Emit trait method bodies in statics

And use this as the target of the default methods or
statically resolved super or $init calls.

The call-site change is predicated on `-Yuse-trait-statics`
as a stepping stone for experimentation / bootstrapping.

I have performed this transformation in the backend,
rather than trying to reflect this in the view from
Scala symbols + ASTs.

```
> ;scalac sandbox/test.scala ; scala Test

[info] Running scala.tools.nsc.MainGenericRunner -usejavacp Test
T
C
[success] Total time: 2 s, completed 04/05/2016 11:07:13 AM
> eval "javap -classpath . -c -private C".!!
[info] ans: String = Compiled from "test.scala"
[info] public class C implements T {
[info]   public C();
[info]     Code:
[info]        0: aload_0
[info]        1: invokespecial #14                 // Method java/lang/Object."<init>":()V
[info]        4: aload_0
[info]        5: invokespecial #17                 // Method T.$init$:()V
[info]        8: getstatic     #23                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
[info]       11: ldc           #24                 // String C
[info]       13: invokevirtual #28                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
[info]       16: aload_0
[info]       17: invokespecial #32                 // Method T.foo:()I
[info]       20: pop
[info]       21: return
[info] }
> ;scalac -Yuse-trait-statics sandbox/test.scala ; scala Test

[info] Running scala.tools.nsc.MainGenericRunner -usejavacp Test
T
C
[success] Total time: 2 s, completed 04/05/2016 11:07:39 AM
> eval "javap -classpath . -c -private C".!!
[info] ans: String = Compiled from "test.scala"
[info] public class C implements T {
[info]   public C();
[info]     Code:
[info]        0: aload_0
[info]        1: invokespecial #14                 // Method java/lang/Object."<init>":()V
[info]        4: aload_0
[info]        5: invokestatic  #18                 // Method T.$init$:(LT;)V
[info]        8: getstatic     #24                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
[info]       11: ldc           #25                 // String C
[info]       13: invokevirtual #29                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
[info]       16: aload_0
[info]       17: invokestatic  #33                 // Method T.foo:(LT;)I
[info]       20: pop
[info]       21: return
[info] }
> eval "javap -classpath . -c -private T".!!
[info] ans: String = Compiled from "test.scala"
[info] public interface T {
[info]   public static int foo(T);
[info]     Code:
[info]        0: iconst_0
[info]        1: ireturn
[info]
[info]   public int foo();
[info]     Code:
[info]        0: aload_0
[info]        1: invokestatic  #15                 // Method foo:(LT;)I
[info]        4: ireturn
[info]
[info]   public static void $init$(T);
[info]     Code:
[info]        0: getstatic     #24                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
[info]        3: ldc           #25                 // String T
[info]        5: invokevirtual #29                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
[info]        8: return
[info]
[info] }
```

src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala

@@ -1060,12 +1060,6 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
       receiverClass.info // ensure types the type is up to date; erasure may add lateINTERFACE to traits
       val receiverName = internalName(receiverClass)
 
-      // super calls are only allowed to direct parents
-      if (style.isSuper && receiverClass.isTraitOrInterface && !cnode.interfaces.contains(receiverName)) {
-        thisBType.info.get.inlineInfo.lateInterfaces += receiverName
-        cnode.interfaces.add(receiverName)
-      }
-
       def needsInterfaceCall(sym: Symbol) = {
         sym.isTraitOrInterface ||
           sym.isJavaDefined && sym.isNonBottomSubClass(definitions.ClassfileAnnotationClass)
@@ -1082,7 +1076,11 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
         case Virtual =>
           if (needsInterfaceCall(receiverClass)) bc.invokeinterface(receiverName, jname, mdescr, pos)
           else                               bc.invokevirtual  (receiverName, jname, mdescr, pos)
-        case Super   =>                      bc.invokespecial  (receiverName, jname, mdescr, pos)
+        case Super   =>
+          if (receiverClass.isTraitOrInterface) {
+            val staticDesc = MethodBType(typeToBType(method.owner.info) :: method.info.paramTypes.map(typeToBType), typeToBType(method.info.resultType)).descriptor
+            bc.invokestatic(receiverName, jname, staticDesc, pos)
+          } else bc.invokespecial  (receiverName, jname, mdescr, pos)
       }
 
       bmType.returnType

src/compiler/scala/tools/nsc/backend/jvm/BCodeHelpers.scala

@@ -11,6 +11,7 @@ import scala.tools.asm
 import scala.tools.nsc.io.AbstractFile
 import GenBCode._
 import BackendReporting._
+import scala.reflect.internal.Flags
 
 /*
  *  Traits encapsulating functionality to convert Scala AST Trees into ASM ClassNodes.
@@ -49,6 +50,9 @@ abstract class BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
     }
   }
 
