Support serialization of collections that are not lists

So far there was an implicit assumption hard-coded that collections are
always lists. However, sets are also collections, and can be serialized
to JSON arrays just like lists.

This change allows to serialize generic collections independently of the
concrete implementation.

Fixes Kotlin#1421.

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

@@ -202,12 +202,17 @@ internal class ReferenceArraySerializer<ElementKlass : Any, Element : ElementKla
     }
 }
 
+@PublishedApi
+internal abstract class CollectionSerializer<E, C: Collection<E>, B>(element: KSerializer<E>) : ListLikeSerializer<E, C, B>(element) {
+    override fun C.collectionSize(): Int = size
+    override fun C.collectionIterator(): Iterator<E> = iterator()
+}
+
 @InternalSerializationApi
 @PublishedApi
-internal class ArrayListSerializer<E>(element: KSerializer<E>) : ListLikeSerializer<E, List<E>, ArrayList<E>>(element) {
+internal class ArrayListSerializer<E>(element: KSerializer<E>) : CollectionSerializer<E, List<E>, ArrayList<E>>(element) {
     override val descriptor: SerialDescriptor = ArrayListClassDesc(element.descriptor)
-    override fun List<E>.collectionSize(): Int = size
-    override fun List<E>.collectionIterator(): Iterator<E> = iterator()
+
     override fun builder(): ArrayList<E> = arrayListOf()
     override fun ArrayList<E>.builderSize(): Int = size
     override fun ArrayList<E>.toResult(): List<E> = this
@@ -219,11 +224,9 @@ internal class ArrayListSerializer<E>(element: KSerializer<E>) : ListLikeSeriali
 @PublishedApi
 internal class LinkedHashSetSerializer<E>(
     eSerializer: KSerializer<E>
-) : ListLikeSerializer<E, Set<E>, LinkedHashSet<E>>(eSerializer) {
-
+) : CollectionSerializer<E, Set<E>, LinkedHashSet<E>>(eSerializer) {
     override val descriptor: SerialDescriptor = LinkedHashSetClassDesc(eSerializer.descriptor)
-    override fun Set<E>.collectionSize(): Int = size
-    override fun Set<E>.collectionIterator(): Iterator<E> = iterator()
+
     override fun builder(): LinkedHashSet<E> = linkedSetOf()
     override fun LinkedHashSet<E>.builderSize(): Int = size
     override fun LinkedHashSet<E>.toResult(): Set<E> = this
@@ -235,11 +238,9 @@ internal class LinkedHashSetSerializer<E>(
 @PublishedApi
 internal class HashSetSerializer<E>(
     eSerializer: KSerializer<E>
-) : ListLikeSerializer<E, Set<E>, HashSet<E>>(eSerializer) {
-
+) : CollectionSerializer<E, Set<E>, HashSet<E>>(eSerializer) {
     override val descriptor: SerialDescriptor = HashSetClassDesc(eSerializer.descriptor)
-    override fun Set<E>.collectionSize(): Int = size
-    override fun Set<E>.collectionIterator(): Iterator<E> = iterator()
+
     override fun builder(): HashSet<E> = HashSet()
     override fun HashSet<E>.builderSize(): Int = size
     override fun HashSet<E>.toResult(): Set<E> = this

formats/json/commonTest/src/kotlinx/serialization/features/CollectionSerializerTest.kt

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2017-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.features
+
+import kotlinx.serialization.*
+import kotlinx.serialization.builtins.*
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class CollectionSerializerTest {
+
+    @Serializable
+    data class CollectionWrapper(
+        val collection: Collection<String>
+    )
+
+    @Test
+    fun testListJson() {
+        val list = listOf("foo", "bar", "foo", "bar")
+
+        val string = Json.encodeToString(CollectionWrapper(list))
+        assertEquals("""{"collection":["foo","bar","foo","bar"]}""", string)
+
+        val wrapper = Json.decodeFromString<CollectionWrapper>(string)
+        assertEquals(list, wrapper.collection)
+    }
+
+    @Test
+    fun testSetJson() {
+        val set = setOf("foo", "bar", "foo", "bar")
+
+        val string = Json.encodeToString(CollectionWrapper(set))
+        assertEquals("""{"collection":["foo","bar"]}""", string)
+
+        val wrapper = Json.decodeFromString<CollectionWrapper>(string)
+        assertEquals(set.toList(), wrapper.collection)
+    }
+}