Added support of external serializers to the FIR

Co-authored-by: Leonid Startsev <leonid.startsev@jetbrains.com>
Merge-request: KT-MR-7334
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrPluginContext.kt

@@ -52,6 +52,8 @@ class Fir2IrPluginContext(
     override val typeTranslator: TypeTranslator
         get() = error(ERROR_MESSAGE)
 
+    override val afterK2: Boolean = true
+
     override val languageVersionSettings: LanguageVersionSettings
         get() = components.session.languageVersionSettings
 

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/extensions/IrPluginContext.kt

@@ -34,6 +34,11 @@ annotation class FirIncompatiblePluginAPI(val hint: String = "")
 interface IrPluginContext : IrGeneratorContext {
     val languageVersionSettings: LanguageVersionSettings
 
+    /**
+     * Indicates that the plugin works after FIR. Effectively it means that all descriptor-based API may contain incorrect and/or incomplete information, and declarations marked with `@FirIncompatibleApi` will throw runtime exceptions.
+     */
+    val afterK2: Boolean
+
     @ObsoleteDescriptorBasedAPI
     val moduleDescriptor: ModuleDescriptor
 

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/extensions/IrPluginContextImpl.kt

@@ -42,6 +42,8 @@ open class IrPluginContextImpl constructor(
     override val symbols: BuiltinSymbolsBase = BuiltinSymbolsBase(irBuiltIns, st)
 ) : IrPluginContext {
 
+    override val afterK2: Boolean = false
+
     override val platform: TargetPlatform? = module.platform
 
     @OptIn(ObsoleteDescriptorBasedAPI::class)

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrPreGenerator.kt

@@ -15,12 +15,9 @@ import org.jetbrains.kotlin.ir.declarations.IrClass
 import org.jetbrains.kotlin.ir.types.*
 import org.jetbrains.kotlin.ir.util.companionObject
 import org.jetbrains.kotlin.name.Name
-import org.jetbrains.kotlin.resolve.isInlineClass
-import org.jetbrains.kotlinx.serialization.compiler.backend.common.SerializerCodegen
 import org.jetbrains.kotlinx.serialization.compiler.extensions.SerializationPluginContext
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerialEntityNames
 import org.jetbrains.kotlinx.serialization.compiler.resolve.bitMaskSlotCount
-import org.jetbrains.kotlinx.serialization.compiler.resolve.hasCompanionObjectAsSerializer
 
 /**
  * Generates only specific declarations, but NOT their bodies.
@@ -43,7 +40,12 @@ class IrPreGenerator(
     private fun preGenerateWriteSelfMethodIfNeeded() {
         if (!irClass.isInternalSerializable) return
         val serializerDescriptor = irClass.classSerializer(compilerContext)?.owner ?: return
-        if (!irClass.shouldHaveSpecificSyntheticMethods { serializerDescriptor.findPluginGeneratedMethod(SerialEntityNames.SAVE) }) return
+        if (!irClass.shouldHaveSpecificSyntheticMethods {
+                serializerDescriptor.findPluginGeneratedMethod(
+                    SerialEntityNames.SAVE,
+                    compilerContext.afterK2
+                )
+            }) return
         if (irClass.findWriteSelfMethod() != null) return
         val method = irClass.addFunction {
             name = SerialEntityNames.WRITE_SELF_NAME
@@ -95,7 +97,7 @@ class IrPreGenerator(
         if (!irClass.isInternalSerializable) return
         // do not add synthetic deserialization constructor if .deserialize method is customized
         if (irClass.hasCompanionObjectAsSerializer && irClass.companionObject()
-                ?.findPluginGeneratedMethod(SerialEntityNames.LOAD) == null
+                ?.findPluginGeneratedMethod(SerialEntityNames.LOAD, compilerContext.afterK2) == null
         ) return
         if (irClass.isValue) return
         if (irClass.findSerializableSyntheticConstructor() != null) return
@@ -133,4 +135,4 @@ class IrPreGenerator(
         }
     }
 
-}
+}

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrPredicates.kt

@@ -57,9 +57,9 @@ internal val IrClass.isInternallySerializableObject: Boolean
     get() = kind == ClassKind.OBJECT && hasSerializableOrMetaAnnotationWithoutArgs()
 
 
-internal fun IrClass.findPluginGeneratedMethod(name: String): IrSimpleFunction? {
+internal fun IrClass.findPluginGeneratedMethod(name: String, afterK2: Boolean): IrSimpleFunction? {
     return this.functions.find {
-        it.name.asString() == name && it.isFromPlugin()
+        it.name.asString() == name && it.isFromPlugin(afterK2)
     }
 }
 
@@ -147,8 +147,13 @@ fun IrClass.getSuperClassNotAny(): IrClass? {
 }
 
 @OptIn(ObsoleteDescriptorBasedAPI::class)
-internal fun IrDeclaration.isFromPlugin(): Boolean =
-    this.origin == IrDeclarationOrigin.GeneratedByPlugin(SerializationPluginKey) || (this.descriptor as? CallableMemberDescriptor)?.kind == CallableMemberDescriptor.Kind.SYNTHESIZED // old FE doesn't specify origin
+internal fun IrDeclaration.isFromPlugin(afterK2: Boolean): Boolean =
+    if (afterK2) {
+        this.origin == IrDeclarationOrigin.GeneratedByPlugin(SerializationPluginKey)
+    } else {
+        // old FE doesn't specify custom origin
+        (this.descriptor as? CallableMemberDescriptor)?.kind == CallableMemberDescriptor.Kind.SYNTHESIZED
+    }
 
 internal fun IrConstructor.isSerializationCtor(): Boolean {
     /*kind == CallableMemberDescriptor.Kind.SYNTHESIZED does not work because DeserializedClassConstructorDescriptor loses its kind*/
@@ -248,4 +253,4 @@ fun IrSimpleType.argumentTypesOrUpperBounds(): List<IrType> {
 }
 
 internal inline fun IrClass.shouldHaveSpecificSyntheticMethods(functionPresenceChecker: () -> IrSimpleFunction?) =
-    !isValue && (isAbstractOrSealedSerializableClass || functionPresenceChecker() != null)
+    !isValue && (isAbstractOrSealedSerializableClass || functionPresenceChecker() != null)

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializableCompanionIrGenerator.kt

@@ -157,7 +157,7 @@ class SerializableCompanionIrGenerator(
             it.valueParameters.size == 1
                     && it.valueParameters.first().isVararg
                     && it.returnType.isKSerializer()
-                    && it.isFromPlugin()
+                    && it.isFromPlugin(compilerContext.afterK2)
         }?.let { return it }
         val kSerializerStarType = compilerContext.getClassFromRuntime(SerialEntityNames.KSERIALIZER_CLASS).starProjectedType
         val f = irClass.addFunction(

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializableIrGenerator.kt

@@ -359,7 +359,7 @@ class SerializableIrGenerator(
 
     private fun generateSyntheticInternalConstructor() {
         val serializerDescriptor = irClass.classSerializer(compilerContext)?.owner ?: return
-        if (irClass.shouldHaveSpecificSyntheticMethods { serializerDescriptor.findPluginGeneratedMethod(LOAD) }) {
+        if (irClass.shouldHaveSpecificSyntheticMethods { serializerDescriptor.findPluginGeneratedMethod(LOAD, compilerContext.afterK2) }) {
             val constrDesc = irClass.constructors.find(IrConstructor::isSerializationCtor) ?: return
             generateInternalConstructor(constrDesc)
         }
@@ -367,7 +367,7 @@ class SerializableIrGenerator(
 
     private fun generateSyntheticMethods() {
         val serializerDescriptor = irClass.classSerializer(compilerContext)?.owner ?: return
-        if (irClass.shouldHaveSpecificSyntheticMethods { serializerDescriptor.findPluginGeneratedMethod(SAVE) }) {
+        if (irClass.shouldHaveSpecificSyntheticMethods { serializerDescriptor.findPluginGeneratedMethod(SAVE, compilerContext.afterK2) }) {
             val func = irClass.findWriteSelfMethod() ?: return
             func.origin = SERIALIZATION_PLUGIN_ORIGIN
             generateWriteSelfMethod(func)

