LambdaLift.scala

/*
 * Scala (https://www.scala-lang.org)
 *
 * Copyright EPFL and Lightbend, Inc.
 *
 * Licensed under Apache License 2.0
 * (http://www.apache.org/licenses/LICENSE-2.0).
 *
 * See the NOTICE file distributed with this work for
 * additional information regarding copyright ownership.
 */

package scala.tools.nsc
package transform

import symtab._
import Flags._
import scala.annotation.tailrec
import scala.collection.mutable
import scala.collection.mutable.{LinkedHashMap, LinkedHashSet}
import scala.tools.nsc.Reporting.WarningCategory.LintPerformance

abstract class LambdaLift extends InfoTransform {
  import global._
  import definitions._

  /** the following two members override abstract members in Transform */
  val phaseName: String = "lambdalift"

  private val lifted = new TypeMap {
    def apply(tp: Type): Type = tp match {
      case TypeRef(NoPrefix, sym, Nil) if sym.isClass && !sym.isPackageClass =>
        typeRef(apply(sym.owner.enclClass.thisType), sym, Nil)
      case ClassInfoType(parents, decls, clazz) =>
        val parents1 = parents mapConserve this
        if (parents1 eq parents) tp
        else ClassInfoType(parents1, decls, clazz)
      case _ =>
        mapOver(tp)
    }
  }

  /** Each scala.runtime.*Ref class has a static method `create(value)` that simply instantiates the Ref to carry that value. */
  private lazy val refCreateMethod: Map[Symbol, Symbol] = {
    mapFrom(allRefClasses.toList)(x => getMemberMethod(x.companionModule, nme.create))
  }

  /** Quite frequently a *Ref is initialized with its zero (e.g., null, 0.toByte, etc.) Method `zero()` of *Ref class encapsulates that pattern. */
  private lazy val refZeroMethod: Map[Symbol, Symbol] = {
    mapFrom(allRefClasses.toList)(x => getMemberMethod(x.companionModule, nme.zero))
  }

  def transformInfo(sym: Symbol, tp: Type): Type =
    if (sym.isCapturedVariable) capturedVariableType(sym, tpe = lifted(tp), erasedTypes = true)
    else lifted(tp)

  protected def newTransformer(unit: CompilationUnit): AstTransformer =
    new LambdaLifter(unit)

  class LambdaLifter(unit: CompilationUnit) extends explicitOuter.OuterPathTransformer(unit) {

    private type SymSet = LinkedHashSet[Symbol]

    /** A map storing free variables of functions and classes */
    private val free = new LinkedHashMap[Symbol, SymSet]

    /** A map storing the free variable proxies of functions and classes */
    private val proxies = new LinkedHashMap[Symbol, List[Symbol]]

    /** A hashtable storing calls between functions */
    private val called = new LinkedHashMap[Symbol, SymSet]

    /** Symbols that are called from an inner class. */
    private val calledFromInner = new LinkedHashSet[Symbol]

    private def newSymSet: LinkedHashSet[Symbol] = new LinkedHashSet[Symbol]

    private def symSet(f: LinkedHashMap[Symbol, SymSet], sym: Symbol): SymSet =
      f.getOrElseUpdate(sym, newSymSet)

    /** The set of symbols that need to be renamed. */
    private val renamable = newSymSet

    /**
     * The new names for free variables proxies. If we simply renamed the
     * free variables, we would transform:
     * {{{
     *   def closure(x: Int) = { () => x }
     * }}}
     *
     * To:
     * {{{
     *   def closure(x$1: Int) = new anonFun$1(this, x$1)
     *   class anonFun$1(outer$: Outer, x$1: Int) { def apply() => x$1 }
     * }}}
     *
     * This is fatally bad for named arguments (0e170e4b), extremely impolite to tools
     * reflecting on the method parameter names in the generated bytecode (scala/bug#6028),
     * and needlessly bothersome to anyone using a debugger.
     *
     * Instead, we transform to:
     * {{{
     *   def closure(x: Int) = new anonFun$1(this, x)
     *   class anonFun$1(outer$: Outer, x$1: Int) { def apply() => x$1 }
     * }}}
     */
    private val proxyNames       = mutable.HashMap[Symbol, Name]()

    /** A flag to indicate whether new free variables have been found */
    private var changedFreeVars: Boolean = _

