Add extension to access original descriptor from one made with SerialDescriptor.nullable (#2633)

It may be required when a custom class implementing SerialDescriptor is used for schema generation.

Closes #2631

Author: Chuckame

core/api/kotlinx-serialization-core.api

@@ -308,6 +308,7 @@ public final class kotlinx/serialization/descriptors/SerialDescriptorsKt {
 	public static synthetic fun buildClassSerialDescriptor$default (Ljava/lang/String;[Lkotlinx/serialization/descriptors/SerialDescriptor;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)Lkotlinx/serialization/descriptors/SerialDescriptor;
 	public static final fun buildSerialDescriptor (Ljava/lang/String;Lkotlinx/serialization/descriptors/SerialKind;[Lkotlinx/serialization/descriptors/SerialDescriptor;Lkotlin/jvm/functions/Function1;)Lkotlinx/serialization/descriptors/SerialDescriptor;
 	public static synthetic fun buildSerialDescriptor$default (Ljava/lang/String;Lkotlinx/serialization/descriptors/SerialKind;[Lkotlinx/serialization/descriptors/SerialDescriptor;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)Lkotlinx/serialization/descriptors/SerialDescriptor;
+	public static final fun getNonNullOriginal (Lkotlinx/serialization/descriptors/SerialDescriptor;)Lkotlinx/serialization/descriptors/SerialDescriptor;
 	public static final fun getNullable (Lkotlinx/serialization/descriptors/SerialDescriptor;)Lkotlinx/serialization/descriptors/SerialDescriptor;
 	public static final fun listSerialDescriptor (Lkotlinx/serialization/descriptors/SerialDescriptor;)Lkotlinx/serialization/descriptors/SerialDescriptor;
 	public static final fun mapSerialDescriptor (Lkotlinx/serialization/descriptors/SerialDescriptor;Lkotlinx/serialization/descriptors/SerialDescriptor;)Lkotlinx/serialization/descriptors/SerialDescriptor;

core/commonMain/src/kotlinx/serialization/descriptors/SerialDescriptors.kt

@@ -223,6 +223,24 @@ public val SerialDescriptor.nullable: SerialDescriptor
         return SerialDescriptorForNullable(this)
     }
 
+/**
+ * Returns non-nullable serial descriptor for the type if this descriptor has been auto-generated (plugin
+ * generated descriptors) or created with `.nullable` extension on a descriptor or serializer.
+ *
+ * Otherwise, returns this.
+ *
+ * It may return nullable descriptor if this descriptor has been created manually as nullable by directly implementing SerialDescriptor interface.
+ *
+ * @see SerialDescriptor.nullable
+ * @see KSerializer.nullable
+ */
+@ExperimentalSerializationApi
+public val SerialDescriptor.nonNullOriginal: SerialDescriptor
+    get() = when (this) {
+        is SerialDescriptorForNullable -> original
+        else -> this
+    }
+
 /**
  * Builder for [SerialDescriptor] for user-defined serializers.
  *

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

@@ -94,4 +94,44 @@ class SerialDescriptorBuilderTest {
         assertTrue(descriptor.isNullable)
         assertEquals("my.Simple?", descriptor.serialName)
     }
+
+    @Test
+    fun testNonNullOriginal() {
+        listOf(
+            buildClassSerialDescriptor("my.Simple") {},
+            Boolean.serializer().descriptor,
+            String.serializer().descriptor,
+            ListSerializer(Int.serializer()).descriptor,
+        ).forEach { originalDescriptor ->
+            // Unwrapping original descriptor when it is not nullable should return the same descriptor (no-op operation)
+            assertSame(originalDescriptor.nonNullOriginal, originalDescriptor)
+
+            // Unwrapping original descriptor when it is nullable should return the original descriptor
+            assertSame(originalDescriptor.nullable.nonNullOriginal, originalDescriptor)
+        }
+
+        // Unwrapping original descriptor of a custom nullable descriptor should return the same descriptor
+        val customNullableDescriptor = CustomNullableDescriptor()
+        assertSame(customNullableDescriptor.nonNullOriginal, customNullableDescriptor)
+
+        // Unwrapping original descriptor of a nullable field should return the original non-null descriptor
+        assertSame(Type.serializer().descriptor.getElementDescriptor(0).nonNullOriginal, String.serializer().descriptor)
+
+    }
+
+    @Serializable
+    class Type(val field: String?)
+
+    private class CustomNullableDescriptor : SerialDescriptor {
+        override val isNullable: Boolean = true
+
+        override val serialName: String = "CustomNullableDescriptor"
+        override val kind: SerialKind = PrimitiveKind.STRING
+        override val elementsCount: Int = 0
+        override fun getElementName(index: Int): String = error("Should not be called")
+        override fun getElementIndex(name: String): Int = error("Should not be called")
+        override fun isElementOptional(index: Int): Boolean = error("Should not be called")
+        override fun getElementAnnotations(index: Int): List<Annotation> = error("Should not be called")
+        override fun getElementDescriptor(index: Int): SerialDescriptor = error("Should not be called")
+    }
 }