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt

@@ -29,7 +29,6 @@ import org.jetbrains.kotlin.ir.types.*
 import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
-import org.jetbrains.kotlin.resolve.isInlineClass
 import org.jetbrains.kotlin.util.OperatorNameConventions
 import org.jetbrains.kotlin.utils.addToStdlib.cast
 import org.jetbrains.kotlinx.serialization.compiler.extensions.SerializationDescriptorSerializerPlugin
@@ -63,7 +62,7 @@ open class SerializerIrGenerator(
     protected val generatedSerialDescPropertyDescriptor = getProperty(
         SerialEntityNames.SERIAL_DESC_FIELD,
         { true }
-    )?.takeIf { it.isFromPlugin() }
+    )?.takeIf { it.isFromPlugin(compilerContext.afterK2) }
 
     protected val anySerialDescProperty = getProperty(
         SerialEntityNames.SERIAL_DESC_FIELD,
@@ -592,15 +591,17 @@ open class SerializerIrGenerator(
         val prop = generatedSerialDescPropertyDescriptor?.let { generateSerializableClassProperty(it); true } ?: false
         if (prop)
             generateSerialDesc()
-        val save = irClass.findPluginGeneratedMethod(SAVE)?.let { generateSave(it); true } ?: false
-        val load = irClass.findPluginGeneratedMethod(LOAD)?.let { generateLoad(it); true } ?: false
-        irClass.findPluginGeneratedMethod(SerialEntityNames.CHILD_SERIALIZERS_GETTER.identifier)?.let { generateChildSerializersGetter(it) }
-        irClass.findPluginGeneratedMethod(SerialEntityNames.TYPE_PARAMS_SERIALIZERS_GETTER.identifier)
+        val withFir = compilerContext.afterK2
+        val save = irClass.findPluginGeneratedMethod(SAVE, withFir)?.let { generateSave(it); true } ?: false
+        val load = irClass.findPluginGeneratedMethod(LOAD, withFir)?.let { generateLoad(it); true } ?: false
+        irClass.findPluginGeneratedMethod(SerialEntityNames.CHILD_SERIALIZERS_GETTER.identifier, withFir)
+            ?.let { generateChildSerializersGetter(it) }
+        irClass.findPluginGeneratedMethod(SerialEntityNames.TYPE_PARAMS_SERIALIZERS_GETTER.identifier, withFir)
             ?.let { generateTypeParamsSerializersGetter(it) }
         if (!prop && (save || load))
             generateSerialDesc()
         if (serializableIrClass.typeParameters.isNotEmpty()) {
-            findSerializerConstructorForTypeArgumentsSerializers(irClass)?.takeIf { it.owner.isFromPlugin() }?.let {
+            findSerializerConstructorForTypeArgumentsSerializers(irClass)?.takeIf { it.owner.isFromPlugin(withFir) }?.let {
                 generateGenericFieldsAndConstructor(it.owner)
             }
         }

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/extensions/SerializationLoweringExtension.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.backend.common.CompilationException
 import org.jetbrains.kotlin.backend.common.extensions.IrGenerationExtension
 import org.jetbrains.kotlin.backend.common.extensions.IrIntrinsicExtension
 import org.jetbrains.kotlin.backend.common.extensions.IrPluginContext
+import org.jetbrains.kotlin.backend.common.extensions.IrPluginContextImpl
 import org.jetbrains.kotlin.backend.common.runOnFilePostfix
 import org.jetbrains.kotlin.backend.jvm.JvmBackendContext
 import org.jetbrains.kotlin.backend.jvm.ir.fileParent

plugins/kotlinx-serialization/kotlinx-serialization.common/src/org/jetbrains/kotlinx/serialization/compiler/resolve/NamingConventions.kt

@@ -198,12 +198,14 @@ object SerializationDependencies {
 }
 
 object SerializersClassIds {
+    val kSerializerId = ClassId(SerializationPackages.packageFqName, SerialEntityNames.KSERIALIZER_NAME)
     val enumSerializerId = ClassId(SerializationPackages.internalPackageFqName, Name.identifier(SpecialBuiltins.enumSerializer))
     val polymorphicSerializerId = ClassId(SerializationPackages.packageFqName, Name.identifier(SpecialBuiltins.polymorphicSerializer))
     val referenceArraySerializerId = ClassId(SerializationPackages.internalPackageFqName, Name.identifier(SpecialBuiltins.referenceArraySerializer))
     val objectSerializerId = ClassId(SerializationPackages.internalPackageFqName, Name.identifier(SpecialBuiltins.objectSerializer))
     val sealedSerializerId = ClassId(SerializationPackages.packageFqName, Name.identifier(SpecialBuiltins.sealedSerializer))
     val contextSerializerId = ClassId(SerializationPackages.packageFqName, Name.identifier(SpecialBuiltins.contextSerializer))
+    val generatedSerializerId = ClassId(SerializationPackages.internalPackageFqName, SerialEntityNames.GENERATED_SERIALIZER_CLASS)
 }
 
 fun findStandardKotlinTypeSerializerName(typeName: String?): String? {

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/SerializationFirResolveExtension.kt

@@ -39,9 +39,8 @@ import org.jetbrains.kotlin.platform.konan.isNative
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerialEntityNames
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerialEntityNames.SERIALIZER_FACTORY_INTERFACE_NAME
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerializationPackages
-
-val generatedSerializerClassId = ClassId(SerializationPackages.internalPackageFqName, SerialEntityNames.GENERATED_SERIALIZER_CLASS)
-val kSerializerClassId = ClassId(SerializationPackages.packageFqName, SerialEntityNames.KSERIALIZER_NAME)
+import org.jetbrains.kotlinx.serialization.compiler.resolve.SerializersClassIds.generatedSerializerId
+import org.jetbrains.kotlinx.serialization.compiler.resolve.SerializersClassIds.kSerializerId
 
 class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGenerationExtension(session) {
 
@@ -62,7 +61,7 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
         with(session) {
             if (classSymbol.shouldHaveGeneratedMethodsInCompanion && !classSymbol.isSerializableObject)
                 result += SpecialNames.DEFAULT_NAME_FOR_COMPANION_OBJECT
-            if (classSymbol.shouldHaveGeneratedSerializer(!runtimeHasEnumSerializerFactory) /* TODO && !classSymbol.hasCompanionObjectAsSerializer*/)
+            if (classSymbol.shouldHaveGeneratedSerializer && !classSymbol.hasCompanionObjectAsSerializer)
                 result += SerialEntityNames.SERIALIZER_CLASS_NAME
         }
 
