Enable a compiler plugin to use the async transform after patmat

Currently, the async transformation is performed during the typer
phase, like all other macros.

We have to levy a few artificial restrictions on whern an async
boundary may be: for instance we don't support await within a
pattern guard. A more natural home for the transform would be
after patterns have been translated.

The test case in this commit shows how to use the async transform
from a custom compiler phase after patmat.

The remainder of the commit updates the implementation to handle
the new tree shapes.

For states that correspond to a label definition, we use `-symbol.id`
as the state ID. This made it easier to emit the forward jumps to when
processing the label application before we had seen the label
definition.

I've also made the transformation more efficient in the way it checks
whether a given tree encloses an `await` call: we traverse the input
tree at the start of the macro, and decorate it with tree attachments
containig the answer to this question. Even after the ANF and state
machine transforms introduce new layers of synthetic trees, the
`containsAwait` code need only traverse shallowly through those
trees to find a child that has the cached answer from the original
traversal.

I had to special case the ANF transform for expressions that always
lead to a label jump: we avoids trying to push an assignment to a result
variable into `if (cond) jump1() else jump2()`, in trees of the form:

```
% cat sandbox/jump.scala
class Test {
  def test = {
    (null: Any) match {
      case _: String => ""
      case _ => ""
    }
  }
}

% qscalac -Xprint:patmat -Xprint-types sandbox/jump.scala
    def test: String = {
      case <synthetic> val x1: Any = (null{Null(null)}: Any){Any};
      case5(){
        if (x1.isInstanceOf{[T0]=> Boolean}[String]{Boolean})
          matchEnd4{(x: String)String}(""{String("")}){String}
        else
          case6{()String}(){String}{String}
      }{String};
      case6(){
        matchEnd4{(x: String)String}(""{String("")}){String}
      }{String};
      matchEnd4(x: String){
        x{String}
      }{String}
    }{String}
```

src/main/scala/scala/async/internal/AnfTransform.scala

@@ -16,16 +16,18 @@ private[async] trait AnfTransform {
   import c.internal._
   import decorators._
 
-  def anfTransform(tree: Tree): Block = {
+  def anfTransform(tree: Tree, owner: Symbol): Block = {
     // Must prepend the () for issue #31.
-    val block = c.typecheck(atPos(tree.pos)(Block(List(Literal(Constant(()))), tree))).setType(tree.tpe)
+    val block = c.typecheck(atPos(tree.pos)(newBlock(List(Literal(Constant(()))), tree))).setType(tree.tpe)
 
     sealed abstract class AnfMode
     case object Anf extends AnfMode
     case object Linearizing extends AnfMode
 
+    val tree1 = adjustTypeOfTranslatedPatternMatches(block, owner)
+
     var mode: AnfMode = Anf
-    typingTransform(block)((tree, api) => {
+    typingTransform(tree1, owner)((tree, api) => {
       def blockToList(tree: Tree): List[Tree] = tree match {
         case Block(stats, expr) => stats :+ expr
         case t                  => t :: Nil
@@ -34,7 +36,7 @@ private[async] trait AnfTransform {
       def listToBlock(trees: List[Tree]): Block = trees match {
         case trees @ (init :+ last) =>
           val pos = trees.map(_.pos).reduceLeft(_ union _)
-          Block(init, last).setType(last.tpe).setPos(pos)
+          newBlock(init, last).setType(last.tpe).setPos(pos)
       }
 
       object linearize {
@@ -66,6 +68,17 @@ private[async] trait AnfTransform {
               stats :+ valDef :+ atPos(tree.pos)(ref1)
 
