Introduce kotlinx-metadata-klib.

`kotlinx-metadata-klib` is an extension of `kotlinx-metadata` that can be used to read and write metadata that is stored inside KLIBs. Note: current version is in its early days and in active development. Almost nothing is stable or properly tested.
2019-10-29 16:21:09 +07:00
parent d91453fb7a
commit 95399b3a41
25 changed files with 1378 additions and 15 deletions
@@ -48,6 +48,9 @@ internal class JvmMetadataExtensions : MetadataExtensions {
        ext.visitEnd()
    }

+    // ModuleFragment is not used by JVM backend.
+    override fun readModuleFragmentExtensions(v: KmModuleFragmentVisitor, proto: ProtoBuf.PackageFragment, c: ReadContext) {}
+
    override fun readFunctionExtensions(v: KmFunctionVisitor, proto: ProtoBuf.Function, c: ReadContext) {
        val ext = v.visitExtensions(JvmFunctionExtensionVisitor.TYPE) as? JvmFunctionExtensionVisitor ?: return
        ext.visit(JvmProtoBufUtil.getJvmMethodSignature(proto, c.strings, c.types)?.wrapAsPublic())
@@ -144,6 +147,13 @@ internal class JvmMetadataExtensions : MetadataExtensions {
        }
    }

+    // PackageFragment is not used by JVM backend.
+    override fun writeModuleFragmentExtensions(
+        type: KmExtensionType,
+        proto: ProtoBuf.PackageFragment.Builder,
+        c: WriteContext
+    ): KmModuleFragmentExtensionVisitor? = null
+
    override fun writeFunctionExtensions(
        type: KmExtensionType, proto: ProtoBuf.Function.Builder, c: WriteContext
    ): KmFunctionExtensionVisitor? {
@@ -262,6 +272,9 @@ internal class JvmMetadataExtensions : MetadataExtensions {

    override fun createPackageExtension(): KmPackageExtension = JvmPackageExtension()

+    override fun createModuleFragmentExtensions(): KmModuleFragmentExtension =
+        error("metadata-jvm doesn't have any extensions for module fragment!")
+
    override fun createFunctionExtension(): KmFunctionExtension = JvmFunctionExtension()