+  def isTraitMethodRequiringStaticImpl(sym: Symbol) =
+    sym.hasAttachment[global.mixer.NeedStaticImpl.type]
+
   /**
    * True if `classSym` is an anonymous class or a local class. I.e., false if `classSym` is a
    * member class. This method is used to decide if we should emit an EnclosingMethod attribute.
@@ -230,58 +234,6 @@ abstract class BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
       sym.isErroneous
     }
 
-  /**
-   * Build the [[InlineInfo]] for a class symbol.
-   */
-  def buildInlineInfoFromClassSymbol(classSym: Symbol, classSymToInternalName: Symbol => InternalName, methodSymToDescriptor: Symbol => String): InlineInfo = {
-    val isEffectivelyFinal = classSym.isEffectivelyFinal
-
-    val sam = {
-      if (classSym.isEffectivelyFinal) None
-      else {
-        // Phase travel necessary. For example, nullary methods (getter of an abstract val) get an
-        // empty parameter list in later phases and would therefore be picked as SAM.
-        val samSym = exitingPickler(definitions.samOf(classSym.tpe))
-        if (samSym == NoSymbol) None
-        else Some(samSym.javaSimpleName.toString + methodSymToDescriptor(samSym))
-      }
-    }
-
-    var warning = Option.empty[ClassSymbolInfoFailureSI9111]
-
-    // Primitive methods cannot be inlined, so there's no point in building a MethodInlineInfo. Also, some
-    // primitive methods (e.g., `isInstanceOf`) have non-erased types, which confuses [[typeToBType]].
-    val methodInlineInfos = classSym.info.decls.iterator.filter(m => m.isMethod && !scalaPrimitives.isPrimitive(m)).flatMap({
-      case methodSym =>
-        if (completeSilentlyAndCheckErroneous(methodSym)) {
-          // Happens due to SI-9111. Just don't provide any MethodInlineInfo for that method, we don't need fail the compiler.
-          if (!classSym.isJavaDefined) devWarning("SI-9111 should only be possible for Java classes")
-          warning = Some(ClassSymbolInfoFailureSI9111(classSym.fullName))
-          None
-        } else {
-          val name      = methodSym.javaSimpleName.toString // same as in genDefDef
-          val signature = name + methodSymToDescriptor(methodSym)
-
-          // In `trait T { object O }`, `oSym.isEffectivelyFinalOrNotOverridden` is true, but the
-          // method is abstract in bytecode, `defDef.rhs.isEmpty`. Abstract methods are excluded
-          // so they are not marked final in the InlineInfo attribute.
-          //
-          // However, due to https://github.com/scala/scala-dev/issues/126, this currently does not
-          // work, the abstract accessor for O will be marked effectivelyFinal.
-          val effectivelyFinal = methodSym.isEffectivelyFinalOrNotOverridden && !methodSym.isDeferred
-
-          val info = MethodInlineInfo(
-            effectivelyFinal                    = effectivelyFinal,
-            annotatedInline                     = methodSym.hasAnnotation(ScalaInlineClass),
-            annotatedNoInline                   = methodSym.hasAnnotation(ScalaNoInlineClass)
-          )
-          Some((signature, info))
-        }
-    }).toMap
-
-    InlineInfo(isEffectivelyFinal, sam, methodInlineInfos, warning)
-  }
-
   /*
    * must-single-thread
    */

src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala

@@ -488,7 +488,24 @@ abstract class BCodeSkelBuilder extends BCodeHelpers {
 
         case ValDef(mods, name, tpt, rhs) => () // fields are added in `genPlainClass()`, via `addClassFields()`
 
-        case dd : DefDef => genDefDef(dd)
+        case dd : DefDef =>
+          val sym = dd.symbol
+          if (isTraitMethodRequiringStaticImpl(sym)) {
+            // Split concrete methods in traits (including mixin constructors) into a static method
+            // with an explicit this parameter, and a non-static forwarder method.
+            val staticDefDef = global.gen.mkStatic(dd, _.cloneSymbol)
+            val forwarderDefDef = {
+              val forwarderBody = Apply(global.gen.mkAttributedRef(staticDefDef.symbol), This(sym.owner).setType(sym.owner.typeConstructor) :: dd.vparamss.head.map(p => global.gen.mkAttributedIdent(p.symbol))).setType(sym.info.resultType)
+              // we don't want to the optimizer to inline the static method into the forwarder. Instead,
+              // the backend has a special case to transitively inline into a callsite of the forwarder
+              // when the forwarder itself is inlined.
+              forwarderBody.updateAttachment(NoInlineCallsiteAttachment)
+              deriveDefDef(dd)(_ => global.atPos(dd.pos)(forwarderBody))
+            }
+            genDefDef(staticDefDef)
+            if (!sym.isMixinConstructor)
+              genDefDef(forwarderDefDef)
+          } else genDefDef(dd)
 
         case Template(_, _, body) => body foreach gen
 

src/compiler/scala/tools/nsc/backend/jvm/BTypes.scala

@@ -225,8 +225,7 @@ abstract class BTypes {
 
     val inlineInfo = inlineInfoFromClassfile(classNode)
 