             case If(cond, thenp, elsep) =>
+              // If we run the ANF transform post patmat, deal with trees like `(if (cond) jump1(){String} else jump2(){String}){String}`
+              // as though it was typed with `Unit`.
+              def isPatMatGeneratedJump(t: Tree): Boolean = t match {
+                case Block(_, expr) => isPatMatGeneratedJump(expr)
+                case If(_, thenp, elsep) => isPatMatGeneratedJump(thenp) && isPatMatGeneratedJump(elsep)
+                case _: Apply if isLabel(t.symbol) => true
+                case _ => false
+              }
+              if (isPatMatGeneratedJump(expr)) {
+                internal.setType(expr, definitions.UnitTpe)
+              }
               // if type of if-else is Unit don't introduce assignment,
               // but add Unit value to bring it into form expected by async transform
               if (expr.tpe =:= definitions.UnitTpe) {
@@ -77,7 +90,7 @@ private[async] trait AnfTransform {
                 def branchWithAssign(orig: Tree) = api.typecheck(atPos(orig.pos) {
                   def cast(t: Tree) = mkAttributedCastPreservingAnnotations(t, tpe(varDef.symbol))
                   orig match {
-                    case Block(thenStats, thenExpr) => Block(thenStats, Assign(Ident(varDef.symbol), cast(thenExpr)))
+                    case Block(thenStats, thenExpr) => newBlock(thenStats, Assign(Ident(varDef.symbol), cast(thenExpr)))
                     case _                          => Assign(Ident(varDef.symbol), cast(orig))
                   }
                 })
@@ -115,7 +128,7 @@ private[async] trait AnfTransform {
           }
         }
 
-        private def defineVar(prefix: String, tp: Type, pos: Position): ValDef = {
+        def defineVar(prefix: String, tp: Type, pos: Position): ValDef = {
           val sym = api.currentOwner.newTermSymbol(name.fresh(prefix), pos, MUTABLE | SYNTHETIC).setInfo(uncheckedBounds(tp))
           valDef(sym, mkZero(uncheckedBounds(tp))).setType(NoType).setPos(pos)
         }
@@ -152,8 +165,7 @@ private[async] trait AnfTransform {
         }
 
         def _transformToList(tree: Tree): List[Tree] = trace(tree) {
-          val containsAwait = tree exists isAwait
-          if (!containsAwait) {
+          if (!containsAwait(tree)) {
             tree match {
               case Block(stats, expr) =>
                 // avoids nested block in `while(await(false)) ...`.
@@ -207,10 +219,11 @@ private[async] trait AnfTransform {
               funStats ++ argStatss.flatten.flatten :+ typedNewApply
 
             case Block(stats, expr)                                    =>
-              (stats :+ expr).flatMap(linearize.transformToList)
+              val trees = stats.flatMap(linearize.transformToList).filterNot(isLiteralUnit) ::: linearize.transformToList(expr)
+              eliminateLabelParameters(trees)
 
             case ValDef(mods, name, tpt, rhs) =>
-              if (rhs exists isAwait) {
+              if (containsAwait(rhs)) {
                 val stats :+ expr = api.atOwner(api.currentOwner.owner)(linearize.transformToList(rhs))
                 stats.foreach(_.changeOwner(api.currentOwner, api.currentOwner.owner))
                 stats :+ treeCopy.ValDef(tree, mods, name, tpt, expr)
@@ -247,7 +260,7 @@ private[async] trait AnfTransform {
               scrutStats :+ treeCopy.Match(tree, scrutExpr, caseDefs)
 
             case LabelDef(name, params, rhs) =>
-              List(LabelDef(name, params, Block(linearize.transformToList(rhs), Literal(Constant(())))).setSymbol(tree.symbol))
+              List(LabelDef(name, params, newBlock(linearize.transformToList(rhs), Literal(Constant(())))).setSymbol(tree.symbol))
 
             case TypeApply(fun, targs) =>
               val funStats :+ simpleFun = linearize.transformToList(fun)
@@ -259,6 +272,44 @@ private[async] trait AnfTransform {
         }
       }
 