    /** Buffers for lifted out classes and methods */
    private val liftedDefs = new LinkedHashMap[Symbol, List[Tree]]

    val delayedInitDummies = new mutable.HashMap[Symbol, Symbol]

    /**
     * For classes capturing locals, LambdaLift uses `local.logicallyEnclosingMember` to decide
     * whether an access to the local is re-written to the field or constructor parameter. If the
     * access is in a constructor statement, the constructor parameter is used.
     *
     * For DelayedInit subclasses, constructor statements end up in the synthetic init method
     * instead of the constructor itself, so the access should go to the field. This method changes
     * `logicallyEnclosingMember` in this case to return a temporary symbol corresponding to that
     * method.
     */
    @tailrec
    private def logicallyEnclosingMember(sym: Symbol): Symbol = {
      if (sym.isLocalDummy) {
        val enclClass = sym.enclClass
        if (enclClass.isSubClass(DelayedInitClass))
          delayedInitDummies.getOrElseUpdate(enclClass, enclClass.newMethod(nme.delayedInit))
        else
          enclClass.primaryConstructor
      } else if (sym.isMethod || sym.isClass || sym == NoSymbol) sym
      else logicallyEnclosingMember(sym.owner)
    }

    private def isSameOwnerEnclosure(sym: Symbol) =
      logicallyEnclosingMember(sym.owner) == logicallyEnclosingMember(currentOwner)

    /** Mark symbol `sym` as being free in `enclosure`, unless `sym`
     *  is defined in `enclosure` or there is a class between `enclosure`s owner
     *  and the owner of `sym`.
     *  Return `true` if there is no class between `enclosure` and
     *  the owner of sym.
     *  pre: sym.isLocalToBlock, (enclosure.isMethod || enclosure.isClass)
     *
     *  The idea of `markFree` is illustrated with an example:
     *
     *  def f(x: int) = {
     *    class C {
     *      class D {
     *        val y = x
     *      }
     *    }
     *  }
     *
     *  In this case `x` is free in the primary constructor of class `C`.
     *  but it is not free in `D`, because after lambda lift the code would be transformed
     *  as follows:
     *
     *  def f(x$0: int) {
     *    class C(x$0: int) {
     *      val x$1 = x$0
     *      class D {
     *        val y = outer.x$1
     *      }
     *    }
     *  }
     */
    private def markFree(sym: Symbol, enclosure: Symbol): Boolean = {
//      println(s"mark free: ${sym.fullLocationString} marked free in $enclosure")
      (enclosure == logicallyEnclosingMember(sym.owner)) || {
        debuglog("%s != %s".format(enclosure, logicallyEnclosingMember(sym.owner)))
        if (enclosure.isPackageClass || !markFree(sym, logicallyEnclosingMember(enclosure.skipConstructor.owner))) false
        else {
          val ss = symSet(free, enclosure)
          if (!ss(sym)) {
            ss += sym
            renamable += sym
            changedFreeVars = true
            debuglog(s"$sym is free in $enclosure")
            if (sym.isVariable && !sym.hasStableFlag) // write-once synthetic case vars are marked STABLE
              sym setFlag CAPTURED
          }
          !enclosure.isClass
        }
      }
    }

    private def markCalled(sym: Symbol, owner: Symbol): Unit = {
//      println(s"mark called: $sym of ${sym.owner} is called by $owner")
      symSet(called, owner) += sym
      if (sym.enclClass != owner.enclClass) calledFromInner += sym
    }

    /** The traverse function */
    private val freeVarTraverser = new InternalTraverser {
      override def traverse(tree: Tree): Unit = {
//       try { //debug
        val sym = tree.symbol
        tree match {
          case ClassDef(_, _, _, _) =>
            liftedDefs(tree.symbol) = Nil
            if (sym.isLocalToBlock) {
              renamable += sym
            }
          case DefDef(_, _, _, _, _, _) =>
            if (sym.isLocalToBlock) {
              renamable += sym
              sym setFlag (PrivateLocal | FINAL)
            } else if (sym.isPrimaryConstructor) {
              symSet(called, sym) += sym.owner
            }
          case Ident(name) =>
            if (sym == NoSymbol) {
              assert(name == nme.WILDCARD, name)
            } else if (sym.isLocalToBlock) {
              val owner = logicallyEnclosingMember(currentOwner)
              if (sym.isTerm && !sym.isMethod) markFree(sym, owner)
              else if (sym.isMethod) markCalled(sym, owner)
                //symSet(called, owner) += sym
            }
          case Select(_, _) =>
            if (sym.isConstructor && sym.owner.isLocalToBlock)
              markCalled(sym, logicallyEnclosingMember(currentOwner))
          case _ =>
        }
        tree.traverse(this)
//       } catch {//debug
//         case ex: Throwable =>
//           Console.println(s"$ex while traversing $tree")
//           throw ex
//       }
      }
    }

