fix field ordering Scala 3 writes macro (#1177)

(cherry picked from commit 68c2665)

Co-authored-by: xuwei-k <6b656e6a69@gmail.com>

Author: mergify[bot]

play-json/shared/src/main/scala-3/play/api/libs/json/JsMacroImpl.scala

@@ -669,11 +669,11 @@ object JsMacroImpl { // TODO: debug
         .toSeq
         .partition { case WritableField(_, _, t) => isOptionalType(t) }
 
-      type ElementAcc = MBuilder[(String, JsValue), Map[String, JsValue]]
+      type ElementAcc = MBuilder[(String, JsValue), List[(String, JsValue)]]
 
       def withIdents[U: Type](f: Expr[ElementAcc] => Expr[U]): Expr[U] =
         '{
-          val ok = Map.newBuilder[String, JsValue]
+          val ok = List.newBuilder[(String, JsValue)]
 
           ${ f('{ ok }) }
         }
@@ -685,7 +685,7 @@ object JsMacroImpl { // TODO: debug
           val fieldMap = withFields(tupled, tupleTpe, tprElements, debug)
 
           withIdents[JsObject] { bufOk =>
-            val values: Seq[Expr[Unit]] = required.map { case WritableField(param, i, pt) =>
+            val values: Seq[(Expr[Unit], Int)] = required.map { case WritableField(param, i, pt) =>
               val pname = param.name
 
               val withField = fieldMap.get(pname) match {
@@ -697,7 +697,7 @@ object JsMacroImpl { // TODO: debug
                   )
               }
 
-              pt.asType match {
+              val expr = pt.asType match {
                 case pTpe @ '[p] =>
                   val writes: Expr[Writes[p]] = resolve(pt) match {
                     case Some((givenWrites, _)) =>
@@ -715,9 +715,10 @@ object JsMacroImpl { // TODO: debug
                     }).asTerm
                   }.asExprOf[Unit]
               }
+              expr -> i
             } // end of required.map
 
-            val extra: Seq[Expr[Unit]] = optional.map {
+            val extra: Seq[(Expr[Unit], Int)] = optional.map {
               case WritableField(param, i, optType @ OptionTypeParameter(pt)) =>
                 val pname = param.name
 
@@ -730,7 +731,7 @@ object JsMacroImpl { // TODO: debug
                     )
                 }
 
-                pt.asType match {
+                val expr = pt.asType match {
                   case pTpe @ '[p] =>
                     val writes: Expr[Writes[p]] = resolve(pt) match {
                       case Some((givenWrites, _)) =>
@@ -757,6 +758,7 @@ object JsMacroImpl { // TODO: debug
                       }).asTerm
                     }.asExprOf[Unit]
                 }
+                expr -> i
             } // end of extra.collect
 
             if (values.isEmpty && extra.isEmpty) {
@@ -766,7 +768,7 @@ object JsMacroImpl { // TODO: debug
 
               '{ JsObject(Map.empty) }
             } else {
-              val fields = values ++ extra
+              val fields = (values ++ extra).sortBy(_._2).map(_._1)
 
               val resExpr = '{ JsObject(${ bufOk }.result()) }
 

play-json/shared/src/test/scala/play/api/libs/json/MacroSpec.scala

@@ -538,10 +538,41 @@ class MacroSpec extends AnyWordSpec with Matchers with org.scalatestplus.scalach
       Json.toJson(GenericCaseClassWithDefault(3, "foo")).mustEqual(expectedJson)
       Json.fromJson(expectedJson).mustEqual(JsSuccess(expected))
     }
+
+    "field ordering" in {
+      // https://github.com/playframework/play-json/issues/1038
+      val instance: OWrites[FieldOrderTest] = Json.writes[FieldOrderTest]
+
+      val value = FieldOrderTest(1, 2, 3, Some(4), 5, 6)
+
+      instance
+        .writes(value)
+        .fields
+        .mustEqual(
+          Seq(
+            "x1" -> 1,
+            "x2" -> 2,
+            "x3" -> 3,
+            "x4" -> 4,
+            "x5" -> 5,
+            "x6" -> 6,
+          ).map { case (k, v) => k -> JsNumber(v) }
+        )
+      assert(instance.writes(value).value.isInstanceOf[ImmutableLinkedHashMap[?, ?]])
+    }
   }
 }
 
 object MacroSpec {
+  case class FieldOrderTest(
+      x1: Int,
+      x2: Int,
+      x3: Int,
+      x4: Option[Int],
+      x5: Int,
+      x6: Int,
+  )
+
   sealed trait Family
   case class Simple(bar: String)            extends Family
   case class Lorem[T](ipsum: T, age: Int)   extends Family