+      // Replace the label parameters on `matchEnd` with use of a `matchRes` temporary variable
+      def eliminateLabelParameters(statsExpr: List[Tree]): List[Tree] = {
+        import internal.{methodType, setInfo}
+        val caseDefToMatchResult = collection.mutable.Map[Symbol, Symbol]()
+
+        val matchResults = collection.mutable.Buffer[Tree]()
+        val statsExpr0 = statsExpr.reverseMap {
+          case ld @ LabelDef(_, param :: Nil, body) =>
+            val matchResult = linearize.defineVar(name.matchRes, param.tpe, ld.pos)
+            matchResults += matchResult
+            caseDefToMatchResult(ld.symbol) = matchResult.symbol
+            val ld2 = treeCopy.LabelDef(ld, ld.name, Nil, body.substituteSymbols(param.symbol :: Nil, matchResult.symbol :: Nil))
+            setInfo(ld.symbol, methodType(Nil, ld.symbol.info.resultType))
+            ld2
+          case t =>
+            if (caseDefToMatchResult.isEmpty) t
+            else typingTransform(t)((tree, api) =>
+              tree match {
+                case Apply(fun, arg :: Nil) if isLabel(fun.symbol) && caseDefToMatchResult.contains(fun.symbol) =>
+                  api.typecheck(atPos(tree.pos)(newBlock(Assign(Ident(caseDefToMatchResult(fun.symbol)), api.recur(arg)) :: Nil, treeCopy.Apply(tree, fun, Nil))))
+                case Block(stats, expr) =>
+                  api.default(tree) match {
+                    case Block(stats, Block(stats1, expr)) =>
+                      treeCopy.Block(tree, stats ::: stats1, expr)
+                    case t => t
+                  }
+                case _ =>
+                  api.default(tree)
+              }
+            )
+        }
+        matchResults.toList match {
+          case Nil => statsExpr0.reverse
+          case r1 :: Nil => (r1 +: statsExpr0.reverse) :+ atPos(tree.pos)(gen.mkAttributedIdent(r1.symbol))
+          case _ => c.error(macroPos, "Internal error: unexpected tree encountered during ANF transform " + statsExpr); statsExpr
+        }
+      }
+
       def anfLinearize(tree: Tree): Block = {
         val trees: List[Tree] = mode match {
           case Anf         => anf._transformToList(tree)

src/main/scala/scala/async/internal/AsyncBase.scala

@@ -43,9 +43,9 @@ abstract class AsyncBase {
                                  (body: c.Expr[T])
                                  (execContext: c.Expr[futureSystem.ExecContext]): c.Expr[futureSystem.Fut[T]] = {
     import c.universe._, c.internal._, decorators._
-    val asyncMacro = AsyncMacro(c, self)
+    val asyncMacro = AsyncMacro(c, self)(body.tree)
 
-    val code = asyncMacro.asyncTransform[T](body.tree, execContext.tree)(c.weakTypeTag[T])
+    val code = asyncMacro.asyncTransform[T](execContext.tree)(c.weakTypeTag[T])
     AsyncUtils.vprintln(s"async state machine transform expands to:\n ${code}")
 
     // Mark range positions for synthetic code as transparent to allow some wiggle room for overlapping ranges

src/main/scala/scala/async/internal/AsyncId.scala

@@ -41,11 +41,11 @@ object AsyncTestLV extends AsyncBase {
  * A trivial implementation of [[FutureSystem]] that performs computations
  * on the current thread. Useful for testing.
  */
+class Box[A] {
+  var a: A = _
+}
 object IdentityFutureSystem extends FutureSystem {
-
-  class Prom[A] {
-    var a: A = _
-  }
+  type Prom[A] = Box[A]
 
   type Fut[A] = A
   type ExecContext = Unit
@@ -57,7 +57,7 @@ object IdentityFutureSystem extends FutureSystem {
 
     def execContext: Expr[ExecContext] = c.Expr[Unit](Literal(Constant(())))
 
