Make empty objects and arrays collapsed in pretty print mode (#2506)

because it is a widespread convention.

Fixes #2502

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

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2017-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json
+
+import kotlinx.serialization.*
+import kotlin.test.*
+
+class JsonPrettyPrintTest : JsonTestBase() {
+    val fmt = Json(default) { prettyPrint = true; encodeDefaults = true }
+
+    @Serializable
+    class Empty
+
+    @Serializable
+    class A(val empty: Empty = Empty())
+
+    @Serializable
+    class B(val prefix: String = "a", val empty: Empty = Empty(), val postfix: String = "b")
+
+    @Serializable
+    class Recursive(val rec: Recursive?, val empty: Empty = Empty())
+
+    @Serializable
+    class WithListRec(val rec: WithListRec?, val l: List<Int> = listOf())
+
+    @Serializable
+    class WithDefaults(val x: String = "x", val y: Int = 0)
+
+    @Test
+    fun testTopLevel() = parametrizedTest { mode ->
+        assertEquals("{}", fmt.encodeToString(Empty(), mode))
+    }
+
+    @Test
+    fun testWithDefaults() = parametrizedTest { mode ->
+        val dropDefaults = Json(fmt) { encodeDefaults = false }
+        val s = "{\n    \"boxed\": {}\n}"
+        assertEquals(s, dropDefaults.encodeToString(Box(WithDefaults()), mode))
+    }
+
+    @Test
+    fun testPlain() = parametrizedTest { mode ->
+        val s = """{
+        |    "empty": {}
+        |}""".trimMargin()
+        assertEquals(s, fmt.encodeToString(A(), mode))
+    }
+
+    @Test
+    fun testInside() = parametrizedTest { mode ->
+        val s = """{
+        |    "prefix": "a",
+        |    "empty": {},
+        |    "postfix": "b"
+        |}""".trimMargin()
+        assertEquals(s, fmt.encodeToString(B(), mode))
+    }
+
+    @Test
+    fun testRecursive() = parametrizedTest { mode ->
+        val obj = Recursive(Recursive(null))
+        val s = "{\n    \"rec\": {\n        \"rec\": null,\n        \"empty\": {}\n    },\n    \"empty\": {}\n}"
+        assertEquals(s, fmt.encodeToString(obj, mode))
+    }
+
+    @Test
+    fun test() = parametrizedTest { mode ->
+        val obj = WithListRec(WithListRec(null), listOf(1, 2, 3))
+        val s =
+            "{\n    \"rec\": {\n        \"rec\": null,\n        \"l\": []\n    },\n    \"l\": [\n        1,\n        2,\n        3\n    ]\n}"
+        assertEquals(s, fmt.encodeToString(obj, mode))
+    }
+}

formats/json/commonMain/src/kotlinx/serialization/json/internal/Composers.kt

@@ -27,6 +27,10 @@ internal open class Composer(@JvmField internal val writer: InternalJsonWriter)
         writingFirst = false
     }
 
+    open fun nextItemIfNotFirst() {
+        writingFirst = false
+    }
+
     open fun space() = Unit
 
     fun print(v: Char) = writer.writeChar(v)
@@ -88,6 +92,11 @@ internal class ComposerWithPrettyPrint(
         repeat(level) { print(json.configuration.prettyPrintIndent) }
     }
 
+    override fun nextItemIfNotFirst() {
+        if (writingFirst) writingFirst = false
+        else nextItem()
+    }
+
     override fun space() {
         print(' ')
     }

formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonEncoder.kt

@@ -96,7 +96,7 @@ internal class StreamingJsonEncoder(
     override fun endStructure(descriptor: SerialDescriptor) {
         if (mode.end != INVALID) {
             composer.unIndent()
-            composer.nextItem()
+            composer.nextItemIfNotFirst()
             composer.print(mode.end)
         }
     }