    override fun createPropertyExtension(): KmPropertyExtension = JvmPropertyExtension()
@@ -0,0 +1,69 @@
+import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
+
+description = "Kotlin Library (KLIB) metadata manipulation library"
+
+plugins {
+    kotlin("jvm")
+    id("jps-compatible")
+}
+
+group = "org.jetbrains.kotlinx"
+
+val deployVersion = findProperty("kotlinxMetadataKlibDeployVersion") as String?
+version = deployVersion ?: "0.0.1-SNAPSHOT"
+
+jvmTarget = "1.6"
+javaHome = rootProject.extra["JDK_16"] as String
+
+sourceSets {
+    "main" { projectDefault() }
+}
+
+val shadows by configurations.creating {
+    isTransitive = false
+}
+configurations.getByName("compileOnly").extendsFrom(shadows)
+configurations.getByName("testCompile").extendsFrom(shadows)
+
+dependencies {
+    compile(kotlinStdlib())
+    shadows(project(":kotlinx-metadata"))
+    shadows(project(":core:metadata"))
+    shadows(project(":compiler:serialization"))
+    shadows(project(":kotlin-util-klib-metadata"))
+    shadows(project(":kotlin-util-klib"))
+    shadows(protobufLite())
+}
+
+if (deployVersion != null) {
+    publish()
+}
+
+noDefaultJar()
+
+tasks.register<ShadowJar>("shadowJar") {
+    callGroovy("manifestAttributes", manifest, project)
+    manifest.attributes["Implementation-Version"] = version
+
+    from(mainSourceSet.output)
+    exclude("**/*.proto")
+    configurations = listOf(shadows)
+    relocate("org.jetbrains.kotlin", "kotlinx.metadata.internal")
+
+    val artifactRef = outputs.files.singleFile
+    runtimeJarArtifactBy(this, artifactRef)
+    addArtifact("runtime", this, artifactRef)
+}
+
+sourcesJar {
+    for (dependency in shadows.dependencies) {
+        if (dependency is ProjectDependency) {
+            val javaPlugin = dependency.dependencyProject.convention.findPlugin(JavaPluginConvention::class.java)
+            if (javaPlugin != null) {
+                from(javaPlugin.sourceSets["main"].allSource)
+            }
+        }
+    }
+}
+
+javadocJar()
@@ -0,0 +1 @@
+kotlinx.metadata.klib.impl.KlibMetadataExtensions
@@ -0,0 +1,16 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib
+
+import kotlinx.metadata.KmAnnotation
+
+class KlibHeader(
+    val moduleName: String,
+    val file: List<KlibSourceFile>,
+    val packageFragmentName: List<String>,
+    val emptyPackage: List<String>,
+    val annotation: List<KmAnnotation>
+)
@@ -0,0 +1,137 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib
+
+import kotlinx.metadata.KmAnnotation
+import kotlinx.metadata.KmModuleFragment
+import kotlinx.metadata.impl.WriteContext
+import kotlinx.metadata.impl.accept
+import kotlinx.metadata.klib.impl.*
+import org.jetbrains.kotlin.backend.common.serialization.metadata.KlibMetadataStringTable
+import org.jetbrains.kotlin.library.metadata.parseModuleHeader
+import org.jetbrains.kotlin.library.metadata.parsePackageFragment
+import org.jetbrains.kotlin.metadata.ProtoBuf
+import org.jetbrains.kotlin.metadata.deserialization.NameResolverImpl
+
+/**
+ * Allows to modify the way fragments of the single package are read by [KlibModuleMetadata.read].
+ * For example, it may be convenient to join fragments into a single one.
+ */
+interface KlibModuleFragmentReadStrategy {
+    fun processModuleParts(parts: List<KmModuleFragment>): List<KmModuleFragment>
+
+    companion object {
+        val DEFAULT = object : KlibModuleFragmentReadStrategy {
+            override fun processModuleParts(parts: List<KmModuleFragment>) =
+                parts
+        }
+    }
+}
+
+/**
+ * Allows to modify the way module fragments are written by [KlibModuleMetadata.write].
+ * For example, splitting big fragments into several small one allows to improve IDE performance.
+ */
+interface KlibModuleFragmentWriteStrategy {
+    fun processPackageParts(parts: List<KmModuleFragment>): List<KmModuleFragment>
+
+    companion object {
+        val DEFAULT = object : KlibModuleFragmentWriteStrategy {
+            override fun processPackageParts(parts: List<KmModuleFragment>): List<KmModuleFragment> =
+                parts
+        }
+    }
+}
+
+/**
+ * Represents the parsed metadata of KLIB.
+ */
+class KlibModuleMetadata(
+    val name: String,
+    val fragments: List<KmModuleFragment>,
+    val annotations: List<KmAnnotation>
+) {
+
+    /**
+     * Serialized representation of module metadata.
+     */
+    class SerializedKlibMetadata(
+        val header: ByteArray,
+        val fragments: List<List<ByteArray>>,
+        val fragmentNames: List<String>
+    )
+
+    /**
+     * Specifies access to library's metadata.
+     */
+    interface MetadataLibraryProvider {
+        val moduleHeaderData: ByteArray
+        fun packageMetadataParts(fqName: String): Set<String>
+        fun packageMetadata(fqName: String, partName: String): ByteArray
+    }
+
+    companion object {
+        /**
+         * Deserializes metadata from the given [library].
+         * @param readStrategy specifies the way module fragments of a single package are modified (e.g. merged) after deserialization.
+         */
+        fun read(
+            library: MetadataLibraryProvider,
+            readStrategy: KlibModuleFragmentReadStrategy = KlibModuleFragmentReadStrategy.DEFAULT
+        ): KlibModuleMetadata {
+            val moduleHeaderProto = parseModuleHeader(library.moduleHeaderData)
+            val nameResolver = NameResolverImpl(moduleHeaderProto.strings, moduleHeaderProto.qualifiedNames)
+            val moduleHeader = moduleHeaderProto.readHeader(nameResolver)
+            val fileIndex = SourceFileIndexReadExtension(moduleHeader.file)
+            val moduleFragments = moduleHeader.packageFragmentName.flatMap { packageFqName ->
+                library.packageMetadataParts(packageFqName).map { part ->
+                    val packageFragment = parsePackageFragment(library.packageMetadata(packageFqName, part))
+                    KmModuleFragment().apply { packageFragment.accept(this, nameResolver, listOf(fileIndex)) }
+                }.let(readStrategy::processModuleParts)
+            }
+            return KlibModuleMetadata(moduleHeader.moduleName, moduleFragments, moduleHeader.annotation)
+        }
+    }
+
+    /**
+     * Writes metadata back to serialized representation.
+     * @param writeStrategy specifies the way module fragments are modified (e.g. split) before serialization.
+     */
+    fun write(
+        writeStrategy: KlibModuleFragmentWriteStrategy = KlibModuleFragmentWriteStrategy.DEFAULT
+    ): SerializedKlibMetadata {
+        val reverseIndex = ReverseSourceFileIndexWriteExtension()
+        val c = WriteContext(KlibMetadataStringTable(), listOf(reverseIndex))
+
+        val groupedFragments = fragments
+            .groupBy(KmModuleFragment::fqNameOrFail)
+            .mapValues { writeStrategy.processPackageParts(it.value) }
+
+        val header = KlibHeader(
+            name,
+            reverseIndex.fileIndex,
+            groupedFragments.map { it.key },
+            groupedFragments.filter { it.value.all(KmModuleFragment::isEmpty) }.map { it.key },
+            annotations
+        )
+        val groupedProtos = groupedFragments.mapValues { (_, fragments) ->
+            fragments.map {
+                KlibModuleFragmentWriter(c.strings as KlibMetadataStringTable, c.contextExtensions).also(it::accept).write()
+            }
+        }
+        return SerializedKlibMetadata(
+            header.writeHeader(c).build().toByteArray(),
+            groupedProtos.map { it.value.map(ProtoBuf.PackageFragment::toByteArray) },
+            header.packageFragmentName
+        )
+    }
+}
+
+private fun KmModuleFragment.fqNameOrFail(): String =
+    fqName ?: error("Module fragment must have a fully-qualified name.")
+
+private fun KmModuleFragment.isEmpty(): Boolean =
+    classes.isEmpty() && (pkg?.let { it.functions.isEmpty() && it.properties.isEmpty() && it.typeAliases.isEmpty() } ?: true)
@@ -0,0 +1,11 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib
+
+class KlibSourceFile(
+    val name: String
+)
+
@@ -0,0 +1,8 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib
+
+class UniqId(val index: Long)
@@ -0,0 +1,224 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib.impl
