Remove experimentality from serializer(java.lang.Type) function family (

#2069)

As it is needed for third-part converter libraries.

Revamp docs for all overloads of serializer().

core/commonMain/src/kotlinx/serialization/Serializers.kt

@@ -18,54 +18,112 @@ import kotlin.reflect.*
 
 /**
  * Retrieves a serializer for the given type [T].
- * This method is a reified version of `serializer(KType)`.
+ * This overload is a reified version of `serializer(KType)`.
+ *
+ * This overload works with full type information, including type arguments and nullability,
+ * and is a recommended way to retrieve a serializer.
+ * For example, `serializer<List<String?>>()` returns [KSerializer] that is able
+ * to serialize and deserialize list of nullable strings — i.e. `ListSerializer(String.serializer().nullable)`.
+ *
+ * Variance of [T]'s type arguments is not used by the serialization and is not taken into account.
+ * Star projections in [T]'s type arguments are prohibited.
+ *
+ * @throws SerializationException if serializer cannot be created (provided [T] or its type argument is not serializable).
+ * @throws IllegalArgumentException if any of [T]'s type arguments contains star projection
  */
 public inline fun <reified T> serializer(): KSerializer<T> {
     return serializer(typeOf<T>()).cast()
 }
 
 /**
- * Retrieves serializer for the given type [T] from the current [SerializersModule] and,
- * if not found, fallbacks to plain [serializer] method.
+ * Retrieves default serializer for the given type [T] and,
+ * if [T] is not serializable, fallbacks to [contextual][SerializersModule.getContextual] lookup.
+ *
+ * This overload works with full type information, including type arguments and nullability,
+ * and is a recommended way to retrieve a serializer.
+ * For example, `serializer<List<String?>>()` returns [KSerializer] that is able
+ * to serialize and deserialize list of nullable strings — i.e. `ListSerializer(String.serializer().nullable)`.
+ *
+ * Variance of [T]'s type arguments is not used by the serialization and is not taken into account.
+ * Star projections in [T]'s type arguments are prohibited.
+ *
+ * @throws SerializationException if serializer cannot be created (provided [T] or its type argument is not serializable).
+ * @throws IllegalArgumentException if any of [T]'s type arguments contains star projection
  */
 public inline fun <reified T> SerializersModule.serializer(): KSerializer<T> {
     return serializer(typeOf<T>()).cast()
 }
 
 /**
  * Creates a serializer for the given [type].
- * [type] argument can be obtained with experimental [typeOf] method.
- *  @throws SerializationException if serializer cannot be created (provided [type] or its type argument is not serializable).
+ * [type] argument is usually obtained with [typeOf] method.
+ *
+ * This overload works with full type information, including type arguments and nullability,
+ * and is a recommended way to retrieve a serializer.
+ * For example, `serializer<typeOf<List<String?>>>()` returns [KSerializer] that is able
+ * to serialize and deserialize list of nullable strings — i.e. `ListSerializer(String.serializer().nullable)`.
+ *
+ * Variance of [type]'s type arguments is not used by the serialization and is not taken into account.
+ * Star projections in [type]'s arguments are prohibited.
+ *
+ * @throws SerializationException if serializer cannot be created (provided [type] or its type argument is not serializable).
+ * @throws IllegalArgumentException if any of [type]'s arguments contains star projection
  */
-@OptIn(ExperimentalSerializationApi::class)
 public fun serializer(type: KType): KSerializer<Any?> = EmptySerializersModule().serializer(type)
 
 /**
- * Creates a serializer for the given [type].
- * [type] argument can be obtained with experimental [typeOf] method.
- * Returns `null` if serializer cannot be created (provided [type] or its type argument is not serializable).
+ * Creates a serializer for the given [type] if possible.
+ * [type] argument is usually obtained with [typeOf] method.
+ *
+ * This overload works with full type information, including type arguments and nullability,
+ * and is a recommended way to retrieve a serializer.
+ * For example, `serializerOrNull<typeOf<List<String?>>>()` returns [KSerializer] that is able
+ * to serialize and deserialize list of nullable strings — i.e. `ListSerializer(String.serializer().nullable)`.
+ *
+ * Variance of [type]'s arguments is not used by the serialization and is not taken into account.
+ * Star projections in [type]'s arguments are prohibited.
+ *
+ * @returns [KSerializer] for the given [type] or `null` if serializer cannot be created (given [type] or its type argument is not serializable).
+ * @throws IllegalArgumentException if any of [type]'s arguments contains star projection
  */
-@OptIn(ExperimentalSerializationApi::class)
 public fun serializerOrNull(type: KType): KSerializer<Any?>? = EmptySerializersModule().serializerOrNull(type)
 
