Rework extension point for checking for already-completed futures

The current extension point assumes that if a future is in the
completed state, a subsequent call to get the already completed
value will succeed. While this assumption holds for
scala.concurrent.Future, it might not hold for a future system
that has semantics like a weak reference.

This commit uses a single call `getCompleted` to query the state
and get the already completed value. This returns null if the
value is not available.

Author: retronym

src/main/scala/scala/async/internal/ExprBuilder.scala

@@ -93,12 +93,15 @@ trait ExprBuilder {
       val fun = This(tpnme.EMPTY)
       val callOnComplete = futureSystemOps.onComplete[Any, Unit](c.Expr[futureSystem.Fut[Any]](awaitable.expr),
         c.Expr[futureSystem.Tryy[Any] => Unit](fun), c.Expr[futureSystem.ExecContext](Ident(name.execContext))).tree
-      val tryGetOrCallOnComplete =
-        if (futureSystemOps.continueCompletedFutureOnSameThread)
-          If(futureSystemOps.isCompleted(c.Expr[futureSystem.Fut[_]](awaitable.expr)).tree,
-            adaptToUnit(ifIsFailureTree[T](futureSystemOps.getCompleted[Any](c.Expr[futureSystem.Fut[Any]](awaitable.expr)).tree) :: Nil),
-            Block(toList(callOnComplete), Return(literalUnit))) :: Nil
-        else
+      val tryGetOrCallOnComplete: List[Tree] =
+        if (futureSystemOps.continueCompletedFutureOnSameThread) {
+          val tempName = name.fresh(name.completed)
+          val initTemp = ValDef(NoMods, tempName, TypeTree(futureSystemOps.tryType[Any]), futureSystemOps.getCompleted[Any](c.Expr[futureSystem.Fut[Any]](awaitable.expr)).tree)
+          val ifTree = If(Apply(Select(Literal(Constant(null)), TermName("ne")), Ident(tempName) :: Nil),
+            adaptToUnit(ifIsFailureTree[T](Ident(tempName)) :: Nil),
+            Block(toList(callOnComplete), Return(literalUnit)))
+          initTemp :: ifTree :: Nil
+        } else
           toList(callOnComplete) ::: Return(literalUnit) :: Nil
       mkHandlerCase(state, stats ++ List(mkStateTree(onCompleteState, symLookup)) ++ tryGetOrCallOnComplete)
     }

src/main/scala/scala/async/internal/FutureSystem.scala

@@ -49,8 +49,10 @@ trait FutureSystem {
                          execContext: Expr[ExecContext]): Expr[Unit]
 
     def continueCompletedFutureOnSameThread = false
-    def isCompleted(future: Expr[Fut[_]]): Expr[Boolean] =
-      throw new UnsupportedOperationException("isCompleted not supported by this FutureSystem")
+
+    /** Return `null` if this future is not yet completed, or `Tryy[A]` with the completed result
+      * otherwise
+      */
     def getCompleted[A: WeakTypeTag](future: Expr[Fut[A]]): Expr[Tryy[A]] =
       throw new UnsupportedOperationException("getCompleted not supported by this FutureSystem")
 
@@ -110,11 +112,8 @@ object ScalaConcurrentFutureSystem extends FutureSystem {
 
     override def continueCompletedFutureOnSameThread: Boolean = true
 
-    override def isCompleted(future: Expr[Fut[_]]): Expr[Boolean] = reify {
-      future.splice.isCompleted
-    }
     override def getCompleted[A: WeakTypeTag](future: Expr[Fut[A]]): Expr[Tryy[A]] = reify {
-      future.splice.value.get
+      if (future.splice.isCompleted) future.splice.value.get else null
     }
 
     def completeProm[A](prom: Expr[Prom[A]], value: Expr[scala.util.Try[A]]): Expr[Unit] = reify {

src/main/scala/scala/async/internal/TransformUtils.scala

@@ -24,6 +24,7 @@ private[async] trait TransformUtils {
     val ifRes         = "ifres"
     val await         = "await"
     val bindSuffix    = "$bind"
+    val completed     = newTermName("completed")
 
     val state         = newTermName("state")
     val result        = newTermName("result")