+
+import kotlinx.metadata.*
+import kotlinx.metadata.impl.extensions.*
+import kotlinx.metadata.klib.*
+
+internal val KmFunction.klibExtensions: KlibFunctionExtension
+    get() = visitExtensions(KlibFunctionExtensionVisitor.TYPE) as KlibFunctionExtension
+
+internal val KmClass.klibExtensions: KlibClassExtension
+    get() = visitExtensions(KlibClassExtensionVisitor.TYPE) as KlibClassExtension
+
+internal val KmType.klibExtensions: KlibTypeExtension
+    get() = visitExtensions(KlibTypeExtensionVisitor.TYPE) as KlibTypeExtension
+
+internal val KmProperty.klibExtensions: KlibPropertyExtension
+    get() = visitExtensions(KlibPropertyExtensionVisitor.TYPE) as KlibPropertyExtension
+
+internal val KmConstructor.klibExtensions: KlibConstructorExtension
+    get() = visitExtensions(KlibConstructorExtensionVisitor.TYPE) as KlibConstructorExtension
+
+internal val KmTypeParameter.klibExtensions: KlibTypeParameterExtension
+    get() = visitExtensions(KlibTypeParameterExtensionVisitor.TYPE) as KlibTypeParameterExtension
+
+internal val KmPackage.klibExtensions: KlibPackageExtension
+    get() = visitExtensions(KlibPackageExtensionVisitor.TYPE) as KlibPackageExtension
+
+internal val KmModuleFragment.klibExtensions: KlibModuleFragmentExtension
+    get() = visitExtensions(KlibModuleFragmentExtensionVisitor.TYPE) as KlibModuleFragmentExtension
+
+internal class KlibFunctionExtension : KlibFunctionExtensionVisitor(), KmFunctionExtension {
+
+    val annotations: MutableList<KmAnnotation> = mutableListOf()
+    var uniqId: UniqId? = null
+    var file: KlibSourceFile? = null
+
+    override fun visitUniqId(uniqId: UniqId) {
+        this.uniqId = uniqId
+    }
+
+    override fun visitAnnotation(annotation: KmAnnotation) {
+        annotations += annotation
+    }
+
+    override fun visitFile(file: KlibSourceFile) {
+        this.file = file
+    }
+
+    override fun accept(visitor: KmFunctionExtensionVisitor) {
+        require(visitor is KlibFunctionExtensionVisitor)
+        annotations.forEach(visitor::visitAnnotation)
+        uniqId?.let(visitor::visitUniqId)
+        file?.let(visitor::visitFile)
+    }
+}
+
+internal class KlibClassExtension : KlibClassExtensionVisitor(), KmClassExtension {
+
+    val annotations: MutableList<KmAnnotation> = mutableListOf()
+    var uniqId: UniqId? = null
+    var file: KlibSourceFile? = null
+
+    override fun visitAnnotation(annotation: KmAnnotation) {
+        annotations += annotation
+    }
+
+    override fun visitUniqId(uniqId: UniqId) {
+        this.uniqId = uniqId
+    }
+
+    override fun visitFile(file: KlibSourceFile) {
+        this.file = file
+    }
+
+    override fun accept(visitor: KmClassExtensionVisitor) {
+        require(visitor is KlibClassExtensionVisitor)
+        annotations.forEach(visitor::visitAnnotation)
+        uniqId?.let(visitor::visitUniqId)
+        file?.let(visitor::visitFile)
+    }
+}
+
+internal class KlibTypeExtension : KlibTypeExtensionVisitor(), KmTypeExtension {
+
+    val annotations: MutableList<KmAnnotation> = mutableListOf()
+
+    override fun visitAnnotation(annotation: KmAnnotation) {
+        annotations += annotation
+    }
+
+    override fun accept(visitor: KmTypeExtensionVisitor) {
+        require(visitor is KlibTypeExtensionVisitor)
+        annotations.forEach(visitor::visitAnnotation)
+    }
+}
+
+internal class KlibPropertyExtension : KlibPropertyExtensionVisitor(), KmPropertyExtension {
+
+    val annotations: MutableList<KmAnnotation> = mutableListOf()
+    val getterAnnotations: MutableList<KmAnnotation> = mutableListOf()
+    val setterAnnotations: MutableList<KmAnnotation> = mutableListOf()
+    var uniqId: UniqId? = null
+    var file: Int? = null
+    var compileTimeValue: KmAnnotationArgument<*>? = null
+
+    override fun visitAnnotation(annotation: KmAnnotation) {
+        annotations += annotation
+    }
+
+    override fun visitGetterAnnotation(annotation: KmAnnotation) {
+        getterAnnotations += annotation
+    }
+
+    override fun visitSetterAnnotation(annotation: KmAnnotation) {
+        setterAnnotations += annotation
+    }
+
+    override fun visitFile(file: Int) {
+        this.file = file
+    }
+
+    override fun visitUniqId(uniqId: UniqId) {
+        this.uniqId = uniqId
+    }
+
+    override fun visitCompileTimeValue(value: KmAnnotationArgument<*>) {
+        this.compileTimeValue = value
+    }
+
+    override fun accept(visitor: KmPropertyExtensionVisitor) {
+        require(visitor is KlibPropertyExtensionVisitor)
+        annotations.forEach(visitor::visitAnnotation)
+        getterAnnotations.forEach(visitor::visitGetterAnnotation)
+        setterAnnotations.forEach(visitor::visitSetterAnnotation)
+        file?.let(visitor::visitFile)
+        uniqId?.let(visitor::visitUniqId)
+        compileTimeValue?.let(visitor::visitCompileTimeValue)
+    }
+}
+
+internal class KlibConstructorExtension : KlibConstructorExtensionVisitor(), KmConstructorExtension {
+
+    val annotations: MutableList<KmAnnotation> = mutableListOf()
+    var uniqId: UniqId? = null
+
+    override fun visitAnnotation(annotation: KmAnnotation) {
+        annotations += annotation
+    }
+
+    override fun visitUniqId(uniqId: UniqId) {
+        this.uniqId = uniqId
+    }
+
+    override fun accept(visitor: KmConstructorExtensionVisitor) {
+        require(visitor is KlibConstructorExtensionVisitor)
+        annotations.forEach(visitor::visitAnnotation)
+        uniqId?.let(visitor::visitUniqId)
+    }
+}
+
+internal class KlibTypeParameterExtension : KlibTypeParameterExtensionVisitor(), KmTypeParameterExtension {
+
+    val annotations: MutableList<KmAnnotation> = mutableListOf()
+    var uniqId: UniqId? = null
+
+    override fun visitAnnotation(annotation: KmAnnotation) {
+        annotations += annotation
+    }
+
+    override fun visitUniqId(uniqId: UniqId) {
+        this.uniqId = uniqId
+    }
+
+    override fun accept(visitor: KmTypeParameterExtensionVisitor) {
+        require(visitor is KlibTypeParameterExtensionVisitor)
+        annotations.forEach(visitor::visitAnnotation)
+        uniqId?.let(visitor::visitUniqId)
+    }
+}
+
+internal class KlibPackageExtension : KlibPackageExtensionVisitor(), KmPackageExtension {
+
+    var fqName: String? = null
+
+    override fun visitFqName(name: String) {
+        fqName = name
+    }
+
+    override fun accept(visitor: KmPackageExtensionVisitor) {
+        require(visitor is KlibPackageExtensionVisitor)
+        fqName?.let(visitor::visitFqName)
+    }
+}
+
+internal class KlibModuleFragmentExtension : KlibModuleFragmentExtensionVisitor(), KmModuleFragmentExtension {
+
+    val moduleFragmentFiles: MutableList<KlibSourceFile> = ArrayList()
+    var fqName: String? = null
+    val className: MutableList<ClassName> = ArrayList()
+
+    override fun visitFile(file: KlibSourceFile) {
+        moduleFragmentFiles += file
+    }
+
+    override fun visitFqName(fqName: String) {
+        this.fqName = fqName
+    }
+
+    override fun visitClassName(className: ClassName) {
+        this.className += className
+    }
+
+    override fun accept(visitor: KmModuleFragmentExtensionVisitor) {
+        require(visitor is KlibModuleFragmentExtensionVisitor)
+        moduleFragmentFiles.forEach(visitor::visitFile)
+        fqName?.let(visitor::visitFqName)
+        className.forEach(visitor::visitClassName)
+    }
+}
@@ -0,0 +1,350 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib.impl
+
+import kotlinx.metadata.*
+import kotlinx.metadata.impl.*
+import kotlinx.metadata.impl.extensions.*
+import kotlinx.metadata.klib.*
+import org.jetbrains.kotlin.library.metadata.KlibMetadataProtoBuf
+import org.jetbrains.kotlin.metadata.ProtoBuf
+import org.jetbrains.kotlin.metadata.deserialization.NameResolverImpl
+import org.jetbrains.kotlin.metadata.deserialization.getExtensionOrNull
+import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.name.FqName
+import org.jetbrains.kotlin.serialization.StringTableImpl
+
+internal class KlibMetadataExtensions : MetadataExtensions {
+
+    private fun ReadContext.getSourceFile(index: Int) =
+        contextExtensions.filterIsInstance<SourceFileIndexReadExtension>().first().getSourceFile(index)
