[KxSerialization] Fixed error accessing the descriptor from companion

Relates #KT-57647

If a serialization descriptor is used in the companion, then when accessing the child elements, an array of cached child serializers may be read.
Since the serialization plugin adds its declarations to the class after the declarations from the source code, the initialization of the array of child serializers occurs after the execution of the user code.
This can lead to N PE errors when trying to access an unfilled cached child serializer.

The solution is to change the order of declarations so that the declaration with the property of cached child serializers comes first.

Merge-request: KT-MR-10545
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

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

@@ -398,9 +398,17 @@ abstract class BaseIrGenerator(private val currentClass: IrClass, final override
         val kSerializerType = kSerializerClass.typeWith(compilerContext.irBuiltIns.anyType)
         val arrayType = compilerContext.irBuiltIns.arrayClass.typeWith(kSerializerType)
 
-        return addValPropertyWithJvmFieldInitializer(arrayType, SerialEntityNames.CACHED_CHILD_SERIALIZERS_PROPERTY_NAME) {
+        val property = addValPropertyWithJvmFieldInitializer(arrayType, SerialEntityNames.CACHED_CHILD_SERIALIZERS_PROPERTY_NAME) {
             createArrayOfExpression(kSerializerType, cacheableSerializers.map { it ?: irNull() })
         }
+
+        if (declarations.removeIf { declaration -> declaration === property }) {
+            // adding the property very first because children can be used even in first constructor
+            declarations.add(0, property)
+        }
+
+
+        return property
     }
 
     /**

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

@@ -32,6 +32,18 @@ sealed class SealedListGeneric {
     var holder = GenericHolder<GenericHolder<List<SealedListGeneric>>>()
 }
 
+@Serializable
+class AccessFromCompanion(@Contextual val category: Any) {
+    companion object {
+        init {
+            serializer().descriptor.getElementDescriptor(0).isNullable
+        }
+        var isNullable = serializer().descriptor.getElementDescriptor(0).isNullable
+        val isNullable2 = serializer<AccessFromCompanion>().descriptor.getElementDescriptor(0).isNullable
+    }
+}
+
+
 fun box(): String {
     // A correctly cached class must be initialized correctly in order to exclude cyclic nesting of caches
     Plain.serializer()
@@ -40,6 +52,7 @@ fun box(): String {
     Abstract.serializer()
     SealedGeneric.serializer()
     SealedListGeneric.serializer()
+    AccessFromCompanion("any")
 
     return "OK"
 }