    /** Compute free variables map `fvs`.
     *  Also assign unique names to all
     *  value/variable/let that are free in some function or class, and to
     *  all class/function symbols that are owned by some function.
     */
    private def computeFreeVars(): Unit = {
      freeVarTraverser.traverse(unit.body)

      do {
        changedFreeVars = false
        for ((caller, callees) <- called ; callee <- callees ; fvs <- free get callee ; fv <- fvs)
          markFree(fv, caller)
      } while (changedFreeVars)

      def renameSym(sym: Symbol): Unit = {
        val originalName = sym.name
        sym setName newName(sym)
        debuglog("renaming in %s: %s => %s".format(sym.owner.fullLocationString, originalName, sym.name))
      }

      def newName(sym: Symbol): Name = {
        val originalName = sym.name
        def freshen(prefix: String): Name =
          if (originalName.isTypeName) unit.freshTypeName(prefix)
          else unit.freshTermName(prefix)

        val join = nme.NAME_JOIN_STRING
        if (sym.isAnonymousFunction && sym.owner.isMethod) {
          freshen("" + sym.name + join + nme.ensureNonAnon(sym.owner.name.toString) + join)
        } else {
          val name = freshen(s"${sym.name}${join}")
          // scala/bug#5652 If the lifted symbol is accessed from an inner class, it will be made public. (where?)
          //         Generating a unique name, mangled with the enclosing full class name (including
          //         package - subclass might have the same name), avoids a VerifyError in the case
          //         that a sub-class happens to lifts out a method with the *same* name.
          if (originalName.isTermName && calledFromInner(sym))
            newTermNameCached(nme.ensureNonAnon(sym.enclClass.fullName('$')) + nme.EXPAND_SEPARATOR_STRING + name)
          else
            name
        }
      }

      val allFree: Set[Symbol] = free.values.flatMap(_.iterator).toSet

      for (sym <- renamable) {
        if (allFree(sym)) proxyNames(sym) = newName(sym)
        else renameSym(sym)
      }

      afterOwnPhase {
        for ((owner, freeValues) <- free.toList) {
          val newFlags = SYNTHETIC | (if (owner.isClass) PARAMACCESSOR else PARAM)

          proxies(owner) =
            for (fv <- freeValues.toList) yield {
              val proxyName = proxyNames.getOrElse(fv, fv.name)
              debuglog(s"new proxy $proxyName in ${owner.fullLocationString}")
              val proxy =
                if (owner.isTrait) {
                  val accessorFlags = newFlags | ACCESSOR | SYNTHESIZE_IMPL_IN_SUBCLASS
                  // scala-dev#408: fields for locals captured in a trait are non-final (created in mixin), as they are
                  // assigned in a trait setter. For safe publication, subclass constructors need to call releaseFence.
                  // We need to add the attachment in lambdalift, as mixin runs after the constructors phase.
                  owner.children.foreach(_.primaryConstructor.updateAttachment(ConstructorNeedsFence))

                  // TODO do we need to preserve pre-erasure info for the accessors (and a NullaryMethodType for the getter)?
                  // can't have a field in the trait, so add a setter
                  val setter = owner.newMethod(nme.expandedSetterName(proxyName.setterName, owner), fv.pos, accessorFlags)
                  setter setInfoAndEnter MethodType(setter.newSyntheticValueParams(List(fv.info)), UnitTpe)

