Add support for Scala 3 type class derivation of Format, Writes and Reads

README.md

@@ -187,6 +187,12 @@ I can also define a `Format` that does both:
 implicit val residentFormat = Json.format[Resident]
 ```
 
+Alternatively, on Scala 3.x, I can declare that my class `derives Reads`, `derives Writes` or `derives Format`:
+
+```scala
+case class Resident(name: String, age: Int, role: Option[String]) derives Format
+```
+
 With the `Reads` and/or `Writes` in scope, I can then easily convert my class using `toJson` and `fromJson`
 
 ### Constructing `Reads` and `Writes`

docs/manual/working/scalaGuide/main/json/ScalaJsonAutomated.md

@@ -74,6 +74,22 @@ Then the macros are able generate `Reads[T]`, `OWrites[T]` or `OFormat[T]`.
 
 @[auto-JSON-sealed-trait](code/ScalaJsonAutomatedSpec.scala)
 
+## Type Class Derivation
+
+When using Scala 3, you can use [Type Class Derivation](https://docs.scala-lang.org/scala3/reference/contextual/derivation.html) to derive `Reads[T]`, `Writes[T]` and `Format[T]`.
+
+This support has exactly the same requirements and limitations as the macros described above.
+
+For example, the following case class automatically derives `Reads[Resident]`:
+
+@[reads-model](code-3/Scala3JsonAutomatedSpec.scala)
+
+You can also use `derives Writes` or `derives Format`.
+
+When deriving `Reads[T]`, `Writes[T]` and `Format[T]` for traits, you must declare that each subclass `derives` the appropriate trait:
+
+@[reads-trait](code-3/Scala3JsonAutomatedSpec.scala)
+
 ## Custom Naming Strategies
 
 To use a custom Naming Strategy you need to define a implicit `JsonConfiguration` object and a `JsonNaming`.

docs/manual/working/scalaGuide/main/json/code-3/Scala3JsonAutomatedSpec.scala

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package scalaguide.json
+
+import org.specs2.mutable.Specification
+import play.api.libs.json._
+
+object Scala3JsonAutomatedSpec {
+  //#reads-model
+  case class Resident(
+    name: String,
+    age: Int,
+    specialism: Option[String]
+  ) derives Reads
+  //#reads-model
+
+  val residentJson = Json.parse(
+    """{
+      "name" : "Fiver",
+      "age" : 4
+    }"""
+  )
+
+  val sampleResident = Resident("Fiver", 4, None)
+
+  //#reads-trait
+  sealed trait Role derives Reads
+  case object Admin                            extends Role derives Reads
+  case class Contributor(organization: String) extends Role derives Reads
+  //#reads-trait
+
+  val contributorJson = Json.obj("_type" -> "scalaguide.json.Scala3JsonAutomatedSpec.Contributor", "organization" -> "Foo")
+  val sampleContributor = Contributor("Foo")
+}
+
+class Scala3JsonAutomatedSpec extends Specification {
+  import Scala3JsonAutomatedSpec._
+
+  "Scala 3 JSON automated" should {
+    "for case class" >> {
+      "derive a Reads" in {
+        residentJson.as[Resident].must_===(sampleResident)
+      }
+    }
+    "for trait" >> {
+      "derive a Reads if every subclass derives Reads" in {
+        contributorJson.as[Role].must_===(sampleContributor)
+      }
+    }
+  }
+}

play-json/shared/src/main/scala-2/play/api/libs/json/DerivedFormat.scala

@@ -0,0 +1,7 @@
+/*
+ * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+private[json] trait DerivedFormat {}

play-json/shared/src/main/scala-2/play/api/libs/json/DerivedReads.scala

@@ -0,0 +1,7 @@
+/*
+ * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+private[json] trait DerivedReads {}

play-json/shared/src/main/scala-2/play/api/libs/json/DerivedWrites.scala

@@ -0,0 +1,7 @@
+/*
+ * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+private[json] trait DerivedWrites {}

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

@@ -0,0 +1,9 @@
+/*
+ * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+private[json] trait DerivedFormat {
+  inline def derived[T]: OFormat[T] = Json.format[T]
+}

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

@@ -0,0 +1,9 @@
+/*
+ * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+private[json] trait DerivedReads {
+  inline def derived[T]: Reads[T] = Json.reads[T]
+}

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

@@ -0,0 +1,9 @@
+/*
+ * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+private[json] trait DerivedWrites {
+  inline def derived[T]: OWrites[T] = Json.writes[T]
+}

play-json/shared/src/main/scala/play/api/libs/json/Format.scala

@@ -64,7 +64,7 @@ object OFormat {
 /**
  * Default Json formatters.
  */
-object Format extends PathFormat with ConstraintFormat with DefaultFormat {
+object Format extends PathFormat with ConstraintFormat with DefaultFormat with DerivedFormat {
   val constraints: ConstraintFormat = this
   val path: PathFormat              = this
 

play-json/shared/src/main/scala/play/api/libs/json/Reads.scala

@@ -166,7 +166,7 @@ trait Reads[A] { self =>
 /**
  * Default deserializer type classes.