+
+    private fun WriteContext.getIndexOf(file: KlibSourceFile) =
+        contextExtensions.filterIsInstance<ReverseSourceFileIndexWriteExtension>().first().getIndexOf(file)
+
+    override fun readClassExtensions(v: KmClassVisitor, proto: ProtoBuf.Class, c: ReadContext) {
+        val extension = v.visitExtensions(KlibClassExtensionVisitor.TYPE) as? KlibClassExtensionVisitor ?: return
+
+        proto.getExtension(KlibMetadataProtoBuf.classAnnotation).forEach { annotation ->
+            extension.visitAnnotation(annotation.readAnnotation(c.strings))
+        }
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.classUniqId)?.let { descriptorUniqId ->
+            extension.visitUniqId(descriptorUniqId.readUniqId())
+        }
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.classFile)?.let {
+            extension.visitFile(c.getSourceFile(it))
+        }
+    }
+
+    override fun readPackageExtensions(v: KmPackageVisitor, proto: ProtoBuf.Package, c: ReadContext) {
+        val extension = v.visitExtensions(KlibPackageExtensionVisitor.TYPE) as? KlibPackageExtensionVisitor ?: return
+
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.packageFqName)?.let {
+            val fqName = (c.strings as NameResolverImpl).getPackageFqName(it)
+            extension.visitFqName(fqName)
+        }
+    }
+
+    override fun readModuleFragmentExtensions(v: KmModuleFragmentVisitor, proto: ProtoBuf.PackageFragment, c: ReadContext) {
+        val extension = v.visitExtensions(KlibModuleFragmentExtensionVisitor.TYPE) as? KlibModuleFragmentExtensionVisitor ?: return
+
+        proto.getExtension(KlibMetadataProtoBuf.packageFragmentFiles)
+            .map { c.getSourceFile(it) }
+            .forEach(extension::visitFile)
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.fqName)?.let(extension::visitFqName)
+        proto.getExtension(KlibMetadataProtoBuf.className)
+            .map(c.strings::getQualifiedClassName)
+            .forEach(extension::visitClassName)
+    }
+
+    override fun readFunctionExtensions(v: KmFunctionVisitor, proto: ProtoBuf.Function, c: ReadContext) {
+        val extension = v.visitExtensions(KlibFunctionExtensionVisitor.TYPE) as? KlibFunctionExtensionVisitor ?: return
+
+        proto.getExtension(KlibMetadataProtoBuf.functionAnnotation).forEach { annotation ->
+            extension.visitAnnotation(annotation.readAnnotation(c.strings))
+        }
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.functionUniqId)?.let { descriptorUniqId ->
+            extension.visitUniqId(descriptorUniqId.readUniqId())
+        }
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.functionFile)?.let {
+            val file = c.getSourceFile(it)
+            extension.visitFile(file)
+        }
+    }
+
+    override fun readPropertyExtensions(v: KmPropertyVisitor, proto: ProtoBuf.Property, c: ReadContext) {
+        val extension = v.visitExtensions(KlibPropertyExtensionVisitor.TYPE) as? KlibPropertyExtensionVisitor ?: return
+
+        proto.getExtension(KlibMetadataProtoBuf.propertyAnnotation).forEach { annotation ->
+            extension.visitAnnotation(annotation.readAnnotation(c.strings))
+        }
+        proto.getExtension(KlibMetadataProtoBuf.propertyGetterAnnotation).forEach { annotation ->
+            extension.visitGetterAnnotation(annotation.readAnnotation(c.strings))
+        }
+        proto.getExtension(KlibMetadataProtoBuf.propertySetterAnnotation).forEach { annotation ->
+            extension.visitSetterAnnotation(annotation.readAnnotation(c.strings))
+        }
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.propertyUniqId)?.let { descriptorUniqId ->
+            extension.visitUniqId(descriptorUniqId.readUniqId())
+        }
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.propertyFile)?.let(extension::visitFile)
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.compileTimeValue)?.let { value ->
+            value.readAnnotationArgument(c.strings)?.let { extension.visitCompileTimeValue(it) }
+        }
+    }
+
+    override fun readConstructorExtensions(v: KmConstructorVisitor, proto: ProtoBuf.Constructor, c: ReadContext) {
+        val extension = v.visitExtensions(KlibConstructorExtensionVisitor.TYPE) as? KlibConstructorExtensionVisitor ?: return
+
+        proto.getExtension(KlibMetadataProtoBuf.constructorAnnotation).forEach { annotation ->
+            extension.visitAnnotation(annotation.readAnnotation(c.strings))
+        }
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.constructorUniqId)?.let { descriptorUniqId ->
+            extension.visitUniqId(descriptorUniqId.readUniqId())
+        }
+    }
+
+    override fun readTypeParameterExtensions(v: KmTypeParameterVisitor, proto: ProtoBuf.TypeParameter, c: ReadContext) {
+        val extension = v.visitExtensions(KlibTypeParameterExtensionVisitor.TYPE) as? KlibTypeParameterExtensionVisitor ?: return
+
+        proto.getExtension(KlibMetadataProtoBuf.typeParameterAnnotation).forEach { annotation ->
+            extension.visitAnnotation(annotation.readAnnotation(c.strings))
+        }
+        proto.getExtensionOrNull(KlibMetadataProtoBuf.typeParamUniqId)?.let { descriptorUniqId ->
+            extension.visitUniqId(descriptorUniqId.readUniqId())
+        }
+    }
+
+    override fun readTypeExtensions(v: KmTypeVisitor, proto: ProtoBuf.Type, c: ReadContext) {
+        val extension = v.visitExtensions(KlibTypeExtensionVisitor.TYPE) as? KlibTypeExtensionVisitor ?: return
+
+        proto.getExtension(KlibMetadataProtoBuf.typeAnnotation).forEach { annotation ->
+            extension.visitAnnotation(annotation.readAnnotation(c.strings))
+        }
+    }
+
+    override fun writeClassExtensions(type: KmExtensionType, proto: ProtoBuf.Class.Builder, c: WriteContext): KmClassExtensionVisitor? {
+        if (type != KlibClassExtensionVisitor.TYPE) return null
+        return object : KlibClassExtensionVisitor() {
+            override fun visitAnnotation(annotation: KmAnnotation) {
+                proto.addExtension(
+                    KlibMetadataProtoBuf.classAnnotation,
+                    annotation.writeAnnotation(c.strings).build()
+                )
+            }
+
+            override fun visitUniqId(uniqId: UniqId) {
+                proto.setExtension(
+                    KlibMetadataProtoBuf.classUniqId,
+                    uniqId.writeUniqId().build()
+                )
+            }
+
+            override fun visitFile(file: KlibSourceFile) {
+                val fileIdx = c.getIndexOf(file)
+                proto.setExtension(KlibMetadataProtoBuf.classFile, fileIdx)
+            }
+        }
+    }
+
+    override fun writePackageExtensions(
+        type: KmExtensionType,
+        proto: ProtoBuf.Package.Builder,
+        c: WriteContext
+    ): KmPackageExtensionVisitor? {
+        if (type != KlibPackageExtensionVisitor.TYPE) return null
+        return object : KlibPackageExtensionVisitor() {
+            override fun visitFqName(name: String) {
+                val nameIdx = (c.strings as StringTableImpl).getPackageFqNameIndex(FqName(name))
+                proto.setExtension(KlibMetadataProtoBuf.packageFqName, nameIdx)
+            }
+        }
+    }
+
+    override fun writeModuleFragmentExtensions(
+        type: KmExtensionType,
+        proto: ProtoBuf.PackageFragment.Builder,
+        c: WriteContext
+    ): KmModuleFragmentExtensionVisitor? {
+        if (type != KlibModuleFragmentExtensionVisitor.TYPE) return null
+        return object : KlibModuleFragmentExtensionVisitor() {
+            override fun visitFile(file: KlibSourceFile) {
+                val fileIdx = c.getIndexOf(file)
+                proto.addExtension(KlibMetadataProtoBuf.packageFragmentFiles, fileIdx)
+            }
+
+            override fun visitFqName(fqName: String) {
+                proto.setExtension(KlibMetadataProtoBuf.fqName, fqName)
+            }
+
+            override fun visitClassName(className: ClassName) {
+                val classNameIdx = (c.strings as StringTableImpl).getQualifiedClassNameIndex(ClassId.fromString(className))
