[K2] Support legacy serialization for custom function type extensions.

For Compose libraries, it is important that libraries built with
K2 in version 2.0 can be read and used by a version 1.9 K1
compiler. That is not possible if custom function types are
in the metadata.

^KT-58456 Fixed

compiler/fir/analysis-tests/test/org/jetbrains/kotlin/test/LoadedMetadataDumpHandler.kt

@@ -9,6 +9,7 @@ import com.intellij.openapi.vfs.StandardFileSystems
 import com.intellij.openapi.vfs.VirtualFileManager
 import org.jetbrains.kotlin.cli.common.prepareJvmSessions
 import org.jetbrains.kotlin.cli.jvm.compiler.VfsBasedProjectEnvironment
+import org.jetbrains.kotlin.config.ApiVersion
 import org.jetbrains.kotlin.config.LanguageVersionSettingsImpl
 import org.jetbrains.kotlin.fir.BinaryModuleData
 import org.jetbrains.kotlin.fir.DependencyListForCliModule
@@ -26,7 +27,9 @@ import org.jetbrains.kotlin.psi.KtFile
 import org.jetbrains.kotlin.resolve.DescriptorUtils
 import org.jetbrains.kotlin.resolve.jvm.platform.JvmPlatformAnalyzerServices
 import org.jetbrains.kotlin.test.backend.handlers.JvmBinaryArtifactHandler
+import org.jetbrains.kotlin.test.directives.LanguageSettingsDirectives.LANGUAGE_VERSION
 import org.jetbrains.kotlin.test.directives.model.RegisteredDirectives
+import org.jetbrains.kotlin.test.directives.model.singleOrZeroValue
 import org.jetbrains.kotlin.test.frontend.classic.ClassicFrontendOutputArtifact
 import org.jetbrains.kotlin.test.frontend.fir.FirOutputArtifact
 import org.jetbrains.kotlin.test.model.*
@@ -42,11 +45,17 @@ class LoadedMetadataDumpHandler(testServices: TestServices) : JvmBinaryArtifactH
 
     override fun processModule(module: TestModule, info: BinaryArtifacts.Jvm) {
         if (testServices.loadedMetadataSuppressionDirective in module.directives) return
+        val languageVersion = module.directives.singleOrZeroValue(LANGUAGE_VERSION)
+        val languageVersionSettings = if (languageVersion != null) {
+            LanguageVersionSettingsImpl(languageVersion, ApiVersion.createByLanguageVersion(languageVersion))
+        } else {
+            LanguageVersionSettingsImpl.DEFAULT
+        }
         val emptyModule = TestModule(
             name = "empty", JvmPlatforms.defaultJvmPlatform, TargetBackend.JVM_IR, FrontendKinds.FIR,
             BackendKinds.IrBackend, ArtifactKinds.Jvm, files = emptyList(),
             allDependencies = listOf(DependencyDescription(module.name, DependencyKind.Binary, DependencyRelation.RegularDependency)),
-            RegisteredDirectives.Empty, LanguageVersionSettingsImpl.DEFAULT
+            RegisteredDirectives.Empty, languageVersionSettings
         )
         val configuration = testServices.compilerConfigurationProvider.getCompilerConfiguration(emptyModule)
         val environment = VfsBasedProjectEnvironment(

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt

@@ -7,7 +7,9 @@ package org.jetbrains.kotlin.fir.serialization
 
 import org.jetbrains.kotlin.builtins.StandardNames
 import org.jetbrains.kotlin.builtins.functions.FunctionTypeKind
+import org.jetbrains.kotlin.builtins.functions.isBuiltin
 import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.config.LanguageVersion
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.constant.EnumValue
 import org.jetbrains.kotlin.constant.IntValue
@@ -823,7 +825,8 @@ class FirElementSerializer private constructor(
                 return lowerBound
             }
             is ConeClassLikeType -> {
-                if (type.functionTypeKind(session) == FunctionTypeKind.SuspendFunction) {
+                val functionTypeKind = type.functionTypeKind(session)
+                if (functionTypeKind == FunctionTypeKind.SuspendFunction) {
                     val runtimeFunctionType = type.suspendFunctionTypeToFunctionTypeWithContinuation(
                         session, StandardClassIds.Continuation
                     )
@@ -831,6 +834,13 @@ class FirElementSerializer private constructor(
                     functionType.flags = Flags.getTypeFlags(true, false)
                     return functionType
                 }
+                if (functionTypeKind?.isBuiltin == false) {
+                    val legacySerializationUntil =
+                        LanguageVersion.fromVersionString(functionTypeKind.serializeAsFunctionWithAnnotationUntil)
+                    if (legacySerializationUntil != null && languageVersionSettings.languageVersion < legacySerializationUntil) {
+                        return typeProto(type.customFunctionTypeToSimpleFunctionType(session))
+                    }
+                }
                 fillFromPossiblyInnerType(builder, type)
                 if (type.hasContextReceivers) {
                     typeAnnotations.addIfNotNull(