Added support for the kotlin.Nothing class as built-in (#1991)

Resolves #614
Resolves #932

Co-authored-by: Paul de Vrieze <pdvrieze@users.noreply.github.com>

Author: shanshin

core/api/kotlinx-serialization-core.api

@@ -166,6 +166,7 @@ public final class kotlinx/serialization/builtins/BuiltinSerializersKt {
 	public static final fun LongArraySerializer ()Lkotlinx/serialization/KSerializer;
 	public static final fun MapEntrySerializer (Lkotlinx/serialization/KSerializer;Lkotlinx/serialization/KSerializer;)Lkotlinx/serialization/KSerializer;
 	public static final fun MapSerializer (Lkotlinx/serialization/KSerializer;Lkotlinx/serialization/KSerializer;)Lkotlinx/serialization/KSerializer;
+	public static final fun NothingSerializer ()Lkotlinx/serialization/KSerializer;
 	public static final fun PairSerializer (Lkotlinx/serialization/KSerializer;Lkotlinx/serialization/KSerializer;)Lkotlinx/serialization/KSerializer;
 	public static final fun SetSerializer (Lkotlinx/serialization/KSerializer;)Lkotlinx/serialization/KSerializer;
 	public static final fun ShortArraySerializer ()Lkotlinx/serialization/KSerializer;
@@ -902,6 +903,15 @@ public abstract class kotlinx/serialization/internal/NamedValueEncoder : kotlinx
 	protected final fun nested (Ljava/lang/String;)Ljava/lang/String;
 }
 
+public final class kotlinx/serialization/internal/NothingSerializer : kotlinx/serialization/KSerializer {
+	public static final field INSTANCE Lkotlinx/serialization/internal/NothingSerializer;
+	public synthetic fun deserialize (Lkotlinx/serialization/encoding/Decoder;)Ljava/lang/Object;
+	public fun deserialize (Lkotlinx/serialization/encoding/Decoder;)Ljava/lang/Void;
+	public fun getDescriptor ()Lkotlinx/serialization/descriptors/SerialDescriptor;
+	public synthetic fun serialize (Lkotlinx/serialization/encoding/Encoder;Ljava/lang/Object;)V
+	public fun serialize (Lkotlinx/serialization/encoding/Encoder;Ljava/lang/Void;)V
+}
+
 public final class kotlinx/serialization/internal/NullableSerializer : kotlinx/serialization/KSerializer {
 	public fun <init> (Lkotlinx/serialization/KSerializer;)V
 	public fun deserialize (Lkotlinx/serialization/encoding/Decoder;)Ljava/lang/Object;

core/commonMain/src/kotlinx/serialization/builtins/BuiltinSerializers.kt

@@ -250,3 +250,12 @@ public fun UShort.Companion.serializer(): KSerializer<UShort> = UShortSerializer
  * The result of serialization is similar to calling [Duration.toIsoString], for deserialization is [Duration.parseIsoString].
  */
 public fun Duration.Companion.serializer(): KSerializer<Duration> = DurationSerializer
+
+/**
+ * Returns serializer for [Nothing].
+ * Throws an exception when trying to encode or decode.
+ *
+ * It is used as a dummy in case it is necessary to pass a type to a parameterized class. At the same time, it is expected that this generic type will not participate in serialization.
+ */
+@ExperimentalSerializationApi
+public fun NothingSerializer(): KSerializer<Nothing> = NothingSerializer

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

@@ -4,6 +4,7 @@
 package kotlinx.serialization.internal
 
 import kotlinx.serialization.KSerializer
+import kotlinx.serialization.SerializationException
 import kotlinx.serialization.descriptors.PrimitiveKind
 import kotlinx.serialization.descriptors.SerialDescriptor
 import kotlinx.serialization.encoding.Decoder
@@ -23,3 +24,16 @@ internal object DurationSerializer : KSerializer<Duration> {
         return Duration.parseIsoString(decoder.decodeString())
     }
 }
+
+@PublishedApi
+internal object NothingSerializer : KSerializer<Nothing> {
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("kotlin.Nothing", PrimitiveKind.INT)
+
+    override fun serialize(encoder: Encoder, value: Nothing) {
+        throw SerializationException("'kotlin.Nothing' cannot be serialized")
+    }
+
+    override fun deserialize(decoder: Decoder): Nothing {
+        throw SerializationException("'kotlin.Nothing' does not have instances")
+    }
+}