+                proto.addExtension(KlibMetadataProtoBuf.className, classNameIdx)
+            }
+        }
+    }
+
+    override fun writeFunctionExtensions(
+        type: KmExtensionType,
+        proto: ProtoBuf.Function.Builder,
+        c: WriteContext
+    ): KmFunctionExtensionVisitor? {
+        if (type != KlibFunctionExtensionVisitor.TYPE) return null
+        return object : KlibFunctionExtensionVisitor() {
+            override fun visitAnnotation(annotation: KmAnnotation) {
+                proto.addExtension(
+                    KlibMetadataProtoBuf.functionAnnotation,
+                    annotation.writeAnnotation(c.strings).build()
+                )
+            }
+
+            override fun visitUniqId(uniqId: UniqId) {
+                proto.setExtension(
+                    KlibMetadataProtoBuf.functionUniqId,
+                    uniqId.writeUniqId().build()
+                )
+            }
+
+            override fun visitFile(file: KlibSourceFile) {
+                val index = c.getIndexOf(file)
+                proto.setExtension(KlibMetadataProtoBuf.functionFile, index)
+            }
+        }
+    }
+
+    override fun writePropertyExtensions(
+        type: KmExtensionType,
+        proto: ProtoBuf.Property.Builder,
+        c: WriteContext
+    ): KmPropertyExtensionVisitor? {
+        if (type != KlibPropertyExtensionVisitor.TYPE) return null
+        return object : KlibPropertyExtensionVisitor() {
+            override fun visitAnnotation(annotation: KmAnnotation) {
+                proto.addExtension(
+                    KlibMetadataProtoBuf.propertyAnnotation,
+                    annotation.writeAnnotation(c.strings).build()
+                )
+            }
+
+            override fun visitGetterAnnotation(annotation: KmAnnotation) {
+                proto.addExtension(
+                    KlibMetadataProtoBuf.propertyGetterAnnotation,
+                    annotation.writeAnnotation(c.strings).build()
+                )
+            }
+
+            override fun visitSetterAnnotation(annotation: KmAnnotation) {
+                proto.addExtension(
+                    KlibMetadataProtoBuf.propertySetterAnnotation,
+                    annotation.writeAnnotation(c.strings).build()
+                )
+            }
+
+            override fun visitUniqId(uniqId: UniqId) {
+                proto.setExtension(
+                    KlibMetadataProtoBuf.propertyUniqId,
+                    uniqId.writeUniqId().build()
+                )
+            }
+
+            override fun visitFile(file: Int) {
+                proto.setExtension(
+                    KlibMetadataProtoBuf.propertyFile,
+                    file
+                )
+            }
+
+            override fun visitCompileTimeValue(value: KmAnnotationArgument<*>) {
+                proto.setExtension(
+                    KlibMetadataProtoBuf.compileTimeValue,
+                    value.writeAnnotationArgument(c.strings).build()
+                )
+            }
+        }
+    }
+
+    override fun writeConstructorExtensions(
+        type: KmExtensionType,
+        proto: ProtoBuf.Constructor.Builder,
+        c: WriteContext
+    ): KmConstructorExtensionVisitor? {
+        if (type != KlibConstructorExtensionVisitor.TYPE) return null
+        return object : KlibConstructorExtensionVisitor() {
+            override fun visitAnnotation(annotation: KmAnnotation) {
+                proto.addExtension(
+                    KlibMetadataProtoBuf.constructorAnnotation,
+                    annotation.writeAnnotation(c.strings).build()
+                )
+            }
+
+            override fun visitUniqId(uniqId: UniqId) {
+                proto.setExtension(
+                    KlibMetadataProtoBuf.constructorUniqId,
+                    uniqId.writeUniqId().build()
+                )
+            }
+        }
+    }
+
+    override fun writeTypeParameterExtensions(
+        type: KmExtensionType,
+        proto: ProtoBuf.TypeParameter.Builder,
+        c: WriteContext
+    ): KmTypeParameterExtensionVisitor? {
+        if (type != KlibTypeParameterExtensionVisitor.TYPE) return null
+        return object : KlibTypeParameterExtensionVisitor() {
+            override fun visitAnnotation(annotation: KmAnnotation) {
+                proto.addExtension(
+                    KlibMetadataProtoBuf.typeParameterAnnotation,
+                    annotation.writeAnnotation(c.strings).build()
+                )
+            }
+
+            override fun visitUniqId(uniqId: UniqId) {
+                proto.setExtension(
+                    KlibMetadataProtoBuf.typeParamUniqId,
+                    uniqId.writeUniqId().build()
+                )
+            }
+        }
+    }
+
+    override fun writeTypeExtensions(type: KmExtensionType, proto: ProtoBuf.Type.Builder, c: WriteContext): KmTypeExtensionVisitor? {
+        if (type != KlibTypeExtensionVisitor.TYPE) return null
+        return object : KlibTypeExtensionVisitor() {
+            override fun visitAnnotation(annotation: KmAnnotation) {
+                proto.addExtension(
+                    KlibMetadataProtoBuf.typeAnnotation,
+                    annotation.writeAnnotation(c.strings).build()
+                )
+            }
+        }
+    }
+
+    override fun createClassExtension(): KmClassExtension =
+        KlibClassExtension()
+
+    override fun createPackageExtension(): KmPackageExtension =
+        KlibPackageExtension()
+
+    override fun createModuleFragmentExtensions(): KmModuleFragmentExtension =
+        KlibModuleFragmentExtension()
+
+    override fun createFunctionExtension(): KmFunctionExtension =
+        KlibFunctionExtension()
+
+    override fun createPropertyExtension(): KmPropertyExtension =
+        KlibPropertyExtension()
+
+    override fun createConstructorExtension(): KmConstructorExtension =
+        KlibConstructorExtension()
+
+    override fun createTypeParameterExtension(): KmTypeParameterExtension =
+        KlibTypeParameterExtension()
+
+    override fun createTypeExtension(): KmTypeExtension =
+        KlibTypeExtension()
+}
@@ -0,0 +1,28 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib.impl
+
+import kotlinx.metadata.impl.readAnnotation
+import kotlinx.metadata.klib.KlibHeader
+import kotlinx.metadata.klib.KlibSourceFile
+import kotlinx.metadata.klib.UniqId
+import org.jetbrains.kotlin.library.metadata.KlibMetadataProtoBuf
+import org.jetbrains.kotlin.metadata.deserialization.NameResolver
+
+internal fun KlibMetadataProtoBuf.DescriptorUniqId.readUniqId(): UniqId =
+    UniqId(index)
+
+internal fun KlibMetadataProtoBuf.Header.readHeader(strings: NameResolver): KlibHeader =
+    KlibHeader(
+        moduleName,
+        fileList.map(KlibMetadataProtoBuf.File::readFile),
+        packageFragmentNameList,
+        emptyPackageList,
+        annotationList.map { it.readAnnotation(strings) }
+    )
+
+internal fun KlibMetadataProtoBuf.File.readFile(): KlibSourceFile =
+    KlibSourceFile(name)
@@ -0,0 +1,15 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib.impl
+
+import kotlinx.metadata.impl.ReadContextExtension
+import kotlinx.metadata.klib.KlibSourceFile
+
+class SourceFileIndexReadExtension(
+    private val files: List<KlibSourceFile>
+) : ReadContextExtension {
+    fun getSourceFile(index: Int): KlibSourceFile = files[index]
+}
@@ -0,0 +1,36 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib.impl
+
+import kotlinx.metadata.impl.WriteContext
+import kotlinx.metadata.impl.writeAnnotation
+import kotlinx.metadata.klib.KlibHeader
+import kotlinx.metadata.klib.KlibSourceFile
+import kotlinx.metadata.klib.UniqId
+import org.jetbrains.kotlin.library.metadata.KlibMetadataProtoBuf
+import org.jetbrains.kotlin.serialization.StringTableImpl
+
+internal fun UniqId.writeUniqId(): KlibMetadataProtoBuf.DescriptorUniqId.Builder =
+    KlibMetadataProtoBuf.DescriptorUniqId.newBuilder().apply {
+        index = this@writeUniqId.index
+    }
+
+internal fun KlibHeader.writeHeader(context: WriteContext): KlibMetadataProtoBuf.Header.Builder =
+    KlibMetadataProtoBuf.Header.newBuilder().also { proto ->
+        val (strings, qualifiedNames) = (context.strings as StringTableImpl).buildProto()
