Do not allow running Expr that came as macro arguments

It is too hard to ensure that the contents of the tree
can be pickled. It would be posible but the current ad-hoc
solution is too brittle and has no good usecases.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/quoted/PickledQuotes.scala

@@ -23,19 +23,6 @@ object PickledQuotes {
 
   /** Pickle the tree of the quoted.Expr */
   def pickleExpr(tree: Tree)(implicit ctx: Context): scala.quoted.Expr[Any] = {
-    // Check that there are no free variables
-    new TreeTraverser {
-      private val definedHere = scala.collection.mutable.Set.empty[Symbol]
-      def traverse(tree: tpd.Tree)(implicit ctx: Context): Unit = tree match {
-        case tree: Ident if tree.symbol.exists && !definedHere(tree.symbol) =>
-          throw new scala.quoted.FreeVariableError(tree.name.toString)
-        case tree: DefTree =>
-          definedHere += tree.symbol
-          traverseChildren(tree)
-        case _ =>
-          traverseChildren(tree)
-      }
-    }.traverse(tree)
     val pickled = pickleQuote(tree)
     scala.runtime.quoted.Unpickler.unpickleExpr(pickled, Nil)
   }
@@ -64,7 +51,7 @@ object PickledQuotes {
         case value: Class[_] => ref(defn.Predef_classOf).appliedToType(classToType(value))
         case value => Literal(Constant(value))
       }
-    case expr: TreeExpr[Tree] @unchecked => expr.tree
+    case expr: TastyTreeExpr[Tree] @unchecked => expr.tree
     case expr: FunctionAppliedTo[_, _] =>
       functionAppliedTo(quotedExprToTree(expr.f), quotedExprToTree(expr.x))
   }

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -22,7 +22,7 @@ import config.Config
 import core.quoted.PickledQuotes
 import scala.quoted
 import scala.quoted.Types.TreeType
-import scala.quoted.Exprs.TreeExpr
+import scala.quoted.Exprs.TastyTreeExpr
 
 /** Unpickler for typed trees
  *  @param reader          the reader from which to unpickle
@@ -1150,7 +1150,7 @@ class TreeUnpickler(reader: TastyReader,
       val args = until(end)(readTerm())
       val splice = splices(idx)
       def wrap(arg: Tree) =
-        if (arg.isTerm) new TreeExpr(arg, PickledQuotes.pickleExpr(arg))
+        if (arg.isTerm) new TastyTreeExpr(arg)
         else new TreeType(arg)
       val reifiedArgs = args.map(wrap)
       if (isType) {

compiler/src/dotty/tools/dotc/quoted/Toolbox.scala

@@ -6,7 +6,7 @@ import dotty.tools.dotc.printing.RefinedPrinter
 
 import scala.quoted.Expr
 import scala.runtime.BoxedUnit
-import scala.quoted.Exprs.{LiftedExpr, TreeExpr}
+import scala.quoted.Exprs.{LiftedExpr, TastyTreeExpr}
 import scala.runtime.quoted._
 
 /** Default runners for quoted expressions */
@@ -24,8 +24,8 @@ object Toolbox {
     def run(expr: Expr[T]): T = expr match {
       case expr: LiftedExpr[T] =>
         expr.value
-      case expr: TreeExpr[Tree] @unchecked =>
-        new QuoteDriver().run(expr.pickled, runSettings)
+      case expr: TastyTreeExpr[Tree] @unchecked =>
+        throw new scala.quoted.QuoteError("Can't run and scala.quoted.Expr coming from a macro argument")
       case _ =>
         new QuoteDriver().run(expr, runSettings)
     }

compiler/src/dotty/tools/dotc/transform/Splicer.scala

@@ -67,7 +67,7 @@ object Splicer {
             assert(!tp.hasAnnotation(defn.InlineParamAnnot))
             // Replace argument by its binding
             val arg1 = bindMap.getOrElse(arg, arg)
-            new scala.quoted.Exprs.TreeExpr(arg1, PickledQuotes.pickleExpr(arg1))
+            new scala.quoted.Exprs.TastyTreeExpr(arg1)
         }
         args1 ::: liftArgs(tp.resType, args.tail)
       case tp: PolyType =>

library/src/scala/quoted/Expr.scala

@@ -33,7 +33,7 @@ object Expr {
 object Exprs {
   /** An Expr backed by a pickled TASTY tree */
   final class TastyExpr[T](val tasty: Pickled, val args: Seq[Any]) extends Expr[T] {
-    override def toString: String = s"Expr(<pickled>)"
+    override def toString: String = s"Expr(<pickled tasty>)"
   }
 
   /** An Expr backed by a lifted value.
@@ -50,9 +50,8 @@ object Exprs {
    *
    *  May contain references to code defined outside this Expr instance.
    */
-  final class TreeExpr[Tree](val tree: Tree, pickle: => Expr[_]) extends quoted.Expr[Any] {
-    def pickled[T]: Expr[T] = pickle.asInstanceOf[Expr[T]]
-    override def toString: String = s"Expr(<raw>)"
+  final class TastyTreeExpr[Tree](val tree: Tree) extends quoted.Expr[Any] {
+    override def toString: String = s"Expr(<tasty tree>)"
   }
 
   /** An Expr representing `'{(~f).apply(~x)}` but it is beta-reduced when the closure is known */

library/src/scala/quoted/FreeVariableError.scala

@@ -1,10 +1,10 @@
 import Macros._
 object Test {
   def main(args: Array[String]): Unit = {
-    println(foo(1))
-    println(foo(1  + 3))
+    println(foo(1)) // error
+    println(foo(1  + 3)) // error
     val x = 3
-    println(foo {
+    println(foo { // error
       val x = 5
       x
     })