 /**
- * Attempts to create a serializer for the given [type] and fallbacks to [contextual][SerializersModule.getContextual]
- * lookup for non-serializable types.
- * [type] argument can be obtained with experimental [typeOf] method.
+ * Retrieves default serializer for the given [type] and,
+ * if [type] is not serializable, fallbacks to [contextual][SerializersModule.getContextual] lookup.
+ * [type] argument is usually obtained with [typeOf] method.
+ *
+ * This overload works with full type information, including type arguments and nullability,
+ * and is a recommended way to retrieve a serializer.
+ * For example, `serializer<typeOf<List<String?>>>()` returns [KSerializer] that is able
+ * to serialize and deserialize list of nullable strings — i.e. `ListSerializer(String.serializer().nullable)`.
+ *
+ * Variance of [type]'s arguments is not used by the serialization and is not taken into account.
+ * Star projections in [type]'s arguments are prohibited.
+ *
  * @throws SerializationException if serializer cannot be created (provided [type] or its type argument is not serializable and is not registered in [this] module).
+ * @throws IllegalArgumentException if any of [type]'s arguments contains star projection
  */
-@OptIn(ExperimentalSerializationApi::class)
 public fun SerializersModule.serializer(type: KType): KSerializer<Any?> =
     serializerByKTypeImpl(type, failOnMissingTypeArgSerializer = true) ?: type.kclass()
         .platformSpecificSerializerNotRegistered()
 
 /**
- * Attempts to create a serializer for the given [type] and fallbacks to [contextual][SerializersModule.getContextual]
- * lookup for non-serializable types.
- * [type] argument can be obtained with experimental [typeOf] method.
- * Returns `null` if serializer cannot be created (provided [type] or its type argument is not serializable and is not registered in [this] module).
+ * Retrieves default serializer for the given [type] and,
+ * if [type] is not serializable, fallbacks to [contextual][SerializersModule.getContextual] lookup.
+ * [type] argument is usually obtained with [typeOf] method.
+ *
+ * This overload works with full type information, including type arguments and nullability,
+ * and is a recommended way to retrieve a serializer.
+ * For example, `serializerOrNull<typeOf<List<String?>>>()` returns [KSerializer] that is able
+ * to serialize and deserialize list of nullable strings — i.e. `ListSerializer(String.serializer().nullable)`.
+ *
+ * Variance of [type]'s arguments is not used by the serialization and is not taken into account.
+ * Star projections in [type]'s arguments are prohibited.
+ *
+ * @returns [KSerializer] for the given [type] or `null` if serializer cannot be created (given [type] or its type argument is not serializable and is not registered in [this] module).
+ * @throws IllegalArgumentException if any of [type]'s arguments contains star projection
  */
-@OptIn(ExperimentalSerializationApi::class)
 public fun SerializersModule.serializerOrNull(type: KType): KSerializer<Any?>? =
     serializerByKTypeImpl(type, failOnMissingTypeArgSerializer = false)
 
@@ -189,7 +247,6 @@ private fun KClass<Any>.builtinParametrizedSerializer(
     typeArguments: List<KType>,
     serializers: List<KSerializer<Any?>>,
 ): KSerializer<out Any>? {
-    // Array is not supported, see KT-32839
     return when (this) {
         Collection::class, List::class, MutableList::class, ArrayList::class -> ArrayListSerializer(serializers[0])
         HashSet::class -> HashSetSerializer(serializers[0])

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

@@ -36,6 +36,7 @@ public inline fun SerializersModule(builderAction: SerializersModuleBuilder.() -
 /**
  * A [SerializersModule] which is empty and returns `null` from each method.
  */
+@Suppress("FunctionName")
 public fun EmptySerializersModule(): SerializersModule = @Suppress("DEPRECATION") EmptySerializersModule
 
 /**

core/jvmMain/src/kotlinx/serialization/SerializersJvm.kt

@@ -17,65 +17,86 @@ import java.lang.reflect.*
 import kotlin.reflect.*
 
 /**
- * Reflectively constructs a serializer for the given reflective Java [type].
- * [serializer] is intended to be used as an interoperability layer for libraries like GSON and Retrofit,
- * that operate with reflective Java [Type] and cannot use [typeOf].
+ * Reflectively retrieves a serializer for the given [type].
  *
- * For application-level serialization, it is recommended to use `serializer<T>()` instead as it is aware of
+ * This overload is intended to be used as an interoperability layer for JVM-centric libraries,
+ * that operate with Java's type tokens and cannot use Kotlin's [KType] or [typeOf].
+ * For application-level serialization, it is recommended to use `serializer<T>()` or `serializer(KType)` instead as it is aware of
  * Kotlin-specific type information, such as nullability, sealed classes and object singletons.
  *
+ * Note that because [Type] does not contain any information about nullability, all created serializers
+ * work only with non-nullable data.
+ *
+ * Not all [Type] implementations are supported.
+ * [type] must be an instance of [Class], [GenericArrayType], [ParameterizedType] or [WildcardType].
+ *
  * @throws SerializationException if serializer cannot be created (provided [type] or its type argument is not serializable).
+ * @throws IllegalArgumentException if an unsupported subclass of [Type] is provided.
  */
-@ExperimentalSerializationApi
 public fun serializer(type: Type): KSerializer<Any> = EmptySerializersModule().serializer(type)
 