@@ -85,31 +84,24 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
     override fun getCallableNamesForClass(classSymbol: FirClassSymbol<*>): Set<Name> {
         val classId = classSymbol.classId
         val result = mutableSetOf<Name>()
-        when (classId.shortClassName) {
-            SpecialNames.DEFAULT_NAME_FOR_COMPANION_OBJECT -> {
-                val origin = classSymbol.origin as? FirDeclarationOrigin.Plugin
-                if (origin?.key == SerializationPluginKey) {
-                    result += SpecialNames.INIT
-                    result += SerialEntityNames.SERIALIZER_PROVIDER_NAME
-                } else {
-                    // TODO: handle user-written companions & named companions
-                }
+
+        val origin = classSymbol.origin as? FirDeclarationOrigin.Plugin
+
+        when {
+            classSymbol.isCompanion && origin?.key == SerializationPluginKey -> {
+                result += SpecialNames.INIT
+                result += SerialEntityNames.SERIALIZER_PROVIDER_NAME
             }
-            SerialEntityNames.SERIALIZER_CLASS_NAME -> {
+
+            classId.shortClassName == SerialEntityNames.SERIALIZER_CLASS_NAME -> {
                 // TODO: check classSymbol for already added functions
-                // TODO: support user-defined serializers?
                 result += setOf(
                     SpecialNames.INIT,
                     SerialEntityNames.SAVE_NAME,
                     SerialEntityNames.LOAD_NAME,
                     SerialEntityNames.SERIAL_DESC_FIELD_NAME
                 )
-                if (classSymbol.resolvedSuperTypes.any {
-                        it.classId == ClassId(
-                            SerializationPackages.internalPackageFqName,
-                            SerialEntityNames.GENERATED_SERIALIZER_CLASS
-                        )
-                    }) {
+                if (classSymbol.resolvedSuperTypes.any { it.classId == generatedSerializerId }) {
                     result += SerialEntityNames.CHILD_SERIALIZERS_GETTER
 
                     if (classSymbol.typeParameterSymbols.isNotEmpty()) {
@@ -118,7 +110,28 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
                 }
             }
 
-            else -> if (with(session) { classSymbol.isSerializableObject }) result += SerialEntityNames.SERIALIZER_PROVIDER_NAME
+            classSymbol.isExternalSerializer -> {
+                if (classSymbol.declarationSymbols.filterIsInstance<FirPropertySymbol>()
+                        .none { it.name == SerialEntityNames.SERIAL_DESC_FIELD_NAME }
+                ) {
+                    result += SerialEntityNames.SERIAL_DESC_FIELD_NAME
+                }
+
+                if (classSymbol.isCompanion) result += SerialEntityNames.SERIALIZER_PROVIDER_NAME
+
+                if (classSymbol.declarationSymbols.filterIsInstance<FirNamedFunctionSymbol>()
+                        .none { it.name == SerialEntityNames.SAVE_NAME }
+                ) {
+                    result += SerialEntityNames.SAVE_NAME // TODO how check parameters?!
+                }
+                if (classSymbol.declarationSymbols.filterIsInstance<FirNamedFunctionSymbol>()
+                        .none { it.name == SerialEntityNames.LOAD_NAME }
+                ) {
+                    result += SerialEntityNames.LOAD_NAME // TODO how check parameters?!
+                }
+            }
+
+            with(session) { classSymbol.isSerializableObject } -> result += SerialEntityNames.SERIALIZER_PROVIDER_NAME
         }
         return result
     }
@@ -135,7 +148,6 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
         return targets.singleOrNull() ?: error("Multiple overrides found for ${callableId.callableName}")
     }
 
-    // TODO: support @Serializer(for)
     @OptIn(SymbolInternals::class)
     override fun generateFunctions(callableId: CallableId, context: MemberGenerationContext?): List<FirNamedFunctionSymbol> {
         val owner = context?.owner ?: return emptyList()
@@ -150,7 +162,7 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
             serializableClass ?: return emptyList()
             return listOf(generateSerializerGetterInCompanion(owner, serializableClass, callableId))
         }
-        if (owner.name != SerialEntityNames.SERIALIZER_CLASS_NAME) return emptyList()
+        if (!owner.isSerializer) return emptyList()
         if (callableId.callableName !in setOf(
                 SpecialNames.INIT,
                 SerialEntityNames.SAVE_NAME,
@@ -193,14 +205,14 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
             valueParameters.addAll(List(serializableClassSymbol.typeParameterSymbols.size) { i ->
                 newSimpleValueParameter(
                     session,
-                    kSerializerClassId.constructClassLikeType(arrayOf(parametersAsArguments[i]), false).toFirResolvedTypeRef(),
+                    kSerializerId.constructClassLikeType(arrayOf(parametersAsArguments[i]), false).toFirResolvedTypeRef(),
                     Name.identifier("${SerialEntityNames.typeArgPrefix}$i")
                 )
             })
 
 
             returnTypeRef = buildResolvedTypeRef {
-                type = kSerializerClassId.constructClassLikeType(
+                type = kSerializerId.constructClassLikeType(
                     arrayOf(serializableClassSymbol.constructType(parametersAsArguments, false)),
                     isNullable = false
                 )
@@ -212,7 +224,7 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
     @OptIn(SymbolInternals::class)
     override fun generateProperties(callableId: CallableId, context: MemberGenerationContext?): List<FirPropertySymbol> {
         val owner = context?.owner ?: return emptyList()
-        if (owner.name != SerialEntityNames.SERIALIZER_CLASS_NAME) return emptyList()
+        if (!owner.isSerializer) return emptyList()
         if (callableId.callableName != SerialEntityNames.SERIAL_DESC_FIELD_NAME) return emptyList()
 
         val target = getFromSupertype(callableId, owner) { it.getProperties(callableId.callableName).filterIsInstance<FirPropertySymbol>() }
@@ -265,7 +277,7 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
                 valueParameters.addAll(owner.typeParameterSymbols.mapIndexed { i, typeParam ->
                     newSimpleValueParameter(
                         session,
-                        kSerializerClassId.constructClassLikeType(arrayOf(typeParam.toConeType()), false).toFirResolvedTypeRef(),
+                        kSerializerId.constructClassLikeType(arrayOf(typeParam.toConeType()), false).toFirResolvedTypeRef(),
                         Name.identifier("${SerialEntityNames.typeArgPrefix}$i")
                     )
                 })
@@ -302,7 +314,7 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
             })
 
             val parametersAsArguments = typeParameters.map { it.toConeType() }.toTypedArray<ConeTypeProjection>()
-            superTypeRefs += generatedSerializerClassId.constructClassLikeType(
+            superTypeRefs += generatedSerializerId.constructClassLikeType(
                 arrayOf(
                     owner.constructType(
                         parametersAsArguments,
@@ -345,6 +357,12 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
         register(FirSerializationPredicates.annotatedWithSerializableOrMeta, FirSerializationPredicates.hasMetaAnnotation)
     }
 
+    private val FirClassSymbol<*>.isSerializer: Boolean
+        get() = name == SerialEntityNames.SERIALIZER_CLASS_NAME || isExternalSerializer
+
+    private val FirClassSymbol<*>.isExternalSerializer: Boolean
+        get() = session.predicateBasedProvider.matches(FirSerializationPredicates.serializerFor, this)
+
     context(FirSession)
     @Suppress("IncorrectFormatting") // KTIJ-22227
     private val FirClassSymbol<*>.companionNeedsSerializerFactory: Boolean

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/SerializationFirSupertypesExtension.kt

@@ -7,12 +7,21 @@ package org.jetbrains.kotlinx.serialization.compiler.fir
 
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.FirClassLikeDeclaration
+import org.jetbrains.kotlin.fir.expressions.FirExpression
+import org.jetbrains.kotlin.fir.expressions.FirGetClassCall
+import org.jetbrains.kotlin.fir.expressions.FirPropertyAccessExpression
 import org.jetbrains.kotlin.fir.extensions.FirDeclarationPredicateRegistrar
 import org.jetbrains.kotlin.fir.extensions.FirSupertypeGenerationExtension
+import org.jetbrains.kotlin.fir.extensions.buildUserTypeFromQualifierParts
 import org.jetbrains.kotlin.fir.extensions.predicateBasedProvider
+import org.jetbrains.kotlin.fir.references.impl.FirSimpleNamedReference
+import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirResolvedTypeRef
+import org.jetbrains.kotlin.fir.types.builder.buildResolvedTypeRef
 import org.jetbrains.kotlin.fir.types.classId
+import org.jetbrains.kotlin.fir.types.constructClassLikeType
 import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlinx.serialization.compiler.fir.FirSerializationPredicates.serializerFor
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerialEntityNames
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerializationPackages
@@ -36,9 +45,33 @@ class SerializationFirSupertypesExtension(session: FirSession) : FirSupertypeGen
         val generatedSerializerClassId = ClassId(SerializationPackages.internalPackageFqName, SerialEntityNames.GENERATED_SERIALIZER_CLASS)
         if (resolvedSupertypes.any { it.type.classId == kSerializerClassId || it.type.classId == generatedSerializerClassId }) return emptyList()
 
