Fix crashers in do/while and while(await(..))

The new tree shapes handled for do/while look like:

    // type checked
    async({
      val b = false;
      doWhile$1(){
        await(());
        if (b)
          doWhile$1()
        else
          ()
      };
      ()
    })

We had to change ExprBuilder to create states for the if/else
that concludes the doWhile body, and also loosen the assertion
that the label jump must be the last thing we see.

We also have to look for more than just `containsAwait` when
deciding whether an `If` needs to be transformed into states;
it might also contain a jump to the enclosing label that is on
the other side of an `await`, and hence needs to be a state
transition instead.

3 files changed, 27 insertions, 3 deletions

diff --git a/src/main/scala/scala/async/internal/AnfTransform.scala b/src/main/scala/scala/async/internal/AnfTransform.scala
index f19a87a..8518cf5 100644
--- a/src/main/scala/scala/async/internal/AnfTransform.scala
+++ b/src/main/scala/scala/async/internal/AnfTransform.scala
@@ -162,7 +162,14 @@ private[async] trait AnfTransform {
       def _transformToList(tree: Tree): List[Tree] = trace(tree) {
         val containsAwait = tree exists isAwait
         if (!containsAwait) {
-          List(tree)
+          tree match {
+            case Block(stats, expr) =>
+              // avoids nested block in `while(await(false)) ...`.
+              // TODO I think `containsAwait` really should return true if the code contains a label jump to an enclosing
+              // while/doWhile and there is an await *anywhere* inside that construct.
+              stats :+ expr
+            case _ => List(tree)
+          }
         } else tree match {
           case Select(qual, sel) =>
             val stats :+ expr = linearize.transformToList(qual)
diff --git a/src/main/scala/scala/async/internal/ExprBuilder.scala b/src/main/scala/scala/async/internal/ExprBuilder.scala
index 85e0953..b0cd914 100644
--- a/src/main/scala/scala/async/internal/ExprBuilder.scala
+++ b/src/main/scala/scala/async/internal/ExprBuilder.scala
@@ -127,7 +127,11 @@ trait ExprBuilder {
     private var nextJumpState: Option[Int] = None
 
     def +=(stat: Tree): this.type = {
-      assert(nextJumpState.isEmpty, s"statement appeared after a label jump: $stat")
+      stat match {
+        case Literal(Constant(())) => // This case occurs in do/while
+        case _ =>
+          assert(nextJumpState.isEmpty, s"statement appeared after a label jump: $stat")
+      }
       def addStat() = stats += stat
       stat match {
         case Apply(fun, Nil) =>
@@ -228,7 +232,7 @@ trait ExprBuilder {
         currState = afterAwaitState
         stateBuilder = new AsyncStateBuilder(currState, symLookup)
 
-      case If(cond, thenp, elsep) if stat exists isAwait =>
+      case If(cond, thenp, elsep) if (stat exists isAwait) || containsForiegnLabelJump(stat) =>
         checkForUnsupportedAwait(cond)
 
         val thenStartState = nextState()
diff --git a/src/main/scala/scala/async/internal/TransformUtils.scala b/src/main/scala/scala/async/internal/TransformUtils.scala
index 71fddaa..e382c62 100644
--- a/src/main/scala/scala/async/internal/TransformUtils.scala
+++ b/src/main/scala/scala/async/internal/TransformUtils.scala
@@ -96,6 +96,19 @@ private[async] trait TransformUtils {
     treeInfo.isExprSafeToInline(tree)
   }
 
+  // `while(await(x))` ... or `do { await(x); ... } while(...)` contain an `If` that loops;
+  // we must break that `If` into states so that it convert the label jump into a state machine
+  // transition
+  final def containsForiegnLabelJump(t: Tree): Boolean = {
+    val labelDefs = t.collect {
+      case ld: LabelDef => ld.symbol
+    }.toSet
+    t.exists {
+      case rt: RefTree => !(labelDefs contains rt.symbol)
+      case _ => false
+    }
+  }
+
   /** Map a list of arguments to:
     * - A list of argument Trees
     * - A list of auxillary results.