+        proto.moduleName = moduleName
+        proto.qualifiedNames = qualifiedNames
+        proto.strings = strings
+        proto.addAllPackageFragmentName(packageFragmentName)
+        proto.addAllFile(file.map { it.writeFile().build() })
+        proto.addAllAnnotation(annotation.map { it.writeAnnotation(context.strings).build() })
+        proto.addAllEmptyPackage(emptyPackage)
+    }
+
+internal fun KlibSourceFile.writeFile(): KlibMetadataProtoBuf.File.Builder =
+    KlibMetadataProtoBuf.File.newBuilder().also { proto ->
+        proto.name = name
+    }
@@ -0,0 +1,45 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib.impl
+
+import kotlinx.metadata.impl.*
+import kotlinx.metadata.klib.KlibSourceFile
+import org.jetbrains.kotlin.backend.common.serialization.metadata.KlibMetadataStringTable
+import org.jetbrains.kotlin.library.metadata.KlibMetadataProtoBuf
+import org.jetbrains.kotlin.metadata.ProtoBuf
+
+class ReverseSourceFileIndexWriteExtension : WriteContextExtension {
+    private val filesReverseIndex = mutableMapOf<KlibSourceFile, Int>()
+
+    val fileIndex: List<KlibSourceFile>
+        get() = filesReverseIndex
+            .map { (file, index) -> index to file }
+            .sortedBy { it.first }
+            .map { it.second }
+
+    fun getIndexOf(file: KlibSourceFile): Int = filesReverseIndex.getOrPut(file) {
+        filesReverseIndex.size
+    }
+}
+
+class KlibModuleFragmentWriter(
+    stringTable: KlibMetadataStringTable,
+    contextExtensions: List<WriteContextExtension> = emptyList()
+) : ModuleFragmentWriter(stringTable, contextExtensions) {
+
+    fun write(): ProtoBuf.PackageFragment =
+        t.build()
+
+    override fun visitEnd() {
+        val isPackageEmpty = if (t.`package` == null) {
+            true
+        } else {
+            t.`package`.let { it.functionCount == 0 && it.propertyCount == 0 && it.typeAliasCount == 0 }
+        }
+        val isEmpty = t.class_Count == 0 && isPackageEmpty
+        t.setExtension(KlibMetadataProtoBuf.isEmpty, isEmpty)
+    }
+}
@@ -0,0 +1,105 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib
+
+import kotlinx.metadata.*
+import kotlinx.metadata.klib.impl.klibExtensions
+
+val KmFunction.annotations: MutableList<KmAnnotation>
+    get() = klibExtensions.annotations
+
+var KmFunction.uniqId: UniqId?
+    get() = klibExtensions.uniqId
+    set(value) {
+        klibExtensions.uniqId = value
+    }
+
+var KmFunction.file: KlibSourceFile?
+    get() = klibExtensions.file
+    set(value) {
+        klibExtensions.file = value
+    }
+
+val KmClass.annotations: MutableList<KmAnnotation>
+    get() = klibExtensions.annotations
+
+var KmClass.uniqId: UniqId?
+    get() = klibExtensions.uniqId
+    set(value) {
+        klibExtensions.uniqId = value
+    }
+
+var KmClass.file: KlibSourceFile?
+    get() = klibExtensions.file
+    set(value) {
+        klibExtensions.file = value
+    }
+
+val KmProperty.annotations: MutableList<KmAnnotation>
+    get() = klibExtensions.annotations
+
+val KmProperty.setterAnnotations: MutableList<KmAnnotation>
+    get() = klibExtensions.setterAnnotations
+
+val KmProperty.getterAnnotations: MutableList<KmAnnotation>
+    get() = klibExtensions.getterAnnotations
+
+var KmProperty.uniqId: UniqId?
+    get() = klibExtensions.uniqId
+    set(value) {
+        klibExtensions.uniqId = value
+    }
+
+var KmProperty.file: Int?
+    get() = klibExtensions.file
+    set(value) {
+        klibExtensions.file = value
+    }
+
+var KmProperty.compileTimeValue: KmAnnotationArgument<*>?
+    get() = klibExtensions.compileTimeValue
+    set(value) {
+        klibExtensions.compileTimeValue = value
+    }
+
+val KmType.annotations: MutableList<KmAnnotation>
+    get() = klibExtensions.annotations
+
+val KmConstructor.annotations: MutableList<KmAnnotation>
+    get() = klibExtensions.annotations
+
+var KmConstructor.uniqId: UniqId?
+    get() = klibExtensions.uniqId
+    set(value) {
+        klibExtensions.uniqId = value
+    }
+
+var KmPackage.fqName: String?
+    get() = klibExtensions.fqName
+    set(value) {
+        klibExtensions.fqName = value
+    }
+
+var KmModuleFragment.fqName: String?
+    get() = klibExtensions.fqName
+    set(value) {
+        klibExtensions.fqName = value
+    }
+
+val KmModuleFragment.className: MutableList<ClassName>?
+    get() = klibExtensions.className
+
+val KmModuleFragment.moduleFragmentFiles: MutableList<KlibSourceFile>?
+    get() = klibExtensions.moduleFragmentFiles
+
+val KmTypeParameter.annotations: MutableList<KmAnnotation>
+    get() = klibExtensions.annotations
+
+var KmTypeParameter.uniqId: UniqId?
+    get() = klibExtensions.uniqId
+    set(value) {
+        klibExtensions.uniqId = value
+    }
@@ -0,0 +1,130 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlinx.metadata.klib
+
+import kotlinx.metadata.*
+
+abstract class KlibFunctionExtensionVisitor : KmFunctionExtensionVisitor {
+
+    abstract fun visitAnnotation(annotation: KmAnnotation)
+
+    abstract fun visitUniqId(uniqId: UniqId)
+
+    abstract fun visitFile(file: KlibSourceFile)
+
+    override val type: KmExtensionType
+        get() = TYPE
+
+    companion object {
+        val TYPE = KmExtensionType(KlibFunctionExtensionVisitor::class)
+    }
+}
+
+abstract class KlibClassExtensionVisitor : KmClassExtensionVisitor {
+
+    abstract fun visitAnnotation(annotation: KmAnnotation)
+
+    abstract fun visitUniqId(uniqId: UniqId)
+
+    abstract fun visitFile(file: KlibSourceFile)
+
+    override val type: KmExtensionType
+        get() = TYPE
+
+    companion object {
+        val TYPE = KmExtensionType(KlibClassExtensionVisitor::class)
+    }
+}
+
+abstract class KlibTypeExtensionVisitor : KmTypeExtensionVisitor {
+
+    abstract fun visitAnnotation(annotation: KmAnnotation)
+
+    override val type: KmExtensionType
+        get() = TYPE
+
+    companion object {
+        val TYPE = KmExtensionType(KlibTypeExtensionVisitor::class)
+    }
+}
+
+abstract class KlibPropertyExtensionVisitor : KmPropertyExtensionVisitor {
+
+    abstract fun visitAnnotation(annotation: KmAnnotation)
+
+    abstract fun visitGetterAnnotation(annotation: KmAnnotation)
+
+    abstract fun visitSetterAnnotation(annotation: KmAnnotation)
+
+    abstract fun visitFile(file: Int)
+
+    abstract fun visitUniqId(uniqId: UniqId)
+
+    abstract fun visitCompileTimeValue(value: KmAnnotationArgument<*>)
+
+    override val type: KmExtensionType
+        get() = TYPE
+
+    companion object {
+        val TYPE = KmExtensionType(KlibPropertyExtensionVisitor::class)
+    }
+}
+
+abstract class KlibConstructorExtensionVisitor : KmConstructorExtensionVisitor {
+
+    abstract fun visitAnnotation(annotation: KmAnnotation)
+
+    abstract fun visitUniqId(uniqId: UniqId)
+
+    override val type: KmExtensionType
+        get() = TYPE
+
+    companion object {
+        val TYPE = KmExtensionType(KlibConstructorExtensionVisitor::class)
+    }
+}
+
+abstract class KlibTypeParameterExtensionVisitor : KmTypeParameterExtensionVisitor {
+
+    abstract fun visitAnnotation(annotation: KmAnnotation)
+
+    abstract fun visitUniqId(uniqId: UniqId)
+
+    override val type: KmExtensionType
+        get() = TYPE
+
+    companion object {
+        val TYPE = KmExtensionType(KlibTypeParameterExtensionVisitor::class)
+    }
+}
+
+abstract class KlibPackageExtensionVisitor : KmPackageExtensionVisitor {
+
+    abstract fun visitFqName(name: String)
+
+    override val type: KmExtensionType
+        get() = TYPE
+
+    companion object {
+        val TYPE = KmExtensionType(KlibPackageExtensionVisitor::class)
+    }
+}
+
+abstract class KlibModuleFragmentExtensionVisitor : KmModuleFragmentExtensionVisitor {
+
+    abstract fun visitFile(file: KlibSourceFile)
+
+    abstract fun visitFqName(fqName: String)
+
+    abstract fun visitClassName(className: ClassName)
+
+    override val type: KmExtensionType
+        get() = TYPE
+
+    companion object {
+        val TYPE = KmExtensionType(KlibModuleFragmentExtensionVisitor::class)
+    }
+}
@@ -64,6 +64,11 @@ interface KmClassExtensionVisitor : KmDeclarationContainerExtensionVisitor
 */
 interface KmPackageExtensionVisitor : KmDeclarationContainerExtensionVisitor

