@transientParam annotation

Author: odersky

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -790,6 +790,8 @@ class Definitions {
   def TASTYLongSignatureAnnot(implicit ctx: Context): ClassSymbol = TASTYLongSignatureAnnotType.symbol.asClass
   lazy val TailrecAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.tailrec")
   def TailrecAnnot(implicit ctx: Context): ClassSymbol = TailrecAnnotType.symbol.asClass
+  lazy val TransientParamAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.transientParam")
+  def TransientParamAnnot(implicit ctx: Context): ClassSymbol = TransientParamAnnotType.symbol.asClass
   lazy val SwitchAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.switch")
   def SwitchAnnot(implicit ctx: Context): ClassSymbol = SwitchAnnotType.symbol.asClass
   lazy val ThrowsAnnotType: TypeRef = ctx.requiredClassRef("scala.throws")

compiler/src/dotty/tools/dotc/transform/Constructors.scala

@@ -224,6 +224,8 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
         dropped += acc
         Nil
       } else {
+        if (acc.hasAnnotation(defn.TransientParamAnnot))
+          ctx.error(em"transient parameter $acc is retained as field in class ${acc.owner}", acc.pos)
         val target = if (acc.is(Method)) acc.field else acc
         if (!target.exists) Nil // this case arises when the parameter accessor is an alias
         else {

library/src/scala/annotation/transientParam.scala

@@ -0,0 +1,20 @@
+/*                     __                                               *\
+**     ________ ___   / /  ___     Scala API                            **
+**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
+**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
+** /____/\___/_/ |_/____/_/ | |                                         **
+**                          |/                                          **
+\*                                                                      */
+
+package scala.annotation
+
+import scala.annotation.meta._
+
+/** An annotation that goes on parameters of classes or traits. It asserts
+ *  that the parameter is used only for initialization and is not kept in
+ *  the class as a field. Violations of this assertion are flagged as
+ *  compile errors. The annotatoon is particularly useful for implicit
+ *  parameters since for these a textual scan is not sufficient to know
+ *  where they are used.
+ */
+@param class transientParam extends scala.annotation.StaticAnnotation

tests/neg/transient-param.scala

@@ -0,0 +1,14 @@
+import annotation.transientParam
+class Context
+
+class Test1()(implicit @transientParam ctx: Context) { // error
+  def test1 = implicitly[Context]
+}
+
+class Test2()(@transientParam ctx: Context) { // error
+  def test1 = ctx
+}
+
+class Test3()(implicit @transientParam ctx: Context) { // OK
+  val foo = implicitly[Context]
+}