 /**
- * Reflectively constructs a serializer for the given reflective Java [type].
- * [serializer] is intended to be used as an interoperability layer for libraries like GSON and Retrofit,
- * that operate with reflective Java [Type] and cannot use [typeOf].
+ * Reflectively retrieves a serializer for the given [type].
  *
- * For application-level serialization, it is recommended to use `serializer<T>()` instead as it is aware of
+ * This overload is intended to be used as an interoperability layer for JVM-centric libraries,
+ * that operate with Java's type tokens and cannot use Kotlin's [KType] or [typeOf].
+ * For application-level serialization, it is recommended to use `serializer<T>()` or `serializer(KType)` instead as it is aware of
  * Kotlin-specific type information, such as nullability, sealed classes and object singletons.
  *
- * Returns `null` if serializer cannot be created (provided [type] or its type argument is not serializable).
+ * Note that because [Type] does not contain any information about nullability, all created serializers
+ * work only with non-nullable data.
+ *
+ * Not all [Type] implementations are supported.
+ * [type] must be an instance of [Class], [GenericArrayType], [ParameterizedType] or [WildcardType].
+ *
+ * @return [KSerializer] for given [type] or `null` if serializer cannot be created (given [type] or its type argument is not serializable).
+ * @throws IllegalArgumentException if an unsupported subclass of [Type] is provided.
  */
-@ExperimentalSerializationApi
 public fun serializerOrNull(type: Type): KSerializer<Any>? = EmptySerializersModule().serializerOrNull(type)
 
 /**
- * Retrieves serializer for the given reflective Java [type] using
- * reflective construction and [contextual][SerializersModule.getContextual] lookup for non-serializable types.
- *
- * [serializer] is intended to be used as an interoperability layer for libraries like GSON and Retrofit,
- * that operate with reflective Java [Type] and cannot use [typeOf].
+ * Retrieves a serializer for the given [type] using
+ * reflective construction and [contextual][SerializersModule.getContextual] lookup as a fallback for non-serializable types.
  *
- * For application-level serialization, it is recommended to use `serializer<T>()` instead as it is aware of
+ * This overload is intended to be used as an interoperability layer for JVM-centric libraries,
+ * that operate with Java's type tokens and cannot use Kotlin's [KType] or [typeOf].
+ * For application-level serialization, it is recommended to use `serializer<T>()` or `serializer(KType)` instead as it is aware of
  * Kotlin-specific type information, such as nullability, sealed classes and object singletons.
  *
+ * Note that because [Type] does not contain any information about nullability, all created serializers
+ * work only with non-nullable data.
+ *
+ * Not all [Type] implementations are supported.
+ * [type] must be an instance of [Class], [GenericArrayType], [ParameterizedType] or [WildcardType].
+ *
  * @throws SerializationException if serializer cannot be created (provided [type] or its type argument is not serializable).
+ * @throws IllegalArgumentException if an unsupported subclass of [Type] is provided.
  */
-@ExperimentalSerializationApi
 public fun SerializersModule.serializer(type: Type): KSerializer<Any> =
     serializerByJavaTypeImpl(type, failOnMissingTypeArgSerializer = true) ?: type.prettyClass()
         .serializerNotRegistered()
 
 /**
- * Retrieves serializer for the given reflective Java [type] using
- * reflective construction and [contextual][SerializersModule.getContextual] lookup for non-serializable types.
- *
- * [serializer] is intended to be used as an interoperability layer for libraries like GSON and Retrofit,
- * that operate with reflective Java [Type] and cannot use [typeOf].
+ * Retrieves a serializer for the given [type] using
+ * reflective construction and [contextual][SerializersModule.getContextual] lookup as a fallback for non-serializable types.
  *
- * For application-level serialization, it is recommended to use `serializer<T>()` instead as it is aware of
+ * This overload is intended to be used as an interoperability layer for JVM-centric libraries,
+ * that operate with Java's type tokens and cannot use Kotlin's [KType] or [typeOf].
+ * For application-level serialization, it is recommended to use `serializer<T>()` or `serializer(KType)` instead as it is aware of
  * Kotlin-specific type information, such as nullability, sealed classes and object singletons.
  *
- * Returns `null` if serializer cannot be created (provided [type] or its type argument is not serializable).
+ * Note that because [Type] does not contain any information about nullability, all created serializers
+ * work only with non-nullable data.
+ *
+ * Not all [Type] implementations are supported.
+ * [type] must be an instance of [Class], [GenericArrayType], [ParameterizedType] or [WildcardType].
+ *
+ * @return [KSerializer] for given [type] or `null` if serializer cannot be created (given [type] or its type argument is not serializable).
+ * @throws IllegalArgumentException if an unsupported subclass of [Type] is provided.
  */
-@ExperimentalSerializationApi
 public fun SerializersModule.serializerOrNull(type: Type): KSerializer<Any>? =
     serializerByJavaTypeImpl(type, failOnMissingTypeArgSerializer = false)
 
