humane reporting of macro impl binding version errors

Macro defs are linked to macro impls by the virtue of MacroImplBinding
structures that are persisted between compilation runs serialized within
instances of macroImpl annotations.

Along with the evolution of our macro engine, we sometimes have to evolve
the format of MacroImplBinding, which means that it has to be versioned.
Version mismatches are checked upon every macro expansion, ensuring that
macros that we expand were compiled with exactly the same version of the
macro engine that we’re running.

That’s all really cool apart from the fact that version mismatches result
in aborting the entire compilation with an obscure message without giving
a hint about the culprits.

This commit improves the situation by providing pretty per-expansion
compilation errors that tell the programmer what macro expansions are
at fault and what macro engines were used to compile them.

Author: xeno-by

src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala

@@ -730,6 +730,11 @@ trait ContextErrors {
         issueNormalTypeError(expandee, s"macro in $role role can only expand into $allowedExpansions")
       }
 
+      def MacroIncompatibleEngineError(macroEngine: String) = {
+        val message = s"macro cannot be expanded, because it was compiled by an incompatible macro engine $macroEngine"
+        issueNormalTypeError(lastTreeToTyper, message)
+      }
+
       case object MacroExpansionException extends Exception with scala.util.control.ControlThrowable
 
       protected def macroExpansionError(expandee: Tree, msg: String, pos: Position = NoPosition) = {

src/compiler/scala/tools/nsc/typechecker/Macros.scala

@@ -123,16 +123,15 @@ trait Macros extends FastTrack with MacroRuntimes with Traces with Helpers {
    *
    *    @scala.reflect.macros.internal.macroImpl(
    *      `macro`(
+   *        "macroEngine" = <current macro engine>,
    *        "isBundle" = false,
    *        "isBlackbox" = true,
    *        "signature" = List(Other),
    *        "methodName" = "impl",
-   *        "versionFormat" = <current version format>,
    *        "className" = "Macros$"))
    */
+  def macroEngine = "v7.0 (implemented in Scala 2.11.0-M8)"
   object MacroImplBinding {
-    val versionFormat = 6.0
-
     def pickleAtom(obj: Any): Tree =
       obj match {
         case list: List[_] => Apply(Ident(ListModule), list map pickleAtom)
@@ -183,12 +182,12 @@ trait Macros extends FastTrack with MacroRuntimes with Traces with Helpers {
       }
 
       val payload = List[(String, Any)](
-        "versionFormat" -> versionFormat,
-        "isBundle"      -> isBundle,
-        "isBlackbox"    -> isBlackbox,
-        "className"     -> className,
-        "methodName"    -> macroImpl.name.toString,
-        "signature"     -> signature
+        "macroEngine" -> macroEngine,
+        "isBundle"    -> isBundle,
+        "isBlackbox"  -> isBlackbox,
+        "className"   -> className,
+        "methodName"  -> macroImpl.name.toString,
+        "signature"   -> signature
       )
 
       // the shape of the nucleus is chosen arbitrarily. it doesn't carry any payload.
@@ -237,8 +236,8 @@ trait Macros extends FastTrack with MacroRuntimes with Traces with Helpers {
         raw.asInstanceOf[T]
       }
 
-      val pickleVersionFormat = unpickle("versionFormat", classOf[Double])
-      if (versionFormat != pickleVersionFormat) fail(s"expected version format $versionFormat, actual $pickleVersionFormat")
+      val macroEngine = unpickle("macroEngine", classOf[String])
+      if (self.macroEngine != macroEngine) typer.TyperErrorGen.MacroIncompatibleEngineError(macroEngine)
 
       val isBundle = unpickle("isBundle", classOf[Boolean])
       val isBlackbox = unpickle("isBlackbox", classOf[Boolean])

test/files/neg/macro-incompatible-macro-engine.check

@@ -0,0 +1,7 @@
+Test_3.scala:2: error: macro cannot be expanded, because it was compiled by an incompatible macro engine vxxx (implemented in the incompatibleMacroEngine plugin)
+  Macros.foo
+  ^
+Test_3.scala:3: error: macro cannot be expanded, because it was compiled by an incompatible macro engine vxxx (implemented in the incompatibleMacroEngine plugin)
+  Macros.foo
+  ^
+two errors found

test/files/neg/macro-incompatible-macro-engine/Macros_2.flags

@@ -0,0 +1 @@
+-Xplugin:.

test/files/neg/macro-incompatible-macro-engine/Macros_2.scala

@@ -0,0 +1,7 @@
+import scala.language.experimental.macros
+import scala.reflect.macros.BlackboxContext
+
+object Macros {
+  def impl(c: BlackboxContext) = c.universe.Literal(c.universe.Constant(()))
+  def foo: Unit = macro impl
+}

test/files/neg/macro-incompatible-macro-engine/Plugin_1.scala

@@ -0,0 +1,35 @@
+package incompatibleMacroEngine
+
+import scala.tools.nsc.Global
+import scala.tools.nsc.plugins.{Plugin => NscPlugin}
+
+class Plugin(val global: Global) extends NscPlugin {
+  import global._
+  import analyzer._
+
+  val name = "incompatibleMacroEngine"
+  val description = "A sample analyzer plugin that crafts a macro impl binding with a non-standard macro engine."
+  val components = Nil
+  addMacroPlugin(MacroPlugin)
+
+  object MacroPlugin extends MacroPlugin {
+    def fixupBinding(tree: Tree) = new Transformer {
+      override def transform(tree: Tree) = {
+        tree match {
+          case Literal(const @ Constant(x)) if tree.tpe == null => tree setType ConstantType(const)
+          case _ if tree.tpe == null => tree setType NoType
+          case _ => ;
+        }
+        super.transform(tree)
+      }
+    }.transform(tree)
+
+    override def pluginsTypedMacroBody(typer: Typer, ddef: DefDef): Option[Tree] = {
+      val result = typedMacroBody(typer, ddef)
+      val List(AnnotationInfo(atp, List(Apply(nucleus, _ :: others)), Nil)) = ddef.symbol.annotations
+      val updatedBinding = Apply(nucleus, Assign(Literal(Constant("macroEngine")), Literal(Constant("vxxx (implemented in the incompatibleMacroEngine plugin)"))) :: others)
+      ddef.symbol.setAnnotations(List(AnnotationInfo(atp, List(fixupBinding(updatedBinding)), Nil)))
+      Some(result)
+    }
+  }
+}

test/files/neg/macro-incompatible-macro-engine/Test_3.scala

@@ -0,0 +1,4 @@
+object Test extends App {
+  Macros.foo
+  Macros.foo
+}

test/files/neg/macro-incompatible-macro-engine/scalac-plugin.xml

@@ -0,0 +1,4 @@
+<plugin>
+  <name>incompatible-macro-engine</name>
+  <classname>incompatibleMacroEngine.Plugin</classname>
+</plugin>