-
         return if (session.predicateBasedProvider.matches(serializerFor, classLikeDeclaration)) {
-            TODO("Support @Serializer(for=...) supertype generation")
+            val getClassArgument = classLikeDeclaration.serializerFor ?: return emptyList()
+            val serializerConeType = resolveConeTypeFromArgument(getClassArgument)
+
+            listOf(
+                buildResolvedTypeRef {
+                    type = kSerializerClassId.constructClassLikeType(arrayOf(serializerConeType), isNullable = false)
+                }
+            )
         } else emptyList()
     }
+
+    // Function helps to resolve class call from annotation argument to `ConeKotlinType`
+    context(TypeResolveServiceContainer)
+    @Suppress("IncorrectFormatting") // KTIJ-22227
+    private fun resolveConeTypeFromArgument(getClassCall: FirGetClassCall): ConeKotlinType {
+        val typeToResolve = buildUserTypeFromQualifierParts(isMarkedNullable = false) {
+            fun visitQualifiers(expression: FirExpression) {
+                if (expression !is FirPropertyAccessExpression) return
+                expression.explicitReceiver?.let { visitQualifiers(it) }
+                expression.qualifierName?.let { part(it) }
+            }
+            visitQualifiers(getClassCall.argument)
+        }
+        return typeResolver.resolveUserType(typeToResolve).type
+    }
+
+    private val FirPropertyAccessExpression.qualifierName: Name?
+        get() = (calleeReference as? FirSimpleNamedReference)?.name
 }

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/SerializationFirUtils.kt

@@ -9,6 +9,7 @@ import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.utils.*
+import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.FirAnnotation
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.arguments
@@ -85,6 +86,10 @@ internal val FirBasedSymbol<*>.serializableWith: ConeKotlinType?
 internal val List<FirAnnotation>.serializableWith: ConeKotlinType?
     get() = serializableAnnotation()?.getKClassArgument(AnnotationParameterNames.WITH)
 
+fun FirAnnotation.getGetKClassArgument(name: Name): FirGetClassCall? {
+    return findArgumentByName(name) as? FirGetClassCall
+}
+
 internal val FirClassSymbol<*>.serializerAnnotation: FirAnnotation?
     get() = getAnnotationByClassId(SerializationAnnotations.serializerAnnotationClassId)
 
@@ -94,6 +99,15 @@ internal val FirClassSymbol<*>.serializerForClass: ConeKotlinType?
         .getAnnotationByClassId(SerializationAnnotations.serializerAnnotationClassId)
         ?.getKClassArgument(AnnotationParameterNames.FOR_CLASS)
 
+internal val FirClassSymbol<*>.serializerFor: FirGetClassCall?
+    get() = resolvedAnnotationsWithArguments
+        .getAnnotationByClassId(SerializationAnnotations.serializerAnnotationClassId)
+        ?.getGetKClassArgument(AnnotationParameterNames.FOR_CLASS)
+
+internal val FirClassLikeDeclaration.serializerFor: FirGetClassCall?
+    get() = getAnnotationByClassId(SerializationAnnotations.serializerAnnotationClassId)
+        ?.getGetKClassArgument(AnnotationParameterNames.FOR_CLASS)
+
 context(FirSession)
 @Suppress("IncorrectFormatting") // KTIJ-22227
 internal val FirClassSymbol<*>.isInternallySerializableObject: Boolean
@@ -172,7 +186,8 @@ val FirClassSymbol<*>.hasCompanionObjectAsSerializer: Boolean
 context(FirSession)
 @Suppress("IncorrectFormatting") // KTIJ-22227
 val FirClassSymbol<*>.hasCustomSerializerOnCompanion: Boolean
-    get() = (this as? FirRegularClassSymbol)?.companionObjectSymbol?.serializerForClass != null
+// TODO It was accepted that the annotation on the companion can take only the current class as an argument. Before the release of FIR, we need to add a new annotation and check gere for its presence
+    get() = (this as? FirRegularClassSymbol)?.companionObjectSymbol?.serializerFor != null
 
 context(FirSession)
 val FirClassSymbol<*>.isEnumWithLegacyGeneratedSerializer: Boolean
@@ -180,7 +195,7 @@ val FirClassSymbol<*>.isEnumWithLegacyGeneratedSerializer: Boolean
 
 context(FirSession)
 val FirClassSymbol<*>.shouldHaveGeneratedSerializer: Boolean
-    get() = (isInternalSerializable && (modality == Modality.FINAL || modality == Modality.OPEN)) || isEnumWithLegacyGeneratedSerializer
+    get() = (isInternalSerializable && isFinalOrOpen()) || isEnumWithLegacyGeneratedSerializer
 
 context(FirSession)
 @Suppress("IncorrectFormatting") // KTIJ-22227

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt

@@ -7,9 +7,7 @@ package org.jetbrains.kotlinx.serialization.compiler.fir.checkers
 
 import org.jetbrains.kotlin.KtSourceElement
 import org.jetbrains.kotlin.config.KotlinCompilerVersion
-import org.jetbrains.kotlin.descriptors.isEnumClass
 import org.jetbrains.kotlin.diagnostics.*
-import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
 import org.jetbrains.kotlin.fir.analysis.checkers.declaration.FirClassChecker
 import org.jetbrains.kotlin.fir.analysis.checkers.isInlineClass
@@ -20,7 +18,6 @@ import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.moduleData
 import org.jetbrains.kotlin.fir.resolve.defaultType
 import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
-import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.*
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.name.*
@@ -33,7 +30,6 @@ import org.jetbrains.kotlinx.serialization.compiler.fir.services.dependencySeria
 import org.jetbrains.kotlinx.serialization.compiler.fir.services.findTypeSerializerOrContextUnchecked
 import org.jetbrains.kotlinx.serialization.compiler.fir.services.serializablePropertiesProvider
 import org.jetbrains.kotlinx.serialization.compiler.fir.services.versionReader
-import org.jetbrains.kotlinx.serialization.compiler.resolve.SerializationAnnotations
 
 object FirSerializationPluginClassChecker : FirClassChecker() {
     private val JAVA_SERIALIZABLE_ID = ClassId.topLevel(FqName("java.io.Serializable"))
@@ -64,7 +60,7 @@ object FirSerializationPluginClassChecker : FirClassChecker() {
     private fun checkExternalSerializer(classSymbol: FirClassSymbol<*>, reporter: DiagnosticReporter) {
         val serializableKType = classSymbol.serializerForClass ?: return
         val serializableClassSymbol = serializableKType.toRegularClassSymbol(session) ?: return
-        val properties = session.serializablePropertiesProvider.getSerializablePropertiesForClass(classSymbol)
+        val properties = session.serializablePropertiesProvider.getSerializablePropertiesForClass(serializableClassSymbol)
         if (!properties.isExternallySerializable) {
             val source = classSymbol.serializerAnnotation?.source ?: classSymbol.source
             val error = if (serializableClassSymbol.moduleData == session.moduleData) {

plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.fir.fail

@@ -1,3 +0,0 @@
-An operation is not implemented: Support @Serializer(for=...) supertype generation
-kotlin.NotImplementedError: An operation is not implemented: Support @Serializer(for=...) supertype generation
-	at org.jetbrains.kotlinx.serialization.compiler.fir.SerializationFirSupertypesExtension.computeAdditionalSupertypes(SerializationFirSupertypesExtension.kt:39)

plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.fir.kt

@@ -1,9 +0,0 @@
-// SKIP_TXT
-
-// FILE: test.kt
-import kotlinx.serialization.*
-
-class Foo(i: Int, val j: Int)
-
-@Serializer(forClass = Foo::class)
-object ExternalSerializer

plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // SKIP_TXT
 
 // FILE: test.kt

plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.fir.kt

@@ -1,9 +0,0 @@
-// This test enshures that analysis ends up without compiler exceptions
-
-import kotlinx.serialization.*
-
-@Serializable
-class Digest() {
-    @Serializer(forClass = Digest::class)
-    companion <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object<!> : KSerializer<Digest> {}
-}

plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // This test enshures that analysis ends up without compiler exceptions
 
 import kotlinx.serialization.*

plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.fir.kt

@@ -1,18 +0,0 @@
-// WITH_STDLIB
-// SKIP_TXT
-
-import kotlinx.serialization.*
-
-class Bar
-@Serializer(forClass = Bar::class)
-<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object BarSerializer<!>: KSerializer<Bar>
-
-class Baz
-@Serializer(forClass = Baz::class)
-<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object BazSerializer<!>: KSerializer<Baz>
-
-@Serializable
-class Foo1(@Polymorphic val i: Baz)
-
-@Serializable
-class Foo2(val li: MutableList<@Serializable(with = BazSerializer::class) Baz>)

plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // WITH_STDLIB
 // SKIP_TXT
 

plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.fir.kt

@@ -5,13 +5,13 @@ import kotlinx.serialization.*
 
 class Bar
 @Serializer(forClass = Bar::class)
-<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object BarSerializer<!>: KSerializer<Bar>
+object BarSerializer: KSerializer<Bar>
 
 class Baz
 @Serializer(forClass = Baz::class)
-<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object BazSerializer<!>: KSerializer<Baz>
+object BazSerializer: KSerializer<Baz>
 @Serializer(forClass = Baz::class)
-<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object NullableBazSerializer<!>: KSerializer<Baz?>
+object NullableBazSerializer: KSerializer<Baz?>
 
 <!SERIALIZER_TYPE_INCOMPATIBLE!>@Serializable(with = BazSerializer::class)<!>
 class Biz(val i: Int)

plugins/kotlinx-serialization/testData/firMembers/externalSerializers.fir.txt

@@ -0,0 +1,55 @@
+FILE: externalSerializers.kt
+    package com.example
+
+    public final class Data : R|kotlin/Any| {
+        public constructor(j: R|kotlin/Int|): R|com/example/Data| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val j: R|kotlin/Int| = R|<local>/j|
+            public get(): R|kotlin/Int|
+
+    }
+    @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/Data|)) public final object ObjectSerializer : R|kotlin/Any|, R|kotlinx/serialization/KSerializer<com/example/Data>| {
+        private constructor(): R|com/example/ObjectSerializer| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|
+            public get(): R|kotlinx/serialization/descriptors/SerialDescriptor|
+
+        public final fun serialize(encoder: R|kotlinx/serialization/encoding/Encoder|, value: R|com/example/Data|): R|kotlin/Unit|
+
+        public final fun deserialize(decoder: R|kotlinx/serialization/encoding/Decoder|): R|com/example/Data|
+
+    }
+    @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/Data|)) public final class ClassSerializer : R|kotlin/Any|, R|kotlinx/serialization/KSerializer<com/example/Data>| {
+        public constructor(): R|com/example/ClassSerializer| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|
+            public get(): R|kotlinx/serialization/descriptors/SerialDescriptor|
+
+        public final fun serialize(encoder: R|kotlinx/serialization/encoding/Encoder|, value: R|com/example/Data|): R|kotlin/Unit|
+
+        public final fun deserialize(decoder: R|kotlinx/serialization/encoding/Decoder|): R|com/example/Data|
+
+    }
+    public final fun box(): R|kotlin/String| {
+        lval encodedForClass: R|kotlin/String| = Q|kotlinx/serialization/json/Json|.R|kotlinx/serialization/json/Json.encodeToString|<R|com/example/Data|>(R|com/example/ClassSerializer.ClassSerializer|(), R|com/example/Data.Data|(Int(1)))
+        when () {
+            !=(R|<local>/encodedForClass|, String({"j":1})) ->  {
+                ^box R|<local>/encodedForClass|
+            }
+        }
+
+        lval encodedForObject: R|kotlin/String| = Q|kotlinx/serialization/json/Json|.R|kotlinx/serialization/json/Json.encodeToString|<R|com/example/Data|>(Q|com/example/ObjectSerializer|, R|com/example/Data.Data|(Int(2)))
+        when () {
+            !=(R|<local>/encodedForObject|, String({"j":2})) ->  {
+                ^box R|<local>/encodedForObject|
+            }
+        }
+
+        ^box String(OK)
+    }

plugins/kotlinx-serialization/testData/firMembers/externalSerializers.kt

@@ -0,0 +1,27 @@
+// WITH_STDLIB
+
+package com.example
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.descriptors.*
+
+
+class Data(val j: Int)
+
+@Serializer(forClass = Data::class)
+object ObjectSerializer
+
+@Serializer(forClass = Data::class)
+class ClassSerializer
+
+fun box(): String {
+    val encodedForClass = Json.encodeToString(ClassSerializer(), Data(1))
+    if (encodedForClass != """{"j":1}""") return encodedForClass
+
+    val encodedForObject = Json.encodeToString(ObjectSerializer, Data(2))
+    if (encodedForObject != """{"j":2}""") return encodedForObject
+
+    return "OK"
+}

plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.fir.txt

@@ -0,0 +1,265 @@
+FILE: serializerViaCompanion.kt
+    package com.example
+
+    @R|kotlinx/serialization/Serializable|() public final data class WithCompanion : R|kotlin/Any| {
+        public constructor(i: R|kotlin/Int|): R|com/example/WithCompanion| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val i: R|kotlin/Int| = R|<local>/i|
+            public get(): R|kotlin/Int|
+
+        @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/WithCompanion|)) public final companion object Companion : R|kotlin/Any|, R|kotlinx/serialization/KSerializer<com/example/WithCompanion>| {
+            private constructor(): R|com/example/WithCompanion.Companion| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final val descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|
+                public get(): R|kotlinx/serialization/descriptors/SerialDescriptor|
+
+            public final fun serialize(encoder: R|kotlinx/serialization/encoding/Encoder|, value: R|com/example/WithCompanion|): R|kotlin/Unit|
+
+            public final fun deserialize(decoder: R|kotlinx/serialization/encoding/Decoder|): R|com/example/WithCompanion|
+
+            public final fun serializer(): R|kotlinx/serialization/KSerializer<com/example/WithCompanion>|
+
+        }
+
+        public final operator fun component1(): R|kotlin/Int|
+
+        public final fun copy(i: R|kotlin/Int| = this@R|com/example/WithCompanion|.R|com/example/WithCompanion.i|): R|com/example/WithCompanion|
+
+    }
+    @R|kotlinx/serialization/Serializable|() public final data class WithNamedCompanion : R|kotlin/Any| {
+        public constructor(i: R|kotlin/Int|): R|com/example/WithNamedCompanion| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val i: R|kotlin/Int| = R|<local>/i|
+            public get(): R|kotlin/Int|
+
+        @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/WithNamedCompanion|)) public final companion object Named : R|kotlin/Any|, R|kotlinx/serialization/KSerializer<com/example/WithNamedCompanion>| {
+            private constructor(): R|com/example/WithNamedCompanion.Named| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final val descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|
+                public get(): R|kotlinx/serialization/descriptors/SerialDescriptor|
+
+            public final fun serialize(encoder: R|kotlinx/serialization/encoding/Encoder|, value: R|com/example/WithNamedCompanion|): R|kotlin/Unit|
+
+            public final fun deserialize(decoder: R|kotlinx/serialization/encoding/Decoder|): R|com/example/WithNamedCompanion|
+
+            public final fun serializer(): R|kotlinx/serialization/KSerializer<com/example/WithNamedCompanion>|
+
+        }
+
+        public final operator fun component1(): R|kotlin/Int|
+
+        public final fun copy(i: R|kotlin/Int| = this@R|com/example/WithNamedCompanion|.R|com/example/WithNamedCompanion.i|): R|com/example/WithNamedCompanion|
+
+    }
+    @R|kotlinx/serialization/Serializable|() public final data class WithExplicitType : R|kotlin/Any| {
+        public constructor(i: R|kotlin/Int|): R|com/example/WithExplicitType| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val i: R|kotlin/Int| = R|<local>/i|
+            public get(): R|kotlin/Int|
+
+        @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/WithExplicitType|)) public final companion object Companion : R|kotlinx/serialization/KSerializer<com/example/WithExplicitType>| {
+            private constructor(): R|com/example/WithExplicitType.Companion| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final val descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|
+                public get(): R|kotlinx/serialization/descriptors/SerialDescriptor|
+
+            public final fun serialize(encoder: R|kotlinx/serialization/encoding/Encoder|, value: R|com/example/WithExplicitType|): R|kotlin/Unit|
+
+            public final fun deserialize(decoder: R|kotlinx/serialization/encoding/Decoder|): R|com/example/WithExplicitType|
+
+            public final fun serializer(): R|kotlinx/serialization/KSerializer<com/example/WithExplicitType>|
+
+        }
+
+        public final operator fun component1(): R|kotlin/Int|
+
+        public final fun copy(i: R|kotlin/Int| = this@R|com/example/WithExplicitType|.R|com/example/WithExplicitType.i|): R|com/example/WithExplicitType|
+
+    }
+    @R|kotlinx/serialization/Serializable|() public final data class PartiallyOverridden : R|kotlin/Any| {
+        public constructor(i: R|kotlin/Int|): R|com/example/PartiallyOverridden| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val i: R|kotlin/Int| = R|<local>/i|
+            public get(): R|kotlin/Int|
+
+        @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/PartiallyOverridden|)) public final companion object Companion : R|kotlinx/serialization/KSerializer<com/example/PartiallyOverridden>| {
+            private constructor(): R|com/example/PartiallyOverridden.Companion| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final override val descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor| = R|kotlinx/serialization/descriptors/buildClassSerialDescriptor|(String(Partially-Overridden), <L> = buildClassSerialDescriptor@fun R|kotlinx/serialization/descriptors/ClassSerialDescriptorBuilder|.<anonymous>(): R|kotlin/Unit| <inline=NoInline>  {
+                this@R|special/anonymous|.R|kotlinx/serialization/descriptors/ClassSerialDescriptorBuilder.element|(String(i), R|kotlinx/serialization/descriptors/PrimitiveSerialDescriptor|(String(i), Q|kotlinx/serialization/descriptors/PrimitiveKind.INT|))
+            }
+            )
+                public get(): R|kotlinx/serialization/descriptors/SerialDescriptor|
+
+            public final override fun serialize(encoder: R|kotlinx/serialization/encoding/Encoder|, value: R|com/example/PartiallyOverridden|): R|kotlin/Unit| {
+                lval compositeOutput: R|kotlinx/serialization/encoding/CompositeEncoder| = R|<local>/encoder|.R|kotlinx/serialization/encoding/Encoder.beginStructure|(Q|com/example/PartiallyOverridden|.R|com/example/PartiallyOverridden.Companion.descriptor|)
+                R|<local>/compositeOutput|.R|kotlinx/serialization/encoding/CompositeEncoder.encodeIntElement|(Q|com/example/PartiallyOverridden|.R|com/example/PartiallyOverridden.Companion.descriptor|, Int(0), R|<local>/value|.R|com/example/PartiallyOverridden.i|.R|kotlin/Int.plus|(Int(10)))
+                R|<local>/compositeOutput|.R|kotlinx/serialization/encoding/CompositeEncoder.endStructure|(Q|com/example/PartiallyOverridden|.R|com/example/PartiallyOverridden.Companion.descriptor|)
+            }
+
+            public final fun deserialize(decoder: R|kotlinx/serialization/encoding/Decoder|): R|com/example/PartiallyOverridden|
+
+            public final fun serializer(): R|kotlinx/serialization/KSerializer<com/example/PartiallyOverridden>|
+
+        }
+
+        public final operator fun component1(): R|kotlin/Int|
+
+        public final fun copy(i: R|kotlin/Int| = this@R|com/example/PartiallyOverridden|.R|com/example/PartiallyOverridden.i|): R|com/example/PartiallyOverridden|
+
+    }
+    @R|kotlinx/serialization/Serializable|() public final data class PartiallyWithoutType : R|kotlin/Any| {
+        public constructor(i: R|kotlin/Int|): R|com/example/PartiallyWithoutType| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val i: R|kotlin/Int| = R|<local>/i|
+            public get(): R|kotlin/Int|
+
+        @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/PartiallyWithoutType|)) public final companion object Companion : R|kotlin/Any|, R|kotlinx/serialization/KSerializer<com/example/PartiallyWithoutType>| {
+            private constructor(): R|com/example/PartiallyWithoutType.Companion| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final override fun deserialize(decoder: R|kotlinx/serialization/encoding/Decoder|): R|com/example/PartiallyWithoutType| {
+                lval dec: R|kotlinx/serialization/encoding/CompositeDecoder| = R|<local>/decoder|.R|kotlinx/serialization/encoding/Decoder.beginStructure|(this@R|com/example/PartiallyWithoutType.Companion|.R|com/example/PartiallyWithoutType.Companion.descriptor|)
+                lvar iv: R|kotlin/Int?| = Null(null)
+                loop@while(Boolean(true)) {
+                    when (lval i: R|kotlin/Int| = R|<local>/dec|.R|kotlinx/serialization/encoding/CompositeDecoder.decodeElementIndex|(this@R|com/example/PartiallyWithoutType.Companion|.R|com/example/PartiallyWithoutType.Companion.descriptor|)) {
+                        ==($subj$, Q|kotlinx/serialization/encoding/CompositeDecoder|.R|kotlinx/serialization/encoding/CompositeDecoder.Companion.DECODE_DONE|) ->  {
+                            break@@@[Boolean(true)]
+                        }
+                        ==($subj$, Int(0)) ->  {