-@OptIn(ExperimentalSerializationApi::class)
 private fun SerializersModule.serializerByJavaTypeImpl(
     type: Type,
     failOnMissingTypeArgSerializer: Boolean = true
@@ -116,15 +137,13 @@ private fun SerializersModule.serializerByJavaTypeImpl(
                 ) as KSerializer<Any>
 
                 else -> {
-                    // probably we should deprecate this method because it can't differ nullable vs non-nullable types
-                    // since it uses Java TypeToken, not Kotlin one
                     val varargs = argsSerializers.map { it as KSerializer<Any?> }
                     reflectiveOrContextual(rootClass as Class<Any>, varargs)
                 }
             }
         }
         is WildcardType -> serializerByJavaTypeImpl(type.upperBounds.first())
-        else -> throw IllegalArgumentException("typeToken should be an instance of Class<?>, GenericArray, ParametrizedType or WildcardType, but actual type is $type ${type::class}")
+        else -> throw IllegalArgumentException("type should be an instance of Class<?>, GenericArrayType, ParametrizedType or WildcardType, but actual argument $type has type ${type::class}")
     }
 
 @OptIn(ExperimentalSerializationApi::class)
@@ -177,5 +196,5 @@ private fun Type.prettyClass(): Class<*> = when (val it = this) {
     is ParameterizedType -> it.rawType.prettyClass()
     is WildcardType -> it.upperBounds.first().prettyClass()
     is GenericArrayType -> it.genericComponentType.prettyClass()
-    else -> throw IllegalArgumentException("typeToken should be an instance of Class<?>, GenericArray, ParametrizedType or WildcardType, but actual type is $it ${it::class}")
+    else -> throw IllegalArgumentException("type should be an instance of Class<?>, GenericArrayType, ParametrizedType or WildcardType, but actual argument $it has type ${it::class}")
 }

formats/json-tests/commonTest/src/kotlinx/serialization/SerializersLookupTest.kt

@@ -101,6 +101,20 @@ class SerializersLookupTest : JsonTestBase() {
         assertSerializedWithType<ArrayList<in Int>>("[1,2,3]", myList)
     }
 
+    @Test
+    fun testStarProjectionsAreProhibited() {
+        val expectedMessage = "Star projections in type arguments are not allowed"
+        assertFailsWithMessage<IllegalArgumentException>(expectedMessage) {
+            serializer<Box<*>>()
+        }
+        assertFailsWithMessage<IllegalArgumentException>(expectedMessage) {
+            serializer(typeOf<Box<*>>())
+        }
+        assertFailsWithMessage<IllegalArgumentException>(expectedMessage) {
+            serializerOrNull(typeOf<Box<*>>())
+        }
+    }
+
     @Test
     fun testNullableTypes() {
         val myList: List<Int?> = listOf(1, null, 3)
@@ -120,6 +134,12 @@ class SerializersLookupTest : JsonTestBase() {
         assertSerializedWithType("""{"first":"1","second":2,"third":{"boxed":42}}""", myTriple)
     }
 
+    @Test
+    fun testLookupDuration() = noLegacyJs {
+        assertNotNull(serializerOrNull(typeOf<Duration>()))
+        assertSame(Duration.serializer(), serializer<Duration>())
+    }
+
     @Test
     fun testCustomGeneric() = noLegacyJs {
         val intBox = Box(42)
@@ -259,13 +279,6 @@ class SerializersLookupTest : JsonTestBase() {
         }
     }
 
-// TODO uncomment when Kotlin 1.7.20 is released
-//    @Test
-//    fun testLookupDuration() = noLegacyJs {
-//        assertNotNull(serializerOrNull(typeOf<Duration>()))
-//        assertSame(Duration.serializer(), serializer<Duration>())
-//    }
-
     private inline fun <reified T> assertSerializedWithType(
         expected: String,
         value: T,