Enabled continuations plugin by default.

We're not buying anything with it off by default.  Added a
-P:continuations:disable option to disable.  -P:continuations:enable
lives on as a no-op.  Review by @odersky.

Author: paulp

build.xml

@@ -649,7 +649,7 @@ QUICK BUILD (QUICK)
     <scalacfork
       destdir="${build-quick.dir}/classes/library"
       compilerpathref="quick.classpath"
-      params="${scalac.args.quick} -Xplugin-require:continuations -P:continuations:enable"
+      params="${scalac.args.quick} -Xplugin-require:continuations"
       srcdir="${src.dir}/continuations/library"
       jvmargs="${scalacfork.jvmargs}">
       <include name="**/*.scala"/>
@@ -1164,7 +1164,7 @@ BOOTSTRAPPING BUILD (STRAP)
     <scalacfork
       destdir="${build-strap.dir}/classes/library"
       compilerpathref="pack.classpath"
-      params="${scalac.args.all} -Xplugin-require:continuations -P:continuations:enable"
+      params="${scalac.args.all} -Xplugin-require:continuations"
       srcdir="${src.dir}/continuations/library"
       jvmargs="${scalacfork.jvmargs}">
       <include name="**/*.scala"/>
@@ -1624,7 +1624,7 @@ BOOTRAPING TEST AND TEST SUITE
     <partest showlog="yes" erroronfailed="yes" javacmd="${java.home}/bin/java"
              timeout="2400000"
              srcdir="${partest.srcdir}"
-             scalacopts="${scalac.args.optimise} -Xplugin-require:continuations -P:continuations:enable">
+             scalacopts="${scalac.args.optimise} -Xplugin-require:continuations">
       <compilerarg value="-Xpluginsdir"/>
       <compilerarg file="${build-quick.dir}/misc/scala-devel/plugins"/>
       <compilationpath>

src/continuations/plugin/scala/tools/selectivecps/CPSUtils.scala

@@ -9,7 +9,7 @@ trait CPSUtils {
   import global._
   import definitions._
 
-  var cpsEnabled = false
+  var cpsEnabled = true
   val verbose: Boolean = System.getProperty("cpsVerbose", "false") == "true"
   def vprintln(x: =>Any): Unit = if (verbose) println(x)
 

src/continuations/plugin/scala/tools/selectivecps/SelectiveCPSPlugin.scala

@@ -26,7 +26,6 @@ class SelectiveCPSPlugin(val global: Global) extends Plugin {
     override val runsBefore = List("uncurry")
   }
 
-
   val components = List[PluginComponent](anfPhase, cpsPhase)
 
   val checker = new CPSAnnotationChecker {
@@ -43,19 +42,17 @@ class SelectiveCPSPlugin(val global: Global) extends Plugin {
   }
 
   // TODO: require -enabled command-line flag
-
   override def processOptions(options: List[String], error: String => Unit) = {
-    var enabled = false
-    for (option <- options) {
-      if (option == "enable") {
-        enabled = true
-      } else {
-        error("Option not understood: "+option)
-      }
+    var enabled = true
+    options foreach {
+      case "enable"    => enabled = true
+      case "disable"   => enabled = false
+      case option      => error("Option not understood: "+option)
     }
     setEnabled(enabled)
   }
 
-  override val optionsHelp: Option[String] =
-    Some("  -P:continuations:enable        Enable continuations")
+  override val optionsHelp: Option[String] = {
+    Some("  -P:continuations:disable        Disable continuations plugin")
+  }
 }