                  // the getter serves as the proxy -- entered below
                  owner.newMethod(proxyName.getterName, fv.pos, accessorFlags | STABLE) setInfo MethodType(Nil, fv.info)
                } else
                  owner.newValue(proxyName.toTermName, fv.pos, newFlags | PrivateLocal) setInfo fv.info

              if (owner.isClass) owner.info.decls enter proxy
              proxy
            }
        }
      }
    }

    private def proxy(sym: Symbol) = {
      def searchIn(enclosure: Symbol): Symbol = {
        if (enclosure eq NoSymbol)
          throw new IllegalArgumentException("Could not find proxy for "+ sym.defString +" in "+ sym.ownerChain +" (currentOwner= "+ currentOwner +" )")
        debuglog("searching for " + sym + "(" + sym.owner + ") in " + enclosure + " " + logicallyEnclosingMember(enclosure))

        val proxyName = proxyNames.getOrElse(sym, sym.name)
        val ps = (proxies get logicallyEnclosingMember(enclosure)).toList.flatten find (_.name == proxyName)
        ps getOrElse searchIn(enclosure.skipConstructor.owner)
      }
      debuglog("proxy %s from %s has logical enclosure %s".format(
        sym.debugLocationString,
        currentOwner.debugLocationString,
        logicallyEnclosingMember(sym.owner).debugLocationString)
      )

      if (isSameOwnerEnclosure(sym)) sym
      else searchIn(currentOwner)
    }

    private def memberRef(sym: Symbol): Tree = {
      val clazz = sym.owner.enclClass
      // println(s"memberRef from $currentClass to $sym in $clazz (currentClass=$currentClass)")
      def prematureSelfReference(): Tree = {
        val what =
          if (clazz.isStaticOwner) clazz.fullLocationString
          else s"the unconstructed `this` of ${clazz.fullLocationString}"
        val msg = s"Implementation restriction: access of ${sym.fullLocationString} from ${currentClass.fullLocationString}, would require illegal premature access to $what"
        reporter.error(curTree.pos, msg)
        EmptyTree
      }
      def qual =
        if (clazz == currentClass) gen.mkAttributedThis(clazz)
        else {
          sym resetFlag (LOCAL | PRIVATE)
          if (isUnderConstruction(clazz)) prematureSelfReference()
          else if (clazz.isStaticOwner) gen.mkAttributedQualifier(clazz.thisType)
          else outerValue match {
            case EmptyTree => prematureSelfReference()
            case o         =>
              val path = outerPath(o, currentClass.outerClass, clazz)
              if (path.tpe <:< clazz.tpeHK) path
              else {
                // scala/bug#9920 The outer accessor might have an erased type of the self type of a trait,
                //         rather than the trait itself. Add a cast if necessary.
                gen.mkAttributedCast(path, clazz.tpeHK)
              }
          }
        }

      qual match {
        case EmptyTree => EmptyTree
        case qual      => Select(qual, sym) setType sym.tpe
      }
    }

    private def proxyRef(sym: Symbol) = {
      val psym = proxy(sym)
      if (psym.isLocalToBlock) gen.mkAttributedIdent(psym)
      else {
        val ref = memberRef(psym)
        if (psym.isMethod) Apply(ref, Nil) setType ref.tpe.resultType
        else ref
      }
    }

    def freeArgsOrNil(sym: Symbol) = free.getOrElse[Iterable[Symbol]](sym, Nil).toList

    private def freeArgs(sym: Symbol): List[Symbol] =
      freeArgsOrNil(sym)

    private def addFreeArgs(pos: Position, sym: Symbol, args: List[Tree]) =
      freeArgs(sym) match {
        case Nil => args
        case fvs => addFree(sym, free = fvs map (fv => atPos(pos)(proxyRef(fv))), original = args)
      }

    def proxiesOrNil(sym: Symbol) = proxies.getOrElse(sym, Nil)

    private def freeParams(sym: Symbol): List[Symbol] =
      proxiesOrNil(sym)

    private def addFreeParams(tree: Tree, sym: Symbol): Tree =
      tree match {
        case DefDef(_, _, _, vparams :: _, _, _) =>
          val ps = freeParams(sym)

          if (ps.isEmpty) tree
          else {
            val paramSyms = cloneSymbols(ps).map(_.setFlag(PARAM))
            val paramDefs = ps map (p => ValDef(p) setPos tree.pos setType NoType)

            sym.updateInfo(lifted(MethodType(addFree(sym, free = paramSyms, original = sym.info.params), sym.info.resultType)))
            copyDefDef(tree)(vparamss = List(addFree(sym, free = paramDefs, original = vparams)))
          }