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

@@ -44,6 +44,7 @@ private val BUILTIN_SERIALIZERS = mapOf(
     Boolean::class to Boolean.serializer(),
     BooleanArray::class to BooleanArraySerializer(),
     Unit::class to Unit.serializer(),
+    Nothing::class to NothingSerializer(),
     Duration::class to Duration.serializer()
 )
 

core/commonTest/src/kotlinx/serialization/BasicTypesSerializationTest.kt

@@ -4,6 +4,7 @@
 
 package kotlinx.serialization
 
+import kotlinx.serialization.builtins.NothingSerializer
 import kotlinx.serialization.builtins.serializer
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
@@ -191,4 +192,20 @@ class BasicTypesSerializationTest {
         assertEquals(Duration.parseIsoString(durationString), other)
     }
 
+    @Test
+    fun testNothingSerialization() {
+        // impossible to deserialize Nothing
+        assertFailsWith(SerializationException::class, "'kotlin.Nothing' does not have instances") {
+            val inp = KeyValueInput(Parser(StringReader("42")))
+            @Suppress("IMPLICIT_NOTHING_TYPE_ARGUMENT_IN_RETURN_POSITION")
+            inp.decodeSerializableValue(NothingSerializer())
+        }
+
+        // it is possible to serialize only `null` for `Nothing?`
+        val sb = StringBuilder()
+        val out = KeyValueOutput(sb)
+        out.encodeNullableSerializableValue(NothingSerializer(), null)
+        assertEquals("null", sb.toString())
+    }
+
 }

docs/builtin-classes.md

@@ -24,6 +24,7 @@ including the standard collections, is built into Kotlin Serialization. This cha
   * [Maps](#maps)
   * [Unit and singleton objects](#unit-and-singleton-objects)
   * [Duration](#duration)
+* [Nothing](#nothing)
 
 <!--- END -->
 
@@ -405,6 +406,33 @@ Duration is serialized as a string in the ISO-8601-2 format.
 
 <!--- TEST -->
 
+
+## Nothing
+
+By default, [Nothing] is a serializable class. However, since there are no instances of this class, it is impossible to encode or decode its values - any attempt will cause an exception.
+
+This serializer is used when syntactically some type is needed, but it is not actually used in serialization. For example, when using parameterized polymorphic base classes:
+```kotlin
+@Serializable
+sealed class ParametrizedParent<out R> {
+    @Serializable
+    data class ChildWithoutParameter(val value: Int) : ParametrizedParent<Nothing>()
+}
+
+fun main() {
+    println(Json.encodeToString(ParametrizedParent.ChildWithoutParameter(42)))
+}
+``` 
+> You can get the full code [here](../guide/example/example-builtin-13.kt).
+
+When encoding, the serializer for the `Nothing` was not used
+
+```text
+{"value":42}
+```
+
+<!--- TEST -->
+
 ---
 
 The next chapter covers [Serializers](serializers.md).
@@ -418,6 +446,7 @@ The next chapter covers [Serializers](serializers.md).
 [Set]: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-set/ 
 [Map]: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-map/ 
 [Duration]: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.time/-duration/
+[Nothing]: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-nothing.html
 
 <!--- MODULE /kotlinx-serialization-core -->
 <!--- INDEX kotlinx-serialization-core/kotlinx.serialization -->

docs/serialization-guide.md

@@ -50,6 +50,7 @@ Once the project is set up, we can start serializing some classes.
   * <a name='maps'></a>[Maps](builtin-classes.md#maps)
   * <a name='unit-and-singleton-objects'></a>[Unit and singleton objects](builtin-classes.md#unit-and-singleton-objects)
   * <a name='duration'></a>[Duration](builtin-classes.md#duration)
+* <a name='nothing'></a>[Nothing](builtin-classes.md#nothing)
 <!--- END -->
 
 **Chapter 3.** [Serializers](serializers.md)

guide/example/example-builtin-13.kt

@@ -0,0 +1,15 @@
+// This file was automatically generated from builtin-classes.md by Knit tool. Do not edit.
+package example.exampleBuiltin13
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable
+sealed class ParametrizedParent<out R> {
+    @Serializable
+    data class ChildWithoutParameter(val value: Int) : ParametrizedParent<Nothing>()
+}
+
+fun main() {
+    println(Json.encodeToString(ParametrizedParent.ChildWithoutParameter(42)))
+}

guide/test/BuiltinClassesTest.kt

@@ -89,4 +89,11 @@ class BuiltinClassesTest {
             "\"PT16M40S\""
         )
     }
+
+    @Test
+    fun testExampleBuiltin13() {
+        captureOutput("ExampleBuiltin13") { example.exampleBuiltin13.main() }.verifyOutputLines(
+            "{\"value\":42}"
+        )
+    }
 }