Added SerialDescriptor object for NothingSerializer (#2150)

Improvement of #932

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

@@ -27,7 +27,7 @@ internal object DurationSerializer : KSerializer<Duration> {
 
 @PublishedApi
 internal object NothingSerializer : KSerializer<Nothing> {
-    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("kotlin.Nothing", PrimitiveKind.INT)
+    override val descriptor: SerialDescriptor = NothingSerialDescriptor
 
     override fun serialize(encoder: Encoder, value: Nothing) {
         throw SerializationException("'kotlin.Nothing' cannot be serialized")

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

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2017-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+@file:OptIn(ExperimentalSerializationApi::class)
+
+package kotlinx.serialization.internal
+
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.descriptors.SerialKind
+import kotlinx.serialization.descriptors.StructureKind
+
+internal object NothingSerialDescriptor : SerialDescriptor {
+    public override val kind: SerialKind = StructureKind.OBJECT
+
+    public override val serialName: String = "kotlin.Nothing"
+
+    override val elementsCount: Int get() = 0
+    override fun getElementName(index: Int): String = error()
+    override fun getElementIndex(name: String): Int = error()
+    override fun isElementOptional(index: Int): Boolean = error()
+    override fun getElementDescriptor(index: Int): SerialDescriptor = error()
+    override fun getElementAnnotations(index: Int): List<Annotation> = error()
+    override fun toString(): String = "NothingSerialDescriptor"
+    override fun equals(other: Any?): Boolean {
+        return this === other
+    }
+
+    override fun hashCode(): Int = serialName.hashCode() + 31 * kind.hashCode()
+    private fun error(): Nothing =
+        throw IllegalStateException("Descriptor for type `kotlin.Nothing` does not have elements")
+}