Revert pull request scala#720 (CPS: enable return expressions in CPS code if they are in tail position)

Reverts commit 0ada070.
Reverts commit 51c92f0.
Reverts commit cdfbe8e.
Reverts commit 796024c.

Author: phaller

src/compiler/scala/tools/nsc/typechecker/Modes.scala

@@ -86,10 +86,6 @@ trait Modes {
    */
   final val TYPEPATmode   = 0x10000
 
-  /** RETmode is set when we are typing a return expression.
-   */
-  final val RETmode       = 0x20000
-  
   final private val StickyModes   = EXPRmode | PATTERNmode | TYPEmode | ALTmode
 
   final def onlyStickyModes(mode: Int) =

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -4024,8 +4024,7 @@ trait Typers extends Modes with Adaptations with Tags {
             ReturnWithoutTypeError(tree, enclMethod.owner)
           } else {
             context.enclMethod.returnsSeen = true
-            val expr1: Tree = typed(expr, EXPRmode | BYVALmode | RETmode, restpt.tpe)
-
+            val expr1: Tree = typed(expr, EXPRmode | BYVALmode, restpt.tpe)
             // Warn about returning a value if no value can be returned.
             if (restpt.tpe.typeSymbol == UnitClass) {
               // The typing in expr1 says expr is Unit (it has already been coerced if

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

@@ -171,9 +171,6 @@ abstract class CPSAnnotationChecker extends CPSUtils with Modes {
             vprintln("yes we can!! (byval)")
             return true
           }
-        } else if ((mode & global.analyzer.RETmode) != 0) {
-          vprintln("yes we can!! (return)")
-          return true
         }
       }
       false
@@ -187,7 +184,6 @@ abstract class CPSAnnotationChecker extends CPSUtils with Modes {
       val patMode   = (mode & global.analyzer.PATTERNmode) != 0
       val exprMode  = (mode & global.analyzer.EXPRmode) != 0
       val byValMode = (mode & global.analyzer.BYVALmode) != 0
-      val retMode   = (mode & global.analyzer.RETmode) != 0
 
       val annotsTree     = cpsParamAnnotation(tree.tpe)
       val annotsExpected = cpsParamAnnotation(pt)
@@ -214,12 +210,6 @@ abstract class CPSAnnotationChecker extends CPSUtils with Modes {
         val res = tree modifyType addMinusMarker
         vprintln("adapted annotations (by val) of " + tree + " to " + res.tpe)
         res
-      } else if (retMode && !hasPlusMarker(tree.tpe) && annotsTree.isEmpty && annotsExpected.nonEmpty) {
-        // add a marker annotation that will make tree.tpe behave as pt, subtyping wise
-        // tree will look like having no annotation
-        val res = tree modifyType (_ withAnnotations List(newPlusMarker()))
-        vprintln("adapted annotations (return) of " + tree + " to " + res.tpe)
-        res        
       } else tree
     }
 
@@ -476,11 +466,6 @@ abstract class CPSAnnotationChecker extends CPSUtils with Modes {
           }
           tpe
 
-        case ret @ Return(expr) =>
-          if (hasPlusMarker(expr.tpe))
-            ret setType expr.tpe
-          ret.tpe
-
         case _ =>
           tpe
       }

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

@@ -3,7 +3,6 @@
 package scala.tools.selectivecps
 
 import scala.tools.nsc.Global
-import scala.collection.mutable.ListBuffer
 
 trait CPSUtils {
   val global: Global
@@ -136,43 +135,4 @@ trait CPSUtils {
       case _ => None
     }
   }
-
-  def isTailReturn(retExpr: Tree, body: Tree): Boolean = {
-    val removed = ListBuffer[Tree]()
-    removeTailReturn(body, removed)
-    removed contains retExpr
-  }
-
-  def removeTailReturn(tree: Tree, removed: ListBuffer[Tree]): Tree = tree match {
-    case Block(stms, r @ Return(expr)) =>
-      removed += r
-      treeCopy.Block(tree, stms, expr)
-
-    case Block(stms, expr) =>
-      treeCopy.Block(tree, stms, removeTailReturn(expr, removed))
-
-    case If(cond, r1 @ Return(thenExpr), r2 @ Return(elseExpr)) =>
-      removed ++= Seq(r1, r2)
-      treeCopy.If(tree, cond, removeTailReturn(thenExpr, removed), removeTailReturn(elseExpr, removed))
-
-    case If(cond, thenExpr, elseExpr) =>
-      treeCopy.If(tree, cond, removeTailReturn(thenExpr, removed), removeTailReturn(elseExpr, removed))
-
-    case Try(block, catches, finalizer) =>
-      treeCopy.Try(tree,
-        removeTailReturn(block, removed),
-        (catches map (t => removeTailReturn(t, removed))).asInstanceOf[List[CaseDef]],
-        removeTailReturn(finalizer, removed))
-
-    case CaseDef(pat, guard, r @ Return(expr)) =>
-      removed += r
-      treeCopy.CaseDef(tree, pat, guard, expr)
-
-    case CaseDef(pat, guard, body) =>
-      treeCopy.CaseDef(tree, pat, guard, removeTailReturn(body, removed))
-
-    case _ =>
-      tree
-  }
-
 }

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

@@ -9,8 +9,6 @@ import scala.tools.nsc.plugins._
 
 import scala.tools.nsc.ast._
 
-import scala.collection.mutable.ListBuffer
-
 /**
  * In methods marked @cps, explicitly name results of calls to other @cps methods
  */
@@ -48,20 +46,10 @@ abstract class SelectiveANFTransform extends PluginComponent with Transform with
         // this would cause infinite recursion. But we could remove the
         // ValDef case here.
 
-        case dd @ DefDef(mods, name, tparams, vparamss, tpt, rhs0) =>
+        case dd @ DefDef(mods, name, tparams, vparamss, tpt, rhs) =>
           debuglog("transforming " + dd.symbol)
 
           atOwner(dd.symbol) {
-            val tailReturns = ListBuffer[Tree]()
-            val rhs = removeTailReturn(rhs0, tailReturns)
-            // throw an error if there is a Return tree which is not in tail position
-            rhs0 foreach {
-              case r @ Return(_) =>
-                if (!tailReturns.contains(r))
-                  unit.error(r.pos, "return expressions in CPS code must be in tail position")
-              case _ => /* do nothing */
-            }
-            
             val rhs1 = transExpr(rhs, None, getExternalAnswerTypeAnn(tpt.tpe))
 
             debuglog("result "+rhs1)
@@ -165,6 +153,7 @@ abstract class SelectiveANFTransform extends PluginComponent with Transform with
       }
     }
 
+
     def transExpr(tree: Tree, cpsA: CPSInfo, cpsR: CPSInfo): Tree = {
       transTailValue(tree, cpsA, cpsR) match {
         case (Nil, b) => b