Clean up code relating to interfaces.

Previous implementation was confused about the meaning of interface (with default methods
or without?).

Now instead of Interface/JavaInterface we have

    PureInterface   - all members are abstract methods
    NoInits         - all members are methods

Author: odersky

src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -26,15 +26,18 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
 
   /** Is tree legal as a member definition of an interface?
    */
-  def isInterfaceMember(tree: Tree): Boolean = unsplice(tree) match {
-    case EmptyTree => true
-    case Import(_, _) => true
-    case TypeDef(_, _, _) => true
-    case DefDef(mods, _, _, _, _, __) => mods.flags is Deferred
-    case ValDef(mods, _, _, _) => mods is Deferred
+  def isPureInterfaceMember(tree: Tree): Boolean = unsplice(tree) match {
+    case EmptyTree | Import(_, _) | TypeDef(_, _, _) => true
+    case DefDef(_, _, _, _, _, rhs) => rhs.isEmpty
+    case ValDef(mods, _, _, rhs) => rhs.isEmpty
     case _ => false
   }
 
+  /** Is tree legal as a member definition of a no-init trait?
+   */
+  def isNoInitMember(tree: Tree): Boolean =
+    isPureInterfaceMember(tree) || unsplice(tree).isInstanceOf[DefDef]
+
   def isOpAssign(tree: Tree) = unsplice(tree) match {
     case Apply(fn, _ :: Nil) =>
       unsplice(fn) match {

src/dotty/tools/dotc/core/Definitions.scala

@@ -191,7 +191,7 @@ class Definitions {
   lazy val ScalaStaticsClass = ScalaStaticsModule.moduleClass.asClass
 
     def staticsMethod(name: PreName) = ctx.requiredMethod(ScalaStaticsClass, name)
-    
+
   lazy val DottyPredefModule = ctx.requiredModule("dotty.DottyPredef")
   lazy val NilModule = ctx.requiredModule("scala.collection.immutable.Nil")
   lazy val PredefConformsClass = ctx.requiredClass("scala.Predef." + tpnme.Conforms)
@@ -201,7 +201,7 @@ class Definitions {
     // needed as a synthetic class because Scala 2.x refers to it in classfiles
     // but does not define it as an explicit class.
     newCompleteClassSymbol(
-      ScalaPackageClass, tpnme.Singleton, Trait | Interface | Final,
+      ScalaPackageClass, tpnme.Singleton, PureInterfaceCreationFlags | Final,
       List(AnyClass.typeRef), EmptyScope)
   lazy val SeqClass: ClassSymbol = ctx.requiredClass("scala.collection.Seq")
     lazy val Seq_apply = ctx.requiredMethod(SeqClass, nme.apply)
@@ -506,7 +506,7 @@ class Definitions {
       val cls = newClassSymbol(
         ScalaPackageClass,
         traitName,
-        Trait | Interface | Synthetic,
+        PureInterfaceCreationFlags | Synthetic,
         completer)
       myLambdaTraits += cls
       cls

src/dotty/tools/dotc/core/Flags.scala

@@ -287,7 +287,7 @@ object Flags {
   /** A trait that has only abstract methods as members
    *  (and therefore can be represented by a Java interface
    */
-  final val Interface = typeFlag(22, "interface")
+  final val PureInterface = typeFlag(22, "interface")
 
   /** Labeled with of abstract & override */
   final val AbsOverride = termFlag(22, "abstract override")
@@ -335,6 +335,9 @@ object Flags {
   final val JavaStaticTerm = JavaStatic.toTermFlags
   final val JavaStaticType = JavaStatic.toTypeFlags
 
+  /** Trait is not an interface, but does not have fields or intialization code */
+  final val NoInits = typeFlag(32, "<noInits>")
+
   /** Variable is accessed from nested function. */
   final val Captured = termFlag(32, "<captured>")
 
@@ -353,9 +356,6 @@ object Flags {
   /** Symbol is a Java-style varargs method */
   final val JavaVarargs = termFlag(37, "<varargs>")
 
-  /** Symbol is a Java default method */
-  final val DefaultMethod = termFlag(38, "<defaultmethod>")
-
   // Flags following this one are not pickled
 
   /** Symbol always defines a fresh named type */
@@ -464,6 +464,9 @@ object Flags {
   /** Accessors always have these flags set */
   final val AccessorCreationFlags = Method | Accessor
 
+  /** Pure interfaces always have these flags */
+  final val PureInterfaceCreationFlags = Trait | NoInits | PureInterface
+
   /** The flags of the self symbol */
   final val SelfSymFlags = Private | Local | Deferred
 
@@ -539,8 +542,11 @@ object Flags {
   /** Is a default parameter in Scala 2*/
   final val DefaultParameter = allOf(Param, DefaultParameterized)
 