+                            R|<local>/iv| = R|<local>/dec|.R|kotlinx/serialization/encoding/CompositeDecoder.decodeIntElement|(this@R|com/example/PartiallyWithoutType.Companion|.R|com/example/PartiallyWithoutType.Companion.descriptor|, R|<local>/i|)
+                        }
+                        else ->  {
+                            throw R|kotlinx/serialization/SerializationException.SerializationException|(<strcat>(String(Unknown index ), R|<local>/i|))
+                        }
+                    }
+
+                }
+
+                R|<local>/dec|.R|kotlinx/serialization/encoding/CompositeDecoder.endStructure|(this@R|com/example/PartiallyWithoutType.Companion|.R|com/example/PartiallyWithoutType.Companion.descriptor|)
+                ^deserialize R|com/example/PartiallyWithoutType.PartiallyWithoutType|(R|<local>/iv|!!.R|kotlin/Int.plus|(Int(10)))
+            }
+
+            public final val descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|
+                public get(): R|kotlinx/serialization/descriptors/SerialDescriptor|
+
+            public final fun serialize(encoder: R|kotlinx/serialization/encoding/Encoder|, value: R|com/example/PartiallyWithoutType|): R|kotlin/Unit|
+
+            public final fun serializer(): R|kotlinx/serialization/KSerializer<com/example/PartiallyWithoutType>|
+
+        }
+
+        public final operator fun component1(): R|kotlin/Int|
+
+        public final fun copy(i: R|kotlin/Int| = this@R|com/example/PartiallyWithoutType|.R|com/example/PartiallyWithoutType.i|): R|com/example/PartiallyWithoutType|
+
+    }
+    @R|kotlinx/serialization/Serializable|() public final data class FullyOverridden : R|kotlin/Any| {
+        public constructor(i: R|kotlin/Int|): R|com/example/FullyOverridden| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val i: R|kotlin/Int| = R|<local>/i|
+            public get(): R|kotlin/Int|
+
+        @R|kotlinx/serialization/Serializer|(forClass = <getClass>(Q|com/example/FullyOverridden|)) public final companion object Companion : R|kotlinx/serialization/KSerializer<com/example/FullyOverridden>| {
+            private constructor(): R|com/example/FullyOverridden.Companion| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final override val descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor| = R|kotlinx/serialization/descriptors/PrimitiveSerialDescriptor|(String(FullyOverridden), Q|kotlinx/serialization/descriptors/PrimitiveKind.STRING|)
+                public get(): R|kotlinx/serialization/descriptors/SerialDescriptor|
+
+            public final override fun serialize(encoder: R|kotlinx/serialization/encoding/Encoder|, value: R|com/example/FullyOverridden|): R|kotlin/Unit| {
+                R|<local>/encoder|.R|kotlinx/serialization/encoding/Encoder.encodeString|(<strcat>(String(i=), R|<local>/value|.R|com/example/FullyOverridden.i|))
+            }
+
+            public final override fun deserialize(decoder: R|kotlinx/serialization/encoding/Decoder|): R|com/example/FullyOverridden| {
+                lval i: R|kotlin/Int| = R|<local>/decoder|.R|kotlinx/serialization/encoding/Decoder.decodeString|().R|kotlin/text/substringAfter|(Char(=)).R|kotlin/text/toInt|()
+                ^deserialize R|com/example/FullyOverridden.FullyOverridden|(R|<local>/i|)
+            }
+
+            public final fun serializer(): R|kotlinx/serialization/KSerializer<com/example/FullyOverridden>|
+
+        }
+
+        public final operator fun component1(): R|kotlin/Int|
+
+        public final fun copy(i: R|kotlin/Int| = this@R|com/example/FullyOverridden|.R|com/example/FullyOverridden.i|): R|com/example/FullyOverridden|
+
+    }
+    public final fun box(): R|kotlin/String| {
+        R|com/example/encodeAndDecode|<R|com/example/WithCompanion|>(Q|com/example/WithCompanion|.R|com/example/WithCompanion.Companion.serializer|(), R|com/example/WithCompanion.WithCompanion|(Int(1)), String({"i":1}))?.{ $subj$.R|kotlin/let|<R|kotlin/String|, R|kotlin/Nothing|>(<L> = let@fun <anonymous>(it: R|kotlin/String|): R|kotlin/Nothing| <inline=Inline, kind=EXACTLY_ONCE>  {
+            ^box R|<local>/it|
+        }
+        ) }
+        R|com/example/encodeAndDecode|<R|com/example/WithNamedCompanion|>(Q|com/example/WithNamedCompanion|.R|com/example/WithNamedCompanion.Named.serializer|(), R|com/example/WithNamedCompanion.WithNamedCompanion|(Int(2)), String({"i":2}))?.{ $subj$.R|kotlin/let|<R|kotlin/String|, R|kotlin/Nothing|>(<L> = let@fun <anonymous>(it: R|kotlin/String|): R|kotlin/Nothing| <inline=Inline, kind=EXACTLY_ONCE>  {
+            ^box R|<local>/it|
+        }
+        ) }
+        R|com/example/encodeAndDecode|<R|com/example/WithExplicitType|>(Q|com/example/WithExplicitType|.R|com/example/WithExplicitType.Companion.serializer|(), R|com/example/WithExplicitType.WithExplicitType|(Int(3)), String({"i":3}))?.{ $subj$.R|kotlin/let|<R|kotlin/String|, R|kotlin/Nothing|>(<L> = let@fun <anonymous>(it: R|kotlin/String|): R|kotlin/Nothing| <inline=Inline, kind=EXACTLY_ONCE>  {
+            ^box R|<local>/it|
+        }
+        ) }
+        R|com/example/encodeAndDecode|<R|com/example/FullyOverridden|>(Q|com/example/FullyOverridden|.R|com/example/FullyOverridden.Companion.serializer|(), R|com/example/FullyOverridden.FullyOverridden|(Int(4)), String("i=4"))?.{ $subj$.R|kotlin/let|<R|kotlin/String|, R|kotlin/Nothing|>(<L> = let@fun <anonymous>(it: R|kotlin/String|): R|kotlin/Nothing| <inline=Inline, kind=EXACTLY_ONCE>  {
+            ^box R|<local>/it|
+        }
+        ) }
+        R|com/example/encodeAndDecode|<R|com/example/PartiallyOverridden|>(Q|com/example/PartiallyOverridden|.R|com/example/PartiallyOverridden.Companion.serializer|(), R|com/example/PartiallyOverridden.PartiallyOverridden|(Int(5)), String({"i":15}), R|com/example/PartiallyOverridden.PartiallyOverridden|(Int(15)))?.{ $subj$.R|kotlin/let|<R|kotlin/String|, R|kotlin/Nothing|>(<L> = let@fun <anonymous>(it: R|kotlin/String|): R|kotlin/Nothing| <inline=Inline, kind=EXACTLY_ONCE>  {
+            ^box R|<local>/it|
+        }
+        ) }
+        when () {
+            !=(Q|com/example/PartiallyOverridden|.R|com/example/PartiallyOverridden.Companion.serializer|().R|SubstitutionOverride<kotlinx/serialization/KSerializer.descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|>|.R|kotlinx/serialization/descriptors/SerialDescriptor.serialName|, String(Partially-Overridden)) ->  {
+                ^box Q|com/example/PartiallyOverridden|.R|com/example/PartiallyOverridden.Companion.serializer|().R|SubstitutionOverride<kotlinx/serialization/KSerializer.descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|>|.R|kotlinx/serialization/descriptors/SerialDescriptor.serialName|
+            }
+        }
+
+        R|com/example/encodeAndDecode|<R|com/example/PartiallyWithoutType|>(Q|com/example/PartiallyWithoutType|.R|com/example/PartiallyWithoutType.Companion.serializer|(), R|com/example/PartiallyWithoutType.PartiallyWithoutType|(Int(6)), String({"i":6}), R|com/example/PartiallyWithoutType.PartiallyWithoutType|(Int(16)))?.{ $subj$.R|kotlin/let|<R|kotlin/String|, R|kotlin/Nothing|>(<L> = let@fun <anonymous>(it: R|kotlin/String|): R|kotlin/Nothing| <inline=Inline, kind=EXACTLY_ONCE>  {
+            ^box R|<local>/it|
+        }
+        ) }
+        when () {
+            !=(Q|com/example/PartiallyWithoutType|.R|com/example/PartiallyWithoutType.Companion.serializer|().R|SubstitutionOverride<kotlinx/serialization/KSerializer.descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|>|.R|kotlinx/serialization/descriptors/SerialDescriptor.serialName|, <getClass>(Q|com/example/PartiallyWithoutType|).R|SubstitutionOverride<kotlin/reflect/KClass.qualifiedName: R|kotlin/String?|>|) ->  {
+                ^box Q|com/example/PartiallyWithoutType|.R|com/example/PartiallyWithoutType.Companion.serializer|().R|SubstitutionOverride<kotlinx/serialization/KSerializer.descriptor: R|kotlinx/serialization/descriptors/SerialDescriptor|>|.R|kotlinx/serialization/descriptors/SerialDescriptor.serialName|
+            }
+        }
+
+        ^box String(OK)
+    }
+    private final fun <T> encodeAndDecode(serializer: R|kotlinx/serialization/KSerializer<T>|, value: R|T|, expectedEncoded: R|kotlin/String|, expectedDecoded: R|T?| = Null(null)): R|kotlin/String?| {
+        lval encoded: R|kotlin/String| = Q|kotlinx/serialization/json/Json|.R|kotlinx/serialization/json/Json.encodeToString|<R|T|>(R|<local>/serializer|, R|<local>/value|)
+        when () {
+            !=(R|<local>/encoded|, R|<local>/expectedEncoded|) ->  {
+                ^encodeAndDecode R|<local>/encoded|
+            }
+        }
+
+        lval decoded: R|T| = Q|kotlinx/serialization/json/Json|.R|kotlinx/serialization/json/Json.decodeFromString|<R|T|>(R|<local>/serializer|, R|<local>/encoded|)
+        when () {
+            !=(R|<local>/decoded|, R|<local>/expectedDecoded| ?: R|<local>/value|) ->  {
+                ^encodeAndDecode <strcat>(String(DECODED=), R|<local>/decoded|.R|kotlin/toString|())
+            }
+        }
+
+        ^encodeAndDecode Null(null)
+    }

plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.kt

@@ -0,0 +1,111 @@
+// WITH_STDLIB
+
+package com.example
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.descriptors.*
+
+
+@Serializable
+data class WithCompanion(val i: Int) {
+    @Serializer(forClass = WithCompanion::class)
+    companion object
+}
+
+@Serializable
+data class WithNamedCompanion(val i: Int) {
+    @Serializer(forClass = WithNamedCompanion::class)
+    companion object Named
+}
+
+@Serializable
+data class WithExplicitType(val i: Int) {
+    @Serializer(forClass = WithExplicitType::class)
+    companion object : KSerializer<WithExplicitType>
+}
+
+@Serializable
+data class PartiallyOverridden(val i: Int) {
+    @Serializer(forClass = PartiallyOverridden::class)
+    companion object : KSerializer<PartiallyOverridden> {
+        override val descriptor: SerialDescriptor = buildClassSerialDescriptor("Partially-Overridden") {
+            element("i", PrimitiveSerialDescriptor("i", PrimitiveKind.INT))
+        }
+
+        override fun serialize(encoder: Encoder, value: PartiallyOverridden) {
+            val compositeOutput = encoder.beginStructure(PartiallyOverridden.descriptor)
+            compositeOutput.encodeIntElement(PartiallyOverridden.descriptor, 0, value.i + 10)
+            compositeOutput.endStructure(PartiallyOverridden.descriptor)
+        }
+    }
+}
+
+@Serializable
+data class PartiallyWithoutType(val i: Int) {
+    @Serializer(forClass = PartiallyWithoutType::class)
+    companion object {
+
+        override fun deserialize(decoder: Decoder): PartiallyWithoutType {
+            val dec: CompositeDecoder = decoder.beginStructure(descriptor)
+            var iv: Int? = null
+            loop@ while (true) {
+                when (val i = dec.decodeElementIndex(descriptor)) {
+                    CompositeDecoder.DECODE_DONE -> break@loop
+                    0 -> iv = dec.decodeIntElement(descriptor, i)
+                    else -> throw SerializationException("Unknown index $i")
+                }
+            }
+            dec.endStructure(descriptor)
+            return PartiallyWithoutType(iv!! + 10)
+        }
+    }
+}
+
+
+@Serializable
+data class FullyOverridden(val i: Int) {
+    @Serializer(forClass = FullyOverridden::class)
+    companion object : KSerializer<FullyOverridden> {
+        override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("FullyOverridden", PrimitiveKind.STRING)
+
+        override fun serialize(encoder: Encoder, value: FullyOverridden) {
+            encoder.encodeString("i=${value.i}")
+        }
+
+        override fun deserialize(decoder: Decoder): FullyOverridden {
+            val i = decoder.decodeString().substringAfter('=').toInt()
+            return FullyOverridden(i)
+        }
+    }
+}
+
+
+fun box(): String {
+    encodeAndDecode(WithCompanion.serializer(), WithCompanion(1), """{"i":1}""")?.let { return it }
+
+    encodeAndDecode(WithNamedCompanion.serializer(), WithNamedCompanion(2), """{"i":2}""")?.let { return it }
+
+    encodeAndDecode(WithExplicitType.serializer(), WithExplicitType(3), """{"i":3}""")?.let { return it }
+
+    encodeAndDecode(FullyOverridden.serializer(), FullyOverridden(4), "\"i=4\"")?.let { return it }
+
+    encodeAndDecode(PartiallyOverridden.serializer(), PartiallyOverridden(5), """{"i":15}""", PartiallyOverridden(15))?.let { return it }
+    if (PartiallyOverridden.serializer().descriptor.serialName != "Partially-Overridden") return PartiallyOverridden.serializer().descriptor.serialName
+
+    encodeAndDecode(PartiallyWithoutType.serializer(), PartiallyWithoutType(6), """{"i":6}""", PartiallyWithoutType(16))?.let { return it }
+    if (PartiallyWithoutType.serializer().descriptor.serialName != PartiallyWithoutType::class.qualifiedName) return PartiallyWithoutType.serializer().descriptor.serialName
+
+    return "OK"
+}
+
+
+private fun <T> encodeAndDecode(serializer: KSerializer<T>, value: T, expectedEncoded: String, expectedDecoded: T? = null): String? {
+    val encoded = Json.encodeToString(serializer, value)
+    if (encoded != expectedEncoded) return encoded
+
+    val decoded = Json.decodeFromString(serializer, encoded)
+    if (decoded != (expectedDecoded ?: value)) return "DECODED=${decoded.toString()}"
+    return null
+}

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirBlackBoxTestGenerated.java

@@ -151,6 +151,12 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
             runTest("plugins/kotlinx-serialization/testData/firMembers/enums.kt");
         }
 
+        @Test
+        @TestMetadata("externalSerializers.kt")
+        public void testExternalSerializers() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/firMembers/externalSerializers.kt");
+        }
+
         @Test
         @TestMetadata("inlineClasses.kt")
         public void testInlineClasses() throws Exception {
@@ -186,5 +192,11 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
         public void testSerializableWith() throws Exception {
             runTest("plugins/kotlinx-serialization/testData/firMembers/serializableWith.kt");
         }
+
+        @Test
+        @TestMetadata("serializerViaCompanion.kt")
+        public void testSerializerViaCompanion() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.kt");
+        }
     }
 }

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirDiagnosticTestGenerated.java

@@ -174,6 +174,12 @@ public class SerializationFirDiagnosticTestGenerated extends AbstractSerializati
             runTest("plugins/kotlinx-serialization/testData/firMembers/enums.kt");
         }
 
+        @Test
+        @TestMetadata("externalSerializers.kt")
+        public void testExternalSerializers() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/firMembers/externalSerializers.kt");
+        }
+
         @Test
         @TestMetadata("inlineClasses.kt")
         public void testInlineClasses() throws Exception {
@@ -209,5 +215,11 @@ public class SerializationFirDiagnosticTestGenerated extends AbstractSerializati
         public void testSerializableWith() throws Exception {
             runTest("plugins/kotlinx-serialization/testData/firMembers/serializableWith.kt");
         }
+
+        @Test
+        @TestMetadata("serializerViaCompanion.kt")
+        public void testSerializerViaCompanion() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.kt");
+        }
     }
 }