+/**
+ * A visitor to visit platform-specific extensions for a module fragment.
+ */
+interface KmModuleFragmentExtensionVisitor : KmExtensionVisitor
+
 /**
 * A visitor to visit platform-specific extensions for a function.
 */
@@ -16,6 +16,8 @@ interface MetadataExtensions {

    fun readPackageExtensions(v: KmPackageVisitor, proto: ProtoBuf.Package, c: ReadContext)

+    fun readModuleFragmentExtensions(v: KmModuleFragmentVisitor, proto: ProtoBuf.PackageFragment, c: ReadContext)
+
    fun readFunctionExtensions(v: KmFunctionVisitor, proto: ProtoBuf.Function, c: ReadContext)

    fun readPropertyExtensions(v: KmPropertyVisitor, proto: ProtoBuf.Property, c: ReadContext)
@@ -30,6 +32,10 @@ interface MetadataExtensions {

    fun writePackageExtensions(type: KmExtensionType, proto: ProtoBuf.Package.Builder, c: WriteContext): KmPackageExtensionVisitor?

+    fun writeModuleFragmentExtensions(
+        type: KmExtensionType, proto: ProtoBuf.PackageFragment.Builder, c: WriteContext
+    ): KmModuleFragmentExtensionVisitor?
+
    fun writeFunctionExtensions(type: KmExtensionType, proto: ProtoBuf.Function.Builder, c: WriteContext): KmFunctionExtensionVisitor?

    fun writePropertyExtensions(type: KmExtensionType, proto: ProtoBuf.Property.Builder, c: WriteContext): KmPropertyExtensionVisitor?
@@ -48,6 +54,8 @@ interface MetadataExtensions {

    fun createPackageExtension(): KmPackageExtension

+    fun createModuleFragmentExtensions(): KmModuleFragmentExtension
+
    fun createFunctionExtension(): KmFunctionExtension

    fun createPropertyExtension(): KmPropertyExtension
@@ -15,6 +15,8 @@ interface KmClassExtension : KmClassExtensionVisitor, KmExtension<KmClassExtensi

 interface KmPackageExtension : KmPackageExtensionVisitor, KmExtension<KmPackageExtensionVisitor>

+interface KmModuleFragmentExtension : KmModuleFragmentExtensionVisitor, KmExtension<KmModuleFragmentExtensionVisitor>
+
 interface KmFunctionExtension : KmFunctionExtensionVisitor, KmExtension<KmFunctionExtensionVisitor>

 interface KmPropertyExtension : KmPropertyExtensionVisitor, KmExtension<KmPropertyExtensionVisitor>
@@ -23,7 +23,7 @@ fun ProtoBuf.Annotation.readAnnotation(strings: NameResolver): KmAnnotation =
        }.toMap()
    )

-private fun ProtoBuf.Annotation.Argument.Value.readAnnotationArgument(strings: NameResolver): KmAnnotationArgument<*>? {
+fun ProtoBuf.Annotation.Argument.Value.readAnnotationArgument(strings: NameResolver): KmAnnotationArgument<*>? {
    if (Flags.IS_UNSIGNED[flags]) {
        return when (type) {
            BYTE -> KmAnnotationArgument.UByteValue(intValue.toByte())
@@ -11,15 +11,23 @@ import org.jetbrains.kotlin.metadata.ProtoBuf
 import org.jetbrains.kotlin.metadata.deserialization.*
 import org.jetbrains.kotlin.metadata.deserialization.Flags as F

+/**
+ * Allows to populate [BasicReadContext] with additional data
+ * that can be used when reading metadata in [MetadataExtensions].
+ */
+interface ReadContextExtension
+
 class ReadContext(
    val strings: NameResolver,
    val types: TypeTable,
    internal val versionRequirements: VersionRequirementTable,
-    private val parent: ReadContext? = null
+    private val parent: ReadContext? = null,
+    val contextExtensions: List<ReadContextExtension> = emptyList()
 ) {
-    internal val extensions = MetadataExtensions.INSTANCES
    private val typeParameterNameToId = mutableMapOf<Int, Int>()

+    internal val extensions = MetadataExtensions.INSTANCES
+
    operator fun get(index: Int): String =
        strings.getString(index)

@@ -30,16 +38,24 @@ class ReadContext(
        typeParameterNameToId[name] ?: parent?.getTypeParameterId(name)

    fun withTypeParameters(typeParameters: List<ProtoBuf.TypeParameter>): ReadContext =
-        ReadContext(strings, types, versionRequirements, this).apply {
+        ReadContext(strings, types, versionRequirements, this, contextExtensions).apply {
            for (typeParameter in typeParameters) {
                typeParameterNameToId[typeParameter.name] = typeParameter.id
            }
        }
 }

-fun ProtoBuf.Class.accept(v: KmClassVisitor, strings: NameResolver) {
-    val c = ReadContext(strings, TypeTable(typeTable), VersionRequirementTable.create(versionRequirementTable))
-        .withTypeParameters(typeParameterList)
+fun ProtoBuf.Class.accept(
+    v: KmClassVisitor,
+    strings: NameResolver,
+    contextExtensions: List<ReadContextExtension> = emptyList()
+) {
+    val c = ReadContext(
+        strings,
+        TypeTable(typeTable),
+        VersionRequirementTable.create(versionRequirementTable),
+        contextExtensions = contextExtensions
+    ).withTypeParameters(typeParameterList)

    v.visit(flags, c.className(fqName))

@@ -87,8 +103,17 @@ fun ProtoBuf.Class.accept(v: KmClassVisitor, strings: NameResolver) {
    v.visitEnd()
 }

-fun ProtoBuf.Package.accept(v: KmPackageVisitor, strings: NameResolver) {
-    val c = ReadContext(strings, TypeTable(typeTable), VersionRequirementTable.create(versionRequirementTable))
+fun ProtoBuf.Package.accept(
+    v: KmPackageVisitor,
+    strings: NameResolver,
+    contextExtensions: List<ReadContextExtension> = emptyList()
+) {
+    val c = ReadContext(
+        strings,
+        TypeTable(typeTable),
+        VersionRequirementTable.create(versionRequirementTable),
+        contextExtensions = contextExtensions
+    )

    v.visitDeclarations(functionList, propertyList, typeAliasList, c)

@@ -99,6 +124,31 @@ fun ProtoBuf.Package.accept(v: KmPackageVisitor, strings: NameResolver) {
    v.visitEnd()
 }

+fun ProtoBuf.PackageFragment.accept(
+    v: KmModuleFragmentVisitor,
+    strings: NameResolver,
+    contextExtensions: List<ReadContextExtension> = emptyList()
+) {
+    val c = ReadContext(
+        strings,
+        TypeTable(ProtoBuf.TypeTable.newBuilder().build()),
+        VersionRequirementTable.EMPTY,
+        contextExtensions = contextExtensions
+    )
+
+    v.visitPackage()?.let { `package`.accept(it, strings, contextExtensions) }
+
+    class_List.forEach { clazz ->
+        v.visitClass()?.let { clazz.accept(it, strings, contextExtensions) }
+    }
+
+    for (extension in c.extensions) {
+        extension.readModuleFragmentExtensions(v, this, c)
+    }
+
+    v.visitEnd()
+}
+
 private fun KmDeclarationContainerVisitor.visitDeclarations(
    functions: List<ProtoBuf.Function>,
    properties: List<ProtoBuf.Property>,
@@ -24,7 +24,7 @@ fun KmAnnotation.writeAnnotation(strings: StringTable): ProtoBuf.Annotation.Buil
        }
    }

-private fun KmAnnotationArgument<*>.writeAnnotationArgument(strings: StringTable): ProtoBuf.Annotation.Argument.Value.Builder =
+fun KmAnnotationArgument<*>.writeAnnotationArgument(strings: StringTable): ProtoBuf.Annotation.Argument.Value.Builder =
    ProtoBuf.Annotation.Argument.Value.newBuilder().apply {
        when (this@writeAnnotationArgument) {
            is KmAnnotationArgument.ByteValue -> {
@@ -6,13 +6,20 @@
 package kotlinx.metadata.impl

 import kotlinx.metadata.*
+import kotlinx.metadata.impl.extensions.MetadataExtensions
 import kotlinx.metadata.impl.extensions.applySingleExtension
 import org.jetbrains.kotlin.metadata.ProtoBuf
 import org.jetbrains.kotlin.metadata.deserialization.VersionRequirement
 import org.jetbrains.kotlin.metadata.serialization.MutableVersionRequirementTable
 import org.jetbrains.kotlin.metadata.serialization.StringTable

