Do not create cacheableChildSerializers unless it is necessary

Creation of this property in the SerializerIrGenerator.<init> can lead to
a 'Serializer not found' internal error when generator is applied to a fully-customized external serializer.
In that case, generator is still created, but none of the generateSave/Load functions are called,
so cacheableChildSerializers(Property) is not necessary.

#KT-57730 Fixed
Fixes Kotlin/kotlinx.serialization#2260

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

@@ -66,11 +66,9 @@ open class SerializerIrGenerator(
         SerialEntityNames.SERIAL_DESC_FIELD
     ) { true }?.takeIf { it.isFromPlugin(compilerContext.afterK2) }
 
-    protected val anySerialDescProperty = getProperty(
+    protected val irAnySerialDescProperty = getProperty(
         SerialEntityNames.SERIAL_DESC_FIELD,
-    ) { true } // remove true?
-
-    protected val irAnySerialDescProperty = anySerialDescProperty
+    ) { true }
 
     fun getProperty(
         name: String,
@@ -83,13 +81,16 @@ open class SerializerIrGenerator(
         private set
 
     // child serializers cached if serializable class is internal
-    private val cachedChildSerializersProperty = if (isGeneratedSerializer)
-        serializableIrClass.companionObject()?.properties?.singleOrNull { it.name == CACHED_CHILD_SERIALIZERS_PROPERTY_NAME }
-    else null
+    private val cachedChildSerializersProperty by lazy {
+        if (isGeneratedSerializer)
+            serializableIrClass.companionObject()?.properties?.singleOrNull { it.name == CACHED_CHILD_SERIALIZERS_PROPERTY_NAME }
+        else null
+    }
 
     // non-object serializers which can be cached
-    private val cacheableChildSerializers =
+    private val cacheableChildSerializers by lazy {
         serializableIrClass.createCachedChildSerializers(properties.serializableProperties).map { it != null }
+    }
 
     // null if was not found — we're in FIR
     private fun findLocalSerializersFieldDescriptors(): List<IrProperty?> {
@@ -230,7 +231,12 @@ open class SerializerIrGenerator(
 
         val allSerializers = serializableProperties.mapIndexed { index, property ->
             requireNotNull(
-                serializerTower(this@SerializerIrGenerator, irFun.dispatchReceiverParameter!!, property, cachedChildSerializerByIndex(index))
+                serializerTower(
+                    this@SerializerIrGenerator,
+                    irFun.dispatchReceiverParameter!!,
+                    property,
+                    cachedChildSerializerByIndex(index)
+                )
             ) { "Property ${property.name} must have a serializer" }
         }
 

plugins/kotlinx-serialization/testData/boxIr/externalSerialierJava.kt

@@ -25,4 +25,4 @@ object URLSerializer : KSerializer<URL> {
 fun box(): String {
     if (URLSerializer.descriptor.toString() != "PrimitiveDescriptor(java.net.URL)") return URLSerializer.descriptor.toString()
     return "OK"
-}
+}

plugins/kotlinx-serialization/testData/boxIr/externalSerializerForClassWithNonSerializableType.kt

@@ -0,0 +1,30 @@
+// TARGET_BACKEND: JVM_IR
+
+// FULL_JDK
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import java.math.BigDecimal
+
+@Serializable(with = DataBigDecimal.Serializer::class)
+data class DataBigDecimal(val value: BigDecimal) {
+
+    @kotlinx.serialization.Serializer(forClass = DataBigDecimal::class)
+    object Serializer : KSerializer<DataBigDecimal> {
+
+        override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("my.DataBigDecimal", PrimitiveKind.STRING)
+
+        override fun deserialize(decoder: Decoder): DataBigDecimal =
+            TODO()
+
+        override fun serialize(encoder: Encoder, value: DataBigDecimal): Unit =
+            TODO()
+    }
+}
+
+fun box(): String {
+    if (DataBigDecimal.Serializer.descriptor.toString() != "PrimitiveDescriptor(my.DataBigDecimal)") return DataBigDecimal.Serializer.descriptor.toString()
+    return "OK"
+}