SI-8947 Additional layers of defence against EmptyTree mutation

As suggested in review:

  - Use `abort` rather than `{error; EmptyTree} when we hit an
    error in reification or tag materialization.
  - Explicitly avoid adding the `MacroExpansionAttachment` to the
    macro expansion if it an `EmptyTree`
  - Emit a `-Xdev` warning if any other code paths find a way to
    mutate attachments in places they shouldn't.

2 files changed, 2 insertions, 3 deletions

diff --git a/src/compiler/scala/reflect/reify/Taggers.scala b/src/compiler/scala/reflect/reify/Taggers.scala
index 093c2bee22..0863ee38f9 100644
--- a/src/compiler/scala/reflect/reify/Taggers.scala
+++ b/src/compiler/scala/reflect/reify/Taggers.scala
@@ -79,8 +79,7 @@ abstract class Taggers {
     try materializer
     catch {
       case ReificationException(pos, msg) =>
-        c.error(pos.asInstanceOf[c.Position], msg) // this cast is a very small price for the sanity of exception handling
-        EmptyTree
+        c.abort(pos.asInstanceOf[c.Position], msg) // this cast is a very small price for the sanity of exception handling
       case UnexpectedReificationException(pos, err, cause) if cause != null =>
         throw cause
     }
diff --git a/src/compiler/scala/tools/nsc/typechecker/StdAttachments.scala b/src/compiler/scala/tools/nsc/typechecker/StdAttachments.scala
index 57f27a05fd..ea44b9dc39 100644
--- a/src/compiler/scala/tools/nsc/typechecker/StdAttachments.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/StdAttachments.scala
@@ -61,7 +61,7 @@ trait StdAttachments {
     val metadata = MacroExpansionAttachment(expandee, expanded)
     expandee updateAttachment metadata
     expanded match {
-      case expanded: Tree => expanded updateAttachment metadata
+      case expanded: Tree if !expanded.isEmpty => expanded updateAttachment metadata
       case _ => // do nothing
     }
   }