        case ClassDef(_, _, _, _) =>
          val freeParamSyms = freeParams(sym)
          val freeParamDefs =
            if (tree.symbol.isTrait) {
              freeParamSyms flatMap { getter =>
                val setter = getter.setterIn(tree.symbol, hasExpandedName = true)
                List(DefDef(getter, EmptyTree) setPos tree.pos setType NoType, DefDef(setter, EmptyTree) setPos tree.pos setType NoType)
              }
            } else freeParamSyms map (p => ValDef(p) setPos tree.pos setType NoType)

          if (freeParamDefs.isEmpty) tree
          else deriveClassDef(tree)(impl => deriveTemplate(impl)(_ ::: freeParamDefs))

        case _ => tree
      }


/*  scala/bug#6231: Something like this will be necessary to eliminate the implementation
 *  restriction from paramGetter above:
 *  We need to pass getters to the interface of an implementation class.
    private def fixTraitGetters(lifted: List[Tree]): List[Tree] =
      for (stat <- lifted) yield stat match {
        case ClassDef(mods, name, tparams, templ @ Template(parents, self, body))
        if stat.symbol.isTrait && !stat.symbol.isImplClass =>
          val iface = stat.symbol
          lifted.find(l => l.symbol.isImplClass && l.symbol.toInterface == iface) match {
            case Some(implDef) =>
              val impl = implDef.symbol
              val implGetters = impl.info.decls.toList filter (_ hasFlag TRANS_FLAG)
              if (implGetters.nonEmpty) {
                val ifaceGetters = implGetters map { ig =>
                  ig resetFlag TRANS_FLAG
                  val getter = ig cloneSymbol iface setFlag DEFERRED
                  iface.info.decls enter getter
                  getter
                }
                val ifaceGetterDefs = ifaceGetters map (DefDef(_, EmptyTree) setType NoType)
                treeCopy.ClassDef(
                  stat, mods, name, tparams,
                  treeCopy.Template(templ, parents, self, body ::: ifaceGetterDefs))
              } else
                stat
            case None =>
              stat
          }
        case _ =>
          stat
      }
*/
    private def liftDef(tree: Tree): Tree = {
      val sym = tree.symbol
      val oldOwner = sym.owner
      if (sym.isMethod && isUnderConstruction(sym.owner.owner)) { // # bug 1909
         if (sym.isModule) { // Yes, it can be a module and a method, see comments on `isModuleNotMethod`!
           // TODO promote to an implementation restriction if we can reason that this *always* leads to VerifyError.
           // See neg/t1909-object.scala
           def msg = s"scala/bug#1909 Unable to STATICally lift $sym, which is defined in the self- or super-constructor call of ${sym.owner.owner}. A VerifyError is likely."
           devWarning(tree.pos, msg)
         } else sym setFlag STATIC
      }

      sym.owner = sym.owner.enclClass
      if (sym.isMethod) sym setFlag LIFTED
      liftedDefs(sym.owner) ::= tree
      // TODO: this modifies the ClassInfotype of the enclosing class, which is associated with another phase (explicitouter).
      // This breaks type history: in a phase travel to before lambda lift, the ClassInfoType will contain lifted classes.
      sym.owner.info.decls enterUnique sym
      debuglog("lifted: " + sym + " from " + oldOwner + " to " + sym.owner)
      EmptyTree
    }

    private def postTransform(tree: Tree, isBoxedRef: Boolean = false): Tree = {
      val sym = tree.symbol
      tree match {
        case _: ClassDef | _: DefDef =>
          val withFreeParams = addFreeParams(tree, sym)
          if (sym.isLocalToBlock) liftDef(withFreeParams)
          else withFreeParams