  */
-object Reads extends ConstraintReads with PathReads with DefaultReads with GeneratedReads {
+object Reads extends ConstraintReads with PathReads with DefaultReads with GeneratedReads with DerivedReads {
   val constraints: ConstraintReads = this
 
   val path: PathReads = this

play-json/shared/src/main/scala/play/api/libs/json/Writes.scala

@@ -237,7 +237,7 @@ object OWrites extends PathWrites with ConstraintWrites {
 /**
  * Default Serializers.
  */
-object Writes extends PathWrites with ConstraintWrites with DefaultWrites with GeneratedWrites {
+object Writes extends PathWrites with ConstraintWrites with DefaultWrites with GeneratedWrites with DerivedWrites {
   val constraints: ConstraintWrites = this
   val path: PathWrites              = this
 

play-json/shared/src/test/scala-3/play/api/libs/json/DerivesSyntaxSpec.scala

@@ -0,0 +1,97 @@
+/*
+ * Copyright (C) from 2022 The Play Framework Contributors <https://github.com/playframework>, 2011-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+import org.scalatest.EitherValues
+import org.scalatest.matchers.must.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+
+final class DerivesSyntaxSpec extends AnyWordSpec with Matchers with EitherValues {
+  "Derives syntax" should {
+    "derive Format using macros" when {
+      "used with standalone case classes" in {
+        val input = DerivesFormat.StandaloneCaseClass(1)
+        val json  = Json.toJson(input)
+        json mustBe Json.obj("intField" -> 1)
+        Json.fromJson[DerivesFormat.StandaloneCaseClass](json).asEither.value mustBe input
+      }
+
+      "used with trait subtypes that are case objects" in {
+        val input = DerivesFormat.SomeCaseObject
+        val json  = Json.toJson[DerivesFormat.SomeTrait](input)
+        json mustBe Json.obj("_type" -> "play.api.libs.json.DerivesFormat.SomeCaseObject")
+        Json.fromJson[DerivesFormat.SomeTrait](json).asEither.value mustBe input
+      }
+
+      "used with trait subtypes that are case classes" in {
+        val input = DerivesFormat.SomeCaseClass("hello")
+        val json  = Json.toJson[DerivesFormat.SomeTrait](input)
+        json mustBe Json.obj("_type" -> "play.api.libs.json.DerivesFormat.SomeCaseClass", "stringField" -> "hello")
+        Json.fromJson[DerivesFormat.SomeTrait](json).asEither.value mustBe input
+      }
+    }
+
+    "derive Reads using macros" when {
+      "used with standalone case classes" in {
+        val input    = Json.obj("intField" -> 16)
+        val expected = DerivesReads.StandaloneCaseClass(16)
+        Json.fromJson[DerivesReads.StandaloneCaseClass](input).asEither.value mustBe expected
+      }
+
+      "used with trait subtypes that are case objects" in {
+        val input    = Json.obj("_type" -> "play.api.libs.json.DerivesReads.SomeCaseObject")
+        val expected = DerivesReads.SomeCaseObject
+        Json.fromJson[DerivesReads.SomeTrait](input).asEither.value mustBe expected
+      }
+
+      "used with trait subtypes that are case classes" in {
+        val input    = Json.obj("_type" -> "play.api.libs.json.DerivesReads.SomeCaseClass", "stringField" -> "abc")
+        val expected = DerivesReads.SomeCaseClass("abc")
+        Json.fromJson[DerivesReads.SomeTrait](input).asEither.value mustBe expected
+      }
+    }
+
+    "derive Writes using macros" when {
+      "used with standalone case classes" in {
+        val input    = DerivesWrites.StandaloneCaseClass(42)
+        val expected = Json.obj("intField" -> 42)
+        Json.toJson(input) mustBe expected
+      }
+
+      "used with trait subtypes that are case objects" in {
+        val input    = DerivesWrites.SomeCaseObject
+        val expected = Json.obj("_type" -> "play.api.libs.json.DerivesWrites.SomeCaseObject")
+        Json.toJson[DerivesWrites.SomeTrait](input) mustBe expected
+      }
+
+      "used with trait subtypes that are case classes" in {
+        val input    = DerivesWrites.SomeCaseClass("def")
+        val expected = Json.obj("_type" -> "play.api.libs.json.DerivesWrites.SomeCaseClass", "stringField" -> "def")
+        Json.toJson[DerivesWrites.SomeTrait](input) mustBe expected
+      }
+    }
+  }
+}
+
+object DerivesFormat {
+  case class StandaloneCaseClass(intField: Int) derives Format
+  sealed trait SomeTrait derives Format
+  case class SomeCaseClass(stringField: String) extends SomeTrait derives Format
+  case object SomeCaseObject                    extends SomeTrait derives Format
+}
+
+object DerivesReads {
+  case class StandaloneCaseClass(intField: Int) derives Reads
+  sealed trait SomeTrait derives Reads
+  case class SomeCaseClass(stringField: String) extends SomeTrait derives Reads
+  case object SomeCaseObject                    extends SomeTrait derives Reads
+}
+
+object DerivesWrites {
+  case class StandaloneCaseClass(intField: Int) derives Writes
+  sealed trait SomeTrait derives Writes
+  case class SomeCaseClass(stringField: String) extends SomeTrait derives Writes
+  case object SomeCaseObject                    extends SomeTrait derives Writes
+}