-    def promType[A: WeakTypeTag]: Type = weakTypeOf[Prom[A]]
+    def promType[A: WeakTypeTag]: Type = weakTypeOf[Box[A]]
     def tryType[A: WeakTypeTag]: Type = weakTypeOf[scala.util.Try[A]]
     def execContextType: Type = weakTypeOf[Unit]
 

src/main/scala/scala/async/internal/AsyncMacro.scala

@@ -1,15 +1,17 @@
 package scala.async.internal
 
 object AsyncMacro {
-  def apply(c0: reflect.macros.Context, base: AsyncBase): AsyncMacro { val c: c0.type } = {
+  def apply(c0: reflect.macros.Context, base: AsyncBase)(body0: c0.Tree): AsyncMacro { val c: c0.type } = {
     import language.reflectiveCalls
     new AsyncMacro { self =>
       val c: c0.type                                             = c0
+      val body: c.Tree = body0
       // This member is required by `AsyncTransform`:
       val asyncBase: AsyncBase                                   = base
       // These members are required by `ExprBuilder`:
       val futureSystem: FutureSystem                             = base.futureSystem
       val futureSystemOps: futureSystem.Ops {val c: self.c.type} = futureSystem.mkOps(c)
+      val containsAwait: c.Tree => Boolean = containsAwaitCached(body0)
     }
   }
 }
@@ -19,7 +21,10 @@ private[async] trait AsyncMacro
   with ExprBuilder with AsyncTransform with AsyncAnalysis with LiveVariables {
 
   val c: scala.reflect.macros.Context
+  val body: c.Tree
+  val containsAwait: c.Tree => Boolean
 
   lazy val macroPos = c.macroApplication.pos.makeTransparent
   def atMacroPos(t: c.Tree) = c.universe.atPos(macroPos)(t)
+
 }

src/main/scala/scala/async/internal/AsyncTransform.scala

@@ -9,7 +9,7 @@ trait AsyncTransform {
 
   val asyncBase: AsyncBase
 
-  def asyncTransform[T](body: Tree, execContext: Tree)
+  def asyncTransform[T](execContext: Tree)
                        (resultType: WeakTypeTag[T]): Tree = {
 
     // We annotate the type of the whole expression as `T @uncheckedBounds` so as not to introduce
@@ -22,7 +22,7 @@ trait AsyncTransform {
     // Transform to A-normal form:
     //  - no await calls in qualifiers or arguments,
     //  - if/match only used in statement position.
-    val anfTree0: Block = anfTransform(body)
+    val anfTree0: Block = anfTransform(body, c.internal.enclosingOwner)
 
     val anfTree = futureSystemOps.postAnfTransform(anfTree0)
 
@@ -35,15 +35,15 @@ trait AsyncTransform {
     val stateMachine: ClassDef = {
       val body: List[Tree] = {
         val stateVar = ValDef(Modifiers(Flag.MUTABLE | Flag.PRIVATE | Flag.LOCAL), name.state, TypeTree(definitions.IntTpe), Literal(Constant(StateAssigner.Initial)))
-        val result = ValDef(NoMods, name.result, TypeTree(futureSystemOps.promType[T](uncheckedBoundsResultTag)), futureSystemOps.createProm[T](uncheckedBoundsResultTag).tree)
+        val resultAndAccessors = mkMutableField(futureSystemOps.promType[T](uncheckedBoundsResultTag), name.result, futureSystemOps.createProm[T](uncheckedBoundsResultTag).tree)
         val execContextValDef = ValDef(NoMods, name.execContext, TypeTree(), execContext)
 
         val apply0DefDef: DefDef = {
           // We extend () => Unit so we can pass this class as the by-name argument to `Future.apply`.
           // See SI-1247 for the the optimization that avoids creation.
           DefDef(NoMods, name.apply, Nil, Nil, TypeTree(definitions.UnitTpe), Apply(Ident(name.apply), literalNull :: Nil))
         }
-        List(emptyConstructor, stateVar, result, execContextValDef) ++ List(applyDefDefDummyBody, apply0DefDef)
+        List(emptyConstructor, stateVar) ++ resultAndAccessors ++ List(execContextValDef) ++ List(applyDefDefDummyBody, apply0DefDef)
       }
 
       val tryToUnit = appliedType(definitions.FunctionClass(1), futureSystemOps.tryType[Any], typeOf[Unit])