        case ValDef(mods, name, tpt, rhs) if sym.isCapturedVariable =>
          val tpt1 = TypeTree(sym.tpe) setPos tpt.pos
          val refTypeSym = sym.tpe.typeSymbol
          val factoryCall = typer.typedPos(rhs.pos) {
            rhs match {
              case EmptyTree =>
                val zeroMSym   = refZeroMethod(refTypeSym)
                gen.mkMethodCall(zeroMSym, Nil)
              case arg =>
                val createMSym = refCreateMethod(refTypeSym)
                gen.mkMethodCall(createMSym, arg :: Nil)
            }
          }
          if (settings.warnCaptured)
            runReporting.warning(tree.pos, s"Modification of variable $name within a closure causes it to be boxed.", LintPerformance, sym)
          treeCopy.ValDef(tree, mods, name, tpt1, factoryCall)
        case ValDef(_, _, _, _) => tree
        case Return(Block(stats, value)) =>
          Block(stats, treeCopy.Return(tree, value)) setType tree.tpe setPos tree.pos
        case Return(expr) =>
          assert(sym == currentMethod, sym)
          tree
        case Apply(fn, args) =>
          treeCopy.Apply(tree, fn, addFreeArgs(tree.pos, sym, args))
        case Assign(Apply(TypeApply(sel @ Select(qual, _), _), List()), rhs) =>
          // eliminate casts introduced by selecting a captured variable field
          // on the lhs of an assignment.
          assert(sel.symbol == Object_asInstanceOf, "asInstanceOf")
          treeCopy.Assign(tree, qual, rhs)
        case Ident(name) =>
          val tree1 =
            if (sym.isTerm && !sym.isLabel)
              if (sym.isMethod)
                atPos(tree.pos)(memberRef(sym))
              else if (sym.isLocalToBlock && !isSameOwnerEnclosure(sym))
                atPos(tree.pos)(proxyRef(sym))
              else tree
            else tree
          if (sym.isCapturedVariable && !isBoxedRef)
            atPos(tree.pos) {
              val tp = tree.tpe
              val elemTree = typer typed Select(tree1 setType sym.tpe, nme.elem)
              if (elemTree.tpe.typeSymbol != tp.typeSymbol) gen.mkAttributedCast(elemTree, tp) else elemTree
            }
          else tree1
        case Block(stats, expr0) =>
          val (lzyVals, rest) = partitionConserve(stats) {
            case stat: ValDef => stat.symbol.isLazy || stat.symbol.isModuleVar
            case _            => false
          }
          if (lzyVals.isEmpty) tree
          else treeCopy.Block(tree, lzyVals ::: rest, expr0)
        case _ =>
          tree
      }
    }

    private def preTransform(tree: Tree) = super.transform(tree) setType lifted(tree.tpe)

    override def transform(tree: Tree): Tree = tree match {
      case Select(ReferenceToBoxed(idt), elem) if elem == nme.elem =>
        postTransform(preTransform(idt), isBoxedRef = false)
      case ReferenceToBoxed(idt) =>
        postTransform(preTransform(idt), isBoxedRef = true)
      case _ =>
        postTransform(preTransform(tree))
    }

    /** Transform statements and add lifted definitions to them. */
    override def transformStats(stats: List[Tree], exprOwner: Symbol): List[Tree] = {
      def addLifted(stat: Tree): Tree = stat match {
        case ClassDef(_, _, _, _) =>
          val lifted = liftedDefs remove stat.symbol match {
            case Some(xs) => xs.reverseIterator.map(addLifted).toList
            case _        => log("unexpectedly no lifted defs for " + stat.symbol) ; Nil
          }
          deriveClassDef(stat)(impl => deriveTemplate(impl)(_ ::: lifted))

        case DefDef(_, _, _, _, _, Block(Nil, expr)) if !stat.symbol.isConstructor =>
          deriveDefDef(stat)(_ => expr)
        case _ =>
          stat
      }
      super.transformStats(stats, exprOwner) map addLifted
    }

    override def transformUnit(unit: CompilationUnit): Unit = {
      computeFreeVars()
      afterOwnPhase {
        super.transformUnit(unit)
      }
      assert(liftedDefs.isEmpty, liftedDefs.keys mkString ", ")
    }
  } // class LambdaLifter

  private def addFree[A](sym: Symbol, free: List[A], original: List[A]): List[A] = {
    val prependFree = (
         !sym.isConstructor // this condition is redundant for now. It will be needed if we remove the second condition in 2.12.x
      && (settings.Ydelambdafy.value == "method" && sym.isDelambdafyTarget) // scala/bug#8359 Makes the lambda body a viable as the target MethodHandle for a call to LambdaMetafactory
    )
    if (prependFree) free ::: original
    else             original ::: free
  }
}