-  /** A Java interface */
-  final val JavaInterface = allOf(JavaDefined, Trait)
+  /** A trait that does not need to be initialized */
+  final val NoInitsTrait = allOf(Trait, NoInits)
+
+  /** A Java interface, potentially with default methods */
+  final val JavaTrait = allOf(JavaDefined, Trait, NoInits)
 
   /** A Java companion object */
   final val JavaModule = allOf(JavaDefined, Module)

src/dotty/tools/dotc/core/pickling/ClassfileConstants.scala

@@ -345,7 +345,7 @@ object ClassfileConstants {
       case JAVA_ACC_SYNTHETIC  => Synthetic
       case JAVA_ACC_STATIC     => JavaStatic
       case JAVA_ACC_ABSTRACT   => if (isAnnotation) EmptyFlags else if (isClass) Abstract else Deferred
-      case JAVA_ACC_INTERFACE  => if (isAnnotation) EmptyFlags else JavaInterface
+      case JAVA_ACC_INTERFACE  => if (isAnnotation) EmptyFlags else PureInterfaceCreationFlags | JavaDefined
       case _                   => EmptyFlags
     }
 

src/dotty/tools/dotc/core/pickling/ClassfileParser.scala

@@ -503,8 +503,9 @@ class ClassfileParser(
           parseExceptions(attrLen)
 
         case tpnme.CodeATTR =>
-          if (sym.owner is Flags.Interface) {
-            sym.setFlag(Flags.DefaultMethod)
+          if (sym.owner is Flags.JavaTrait) {
+            sym.resetFlag(Flags.Deferred)
+            sym.owner.resetFlag(Flags.PureInterface)
             ctx.log(s"$sym in ${sym.owner} is a java8+ default method.")
           }
           in.skip(attrLen)

src/dotty/tools/dotc/core/pickling/PickleBuffer.scala

@@ -220,7 +220,7 @@ object PickleBuffer {
       DEFERRED_PKL -> Deferred,
       FINAL_PKL -> Final,
       METHOD_PKL -> Method,
-      INTERFACE_PKL -> Interface,
+      INTERFACE_PKL -> PureInterface,
       MODULE_PKL -> Module,
       IMPLICIT_PKL -> Implicit,
       SEALED_PKL -> Sealed,

src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -296,7 +296,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
   /** String representation of symbol's definition key word */
   protected def keyString(sym: Symbol): String = {
     val flags = sym.flagsUNSAFE
-    if (flags is JavaInterface) "interface"
+    if (flags is JavaTrait) "interface"
     else if ((flags is Trait) && !(flags is ImplClass)) "trait"
     else if (sym.isClass) "class"
     else if (sym.isType) "type"

src/dotty/tools/dotc/transform/ExplicitOuter.scala

@@ -144,11 +144,11 @@ object ExplicitOuter {
     nme.OUTER.expandedName(cls)
 
   /** Class needs an outer pointer, provided there is a reference to an outer this in it. */
-  def needsOuterIfReferenced(cls: ClassSymbol)(implicit ctx: Context): Boolean = !(
-    cls.isStatic ||
-    cls.owner.enclosingClass.isStaticOwner ||
-    cls.is(Interface)
-  )
+  def needsOuterIfReferenced(cls: ClassSymbol)(implicit ctx: Context): Boolean =
+    !(cls.isStatic ||
+      cls.owner.enclosingClass.isStaticOwner ||
+      cls.is(PureInterface)
+     )
 
   /** Class unconditionally needs an outer pointer. This is the case if
    *  the class needs an outer pointer if referenced and one of the following holds:

src/dotty/tools/dotc/transform/Mixin.scala

@@ -134,7 +134,7 @@ class Mixin extends MiniPhaseTransform with SymTransformer { thisTransform =>
       case Some(call) =>
         if (defn.PhantomClasses.contains(baseCls)) Nil else call :: Nil
       case None =>
-        if (baseCls.is(Interface) || defn.PhantomClasses.contains(baseCls)) Nil
+        if (baseCls.is(NoInitsTrait) || defn.PhantomClasses.contains(baseCls)) Nil
         else {
           //println(i"synth super call ${baseCls.primaryConstructor}: ${baseCls.primaryConstructor.info}")
           superRef(baseCls.primaryConstructor).appliedToNone :: Nil

src/dotty/tools/dotc/typer/Namer.scala

@@ -492,6 +492,13 @@ class Namer { typer: Typer =>
 
       index(rest)(inClassContext(selfInfo))
       denot.info = ClassInfo(cls.owner.thisType, cls, parentRefs, decls, selfInfo)
+      if (cls is Trait) {
+        if (body forall isNoInitMember) {
+          cls.setFlag(NoInits)
+          if (body forall isPureInterfaceMember)
+            cls.setFlag(PureInterface)
+        }
+      }
     }
   }