diff --git a/main-settings/src/main/scala/sbt/std/TaskMacro.scala b/main-settings/src/main/scala/sbt/std/TaskMacro.scala
index 1390145a8..c12900150 100644
--- a/main-settings/src/main/scala/sbt/std/TaskMacro.scala
+++ b/main-settings/src/main/scala/sbt/std/TaskMacro.scala
@@ -70,13 +70,11 @@ object TaskMacro {
   final val TransformInitName = "transform"
   final val InputTaskCreateDynName = "createDyn"
   final val InputTaskCreateFreeName = "createFree"
-  final val append1Migration = "Use `lhs += { x.value }`."
-  final val appendNMigration = "Use `lhs ++= { x.value }`."
+  final val append1Migration = "`<+=` operator is deprecated. Use `lhs += { x.value }`."
+  final val appendNMigration = "`<++=` operator is deprecated. Use `lhs ++= { x.value }`."
   final val assignMigration =
-    """Use `key := { x.value }` or `key ~= (old => { newValue })`.
-      |The RHS of `<<=` takes an `Initialize[_]` expression, which can be converted to `:=` style
-      |by wrapping the expression in parenthesis, and calling `.value` at the end.
-      |For example, `key := (key.dependsOn(compile in Test)).value`.""".stripMargin
+    """`<<=` operator is deprecated. Use `key := { x.value }` or `key ~= (old => { newValue })`.
+      |See http://www.scala-sbt.org/0.13/docs/Migrating-from-sbt-012x.html""".stripMargin
 
   def taskMacroImpl[T: c.WeakTypeTag](c: blackbox.Context)(t: c.Expr[T]): c.Expr[Initialize[Task[T]]] =
     Instance.contImpl[T, Id](c, FullInstance, FullConvert, MixedBuilder)(Left(t), Instance.idTransform[c.type])
diff --git a/main/src/main/scala/sbt/Defaults.scala b/main/src/main/scala/sbt/Defaults.scala
index d05bfbe05..6dab937c3 100755
--- a/main/src/main/scala/sbt/Defaults.scala
+++ b/main/src/main/scala/sbt/Defaults.scala
@@ -1091,8 +1091,7 @@ object Defaults extends BuildCommon {
 
   lazy val runnerSettings: Seq[Setting[_]] = Seq(runnerTask)
   lazy val baseTasks: Seq[Setting[_]] = projectTasks ++ packageBase
-
-  lazy val configSettings: Seq[Setting[_]] = Classpaths.configSettings ++ configTasks ++ configPaths ++ packageConfig ++ Classpaths.compilerPluginConfig
+  lazy val configSettings: Seq[Setting[_]] = Classpaths.configSettings ++ configTasks ++ configPaths ++ packageConfig ++ Classpaths.compilerPluginConfig ++ deprecationSettings
 
   lazy val compileSettings: Seq[Setting[_]] = configSettings ++ (mainRunMainTask +: mainRunTask +: addBaseSources) ++ Classpaths.addUnmanagedLibrary
   lazy val testSettings: Seq[Setting[_]] = configSettings ++ testTasks
@@ -1107,7 +1106,17 @@ object Defaults extends BuildCommon {
       baseDirectory := thisProject.value.base,
       target := baseDirectory.value / "target"
     )
-
+  // build.sbt is treated a Scala source of metabuild, so to enable deprecation flag on build.sbt we set the option here.
+  lazy val deprecationSettings: Seq[Setting[_]] =
+    inConfig(Compile)(Seq(
+      scalacOptions := {
+        val old = scalacOptions.value
+        val existing = old.toSet
+        val d = "-deprecation"
+        if (sbtPlugin.value && !existing(d)) d :: old.toList
+        else old
+      }
+    ))
 }
 object Classpaths {
   import Keys._