Partially implement storing the map from polymorphic subclasses to polymorphic numbers in SerializersModule

This may not improve performance much so it's dropped for now.

Author: ShreckYe

core/commonMain/src/kotlinx/serialization/internal/AbstractPolymorphicSerializer.kt

@@ -31,13 +31,17 @@ public abstract class AbstractPolymorphicSerializer<T : Any> internal constructo
 
     public final override fun serialize(encoder: Encoder, value: T) {
         val actualSerializer = findPolymorphicSerializer(encoder, value)
+        val serialPolymorphicNumberInSerializersModule = if (descriptor.useSerialPolymorphicNumbers)
+            findSerialPolymorphicNumberOrNull(encoder, value)
+        else
+            null
         encoder.encodeStructure(descriptor) {
             if (descriptor.useSerialPolymorphicNumbers)
                 encodeIntElement(
                     descriptor,
                     0,
-                    // it seems not possible to cache this with the current implementation that serializers are completely separated from serializers modules
-                    actualSerializer.descriptor.serialPolymorphicNumberByBaseClass.getValue(baseClass)
+                    serialPolymorphicNumberInSerializersModule
+                        ?: actualSerializer.descriptor.serialPolymorphicNumberByBaseClass.getValue(baseClass)
                 )
             else
                 encodeStringElement(descriptor, 0, actualSerializer.descriptor.serialName)
@@ -125,6 +129,16 @@ public abstract class AbstractPolymorphicSerializer<T : Any> internal constructo
         value: T
     ): SerializationStrategy<T>? =
         encoder.serializersModule.getPolymorphic(baseClass, value)
+
+    /**
+     *  TODO
+     */
+    @InternalSerializationApi
+    public fun <T : Any> AbstractPolymorphicSerializer<T>.findSerialPolymorphicNumberOrNull(
+        encoder: Encoder,
+        value: T
+    ): Int? =
+        encoder.serializersModule.getSerialPolymorphicNumber(baseClass, value)
 }
 
 @JvmName("throwSubtypeNotRegistered")

core/commonMain/src/kotlinx/serialization/modules/SerializersModule.kt

@@ -69,10 +69,8 @@ public sealed class SerializersModule {
         baseClass: KClass<in T>, serializedNumber: Int?
     ): DeserializationStrategy<T>?
 
-    // TODO remove or use
-    // TODO old design for cashing serializers by number in `AbstractPolymorphicSerializer` which probably doesn't work and is not needed
     @ExperimentalSerializationApi
-    public abstract fun <T : Any> getPolymorphicForAllSubclasses(baseClass: KClass<T>): Map<KClass<out T>, KSerializer<out T>>
+    public abstract fun <T : Any> getSerialPolymorphicNumber(baseClass: KClass<in T>, value: T): Int?
 
     /**
      * Copies contents of this module to the given [collector].
@@ -203,10 +201,9 @@ internal class SerialModuleImpl(
         )
     }
 
-    // TODO remove or use
-    @ExperimentalSerializationApi
-    override fun <T : Any> getPolymorphicForAllSubclasses(baseClass: KClass<T>): Map<KClass<out T>, KSerializer<out T>> =
-        polyBase2Serializers.getValue(baseClass) as Map<KClass<out T>, KSerializer<out T>>
+    override fun <T : Any> getSerialPolymorphicNumber(baseClass: KClass<in T>, value: T): Int? {
+        TODO("Not yet implemented")
+    }
 
     override fun <T : Any> getContextual(kClass: KClass<T>, typeArgumentsSerializers: List<KSerializer<*>>): KSerializer<T>? {
         return (class2ContextualFactory[kClass]?.invoke(typeArgumentsSerializers)) as? KSerializer<T>?