     * * 2 - this class file is a compiled Kotlin script source file (.kts).
     * * 3 - the metadata of this class file is not supposed to be read by the compiler, whose major.minor version is less than
-     *   the major.minor version of this metadata ([mv]).
+     *   the major.minor version of this metadata ([metadataVersion]).
+     * * 4 - this class file is compiled with the new Kotlin compiler backend introduced in Kotlin 1.4.
+     * * 5 - if the class file is compiled with the new Kotlin compiler backend, the metadata has been verified by the author and
+     *   no metadata incompatibility diagnostic should be reported at the call site.
     */
    @SinceKotlin("1.1")
    @get:JvmName("xi")