-class WriteContext(val strings: StringTable) {
+/**
+ * Allows to populate [WriteContext] with additional data
+ * that can be used when writing metadata in [MetadataExtensions].
+ */
+interface WriteContextExtension
+
+open class WriteContext(val strings: StringTable, val contextExtensions: List<WriteContextExtension> = emptyList()) {
    val versionRequirements: MutableVersionRequirementTable = MutableVersionRequirementTable()

    operator fun get(string: String): Int =
@@ -393,9 +400,9 @@ private fun writeEffectExpression(c: WriteContext, output: (ProtoBuf.Expression.
        }
    }

-open class ClassWriter(stringTable: StringTable) : KmClassVisitor() {
+open class ClassWriter(stringTable: StringTable, contextExtensions: List<WriteContextExtension> = emptyList()) : KmClassVisitor() {
    protected val t = ProtoBuf.Class.newBuilder()!!
-    protected val c = WriteContext(stringTable)
+    protected val c: WriteContext = WriteContext(stringTable, contextExtensions)

    override fun visit(flags: Flags, name: ClassName) {
        if (flags != ProtoBuf.Class.getDefaultInstance().flags) {
@@ -455,9 +462,9 @@ open class ClassWriter(stringTable: StringTable) : KmClassVisitor() {
    }
 }

-open class PackageWriter(stringTable: StringTable) : KmPackageVisitor() {
+open class PackageWriter(stringTable: StringTable, contextExtensions: List<WriteContextExtension> = emptyList()) : KmPackageVisitor() {
    protected val t = ProtoBuf.Package.newBuilder()!!
-    protected val c = WriteContext(stringTable)
+    protected val c: WriteContext = WriteContext(stringTable, contextExtensions)

    override fun visitFunction(flags: Flags, name: String): KmFunctionVisitor? =
        writeFunction(c, flags, name) { t.addFunction(it) }
@@ -480,6 +487,31 @@ open class PackageWriter(stringTable: StringTable) : KmPackageVisitor() {
    }
 }

+open class ModuleFragmentWriter(stringTable: StringTable, contextExtensions: List<WriteContextExtension> = emptyList()) :
+    KmModuleFragmentVisitor() {
+    protected val t = ProtoBuf.PackageFragment.newBuilder()
+    protected val c: WriteContext = WriteContext(stringTable, contextExtensions)
+
+    override fun visitPackage(): KmPackageVisitor? = object : PackageWriter(c.strings, c.contextExtensions) {
+        override fun visitEnd() {
+            super.visitEnd()
+            this@ModuleFragmentWriter.t.setPackage(t)
+        }
+    }
+
+    override fun visitClass(): KmClassVisitor? = object : ClassWriter(c.strings, c.contextExtensions) {
+        override fun visitEnd() {
+            super.visitEnd()
+            this@ModuleFragmentWriter.t.addClass_(t)
+        }
+    }
+
+    override fun visitExtensions(type: KmExtensionType): KmModuleFragmentExtensionVisitor? =
+        applySingleExtension(type) {
+            writeModuleFragmentExtensions(type, t, c)
+        }
+}
+
 open class LambdaWriter(stringTable: StringTable) : KmLambdaVisitor() {
    protected var t: ProtoBuf.Function.Builder? = null
    protected val c = WriteContext(stringTable)
@@ -217,6 +217,46 @@ class KmPackage : KmPackageVisitor(), KmDeclarationContainer {
    }
 }

+/**
+ * Represents a Kotlin module fragment. This is used to represent metadata of a part of a module on platforms other than JVM.
+ */
+class KmModuleFragment : KmModuleFragmentVisitor() {
+
+    /**
+     * Top-level functions, type aliases and properties in the module fragment.
+     */
+    var pkg: KmPackage? = null
+
+    /**
+     * Classes in the module fragment.
+     */
+    val classes: MutableList<KmClass> = ArrayList()
+
+    private val extensions: List<KmModuleFragmentExtension> =
+        MetadataExtensions.INSTANCES.map(MetadataExtensions::createModuleFragmentExtensions)
+
+    override fun visitPackage(): KmPackageVisitor? =
+        KmPackage().also { pkg = it }
+
+    override fun visitExtensions(type: KmExtensionType): KmModuleFragmentExtensionVisitor? =
+        extensions.singleOfType(type)
+
+    override fun visitClass(): KmClassVisitor? =
+        KmClass().addTo(classes)
+
+    /**
+     * Populates the given visitor with data in this module fragment.
+     *
+     * @param visitor the visitor which will visit data in the module fragment.
+     */
+    fun accept(visitor: KmModuleFragmentVisitor) {
+        pkg?.let { visitor.visitPackage()?.let(it::accept) }
+        classes.forEach { visitor.visitClass()?.let(it::accept) }
+        extensions.forEach { visitor.visitExtensions(it.type)?.let(it::accept) }
+        visitor.visitEnd()
+    }
+}
+
 /**
 * Represents a synthetic class generated for a Kotlin lambda.
 */
@@ -179,6 +179,42 @@ abstract class KmPackageVisitor @JvmOverloads constructor(delegate: KmPackageVis
    }
 }

+/**
+ * A visitor to visit module fragments. The module fragment can have no more than one package, and any number of classes,
+ * and must have at least one declaration.
+ *
+ * When using this class, [visitEnd] must be called exactly once and after calls to all other visit* methods.
+ */
+abstract class KmModuleFragmentVisitor @JvmOverloads constructor(private val delegate: KmModuleFragmentVisitor? = null) {
+
+    /**
+     * Visits a package within the module fragment.
+     */
+    open fun visitPackage(): KmPackageVisitor? =
+        delegate?.visitPackage()
+
+    /**
+     * Visits a class within the module fragment.
+     */
+    open fun visitClass(): KmClassVisitor? =
+        delegate?.visitClass()
+
+    /**
+     * Visits the extensions of the given type on the module fragment.
+     *
+     * @param type the type of extension visitor to be returned.
+     */
+    open fun visitExtensions(type: KmExtensionType): KmModuleFragmentExtensionVisitor? =
+        delegate?.visitExtensions(type)
+
+    /**
+     * Visits the end of the module fragment.
+     */
+    open fun visitEnd() {
+        delegate?.visitEnd()
+    }
+}
+
 /**
 * A visitor to visit the metadata of a synthetic class generated for a Kotlin lambda.
 *
@@ -159,6 +159,7 @@ include ":kotlin-build-common",
        ":kotlin-source-map-loader",
        ":kotlinx-metadata",
        ":kotlinx-metadata-jvm",
+        ":kotlinx-metadata-klib",
        ":prepare:build.version",
        ":kotlin-build-common",
        ":prepare:formatter",
@@ -324,6 +325,7 @@ project(':kotlin-reflect').projectDir = "$rootDir/libraries/reflect" as File
 project(':kotlin-reflect-api').projectDir = "$rootDir/libraries/reflect/api" as File
 project(':kotlinx-metadata').projectDir = "$rootDir/libraries/kotlinx-metadata" as File
 project(':kotlinx-metadata-jvm').projectDir = "$rootDir/libraries/kotlinx-metadata/jvm" as File
+project(':kotlinx-metadata-klib').projectDir = "$rootDir/libraries/kotlinx-metadata/klib" as File
 project(':kotlin-compiler').projectDir = "$rootDir/prepare/compiler" as File
 project(':kotlin-compiler-embeddable').projectDir = "$rootDir/prepare/compiler-embeddable" as File
 project(':kotlin-compiler-client-embeddable').projectDir = "$rootDir/prepare/compiler-client-embeddable" as File
				`@@ -0,0 +1 @@`
				`kotlinx.metadata.klib.impl.KlibMetadataExtensions`