-    val classfileInterfaces: List[ClassBType] = classNode.interfaces.asScala.map(classBTypeFromParsedClassfile)(collection.breakOut)
-    val interfaces = classfileInterfaces.filterNot(i => inlineInfo.lateInterfaces.contains(i.internalName))
+    val interfaces: List[ClassBType] = classNode.interfaces.asScala.map(classBTypeFromParsedClassfile)(collection.breakOut)
 
     classBType.info = Right(ClassInfo(superClass, interfaces, flags, nestedClasses, nestedInfo, inlineInfo))
     classBType
@@ -1147,25 +1146,6 @@ object BTypes {
                               sam: Option[String],
                               methodInfos: Map[String, MethodInlineInfo],
                               warning: Option[ClassInlineInfoWarning]) {
-    /**
-     * A super call (invokespecial) to a default method T.m is only allowed if the interface T is
-     * a direct parent of the class. Super calls are introduced for example in Mixin when generating
-     * forwarder methods:
-     *
-     *   trait T { override def clone(): Object = "hi" }
-     *   trait U extends T
-     *   class C extends U
-     *
-     * The class C gets a forwarder that invokes T.clone(). During code generation the interface T
-     * is added as direct parent to class C. Note that T is not a (direct) parent in the frontend
-     * type of class C.
-     *
-     * All interfaces that are added to a class during code generation are added to this buffer and
-     * stored in the InlineInfo classfile attribute. This ensures that the ClassBTypes for a
-     * specific class is the same no matter if it's constructed from a Symbol or from a classfile.
-     * This is tested in BTypesFromClassfileTest.
-     */
-    val lateInterfaces: ListBuffer[InternalName] = ListBuffer.empty
   }
 
   val EmptyInlineInfo = InlineInfo(false, None, Map.empty, None)

src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala

@@ -509,7 +509,7 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
    * classfile attribute.
    */
   private def buildInlineInfo(classSym: Symbol, internalName: InternalName): InlineInfo = {
-    def buildFromSymbol = buildInlineInfoFromClassSymbol(classSym, classBTypeFromSymbol(_).internalName, methodBTypeFromSymbol(_).descriptor)
+    def buildFromSymbol = buildInlineInfoFromClassSymbol(classSym)
 
     // phase travel required, see implementation of `compiles`. for nested classes, it checks if the
     // enclosingTopLevelClass is being compiled. after flatten, all classes are considered top-level,
@@ -530,6 +530,73 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
     }
   }
 
+  /**
+   * Build the [[InlineInfo]] for a class symbol.
+   */
+  def buildInlineInfoFromClassSymbol(classSym: Symbol): InlineInfo = {
+    val isEffectivelyFinal = classSym.isEffectivelyFinal
+
+    val sam = {
+      if (classSym.isEffectivelyFinal) None
+      else {
+        // Phase travel necessary. For example, nullary methods (getter of an abstract val) get an
+        // empty parameter list in later phases and would therefore be picked as SAM.
+        val samSym = exitingPickler(definitions.samOf(classSym.tpe))
+        if (samSym == NoSymbol) None
+        else Some(samSym.javaSimpleName.toString + methodBTypeFromSymbol(samSym).descriptor)
+      }
+    }
+
+    var warning = Option.empty[ClassSymbolInfoFailureSI9111]
+
+    // Primitive methods cannot be inlined, so there's no point in building a MethodInlineInfo. Also, some
+    // primitive methods (e.g., `isInstanceOf`) have non-erased types, which confuses [[typeToBType]].
+    val methodInlineInfos = classSym.info.decls.iterator.filter(m => m.isMethod && !scalaPrimitives.isPrimitive(m)).flatMap({
+      case methodSym =>
+        if (completeSilentlyAndCheckErroneous(methodSym)) {
+          // Happens due to SI-9111. Just don't provide any MethodInlineInfo for that method, we don't need fail the compiler.
+          if (!classSym.isJavaDefined) devWarning("SI-9111 should only be possible for Java classes")
+          warning = Some(ClassSymbolInfoFailureSI9111(classSym.fullName))
+          Nil
+        } else {
+          val name      = methodSym.javaSimpleName.toString // same as in genDefDef
+          val signature = name + methodBTypeFromSymbol(methodSym).descriptor
+
+          // In `trait T { object O }`, `oSym.isEffectivelyFinalOrNotOverridden` is true, but the
+          // method is abstract in bytecode, `defDef.rhs.isEmpty`. Abstract methods are excluded
+          // so they are not marked final in the InlineInfo attribute.
+          //
+          // However, due to https://github.com/scala/scala-dev/issues/126, this currently does not
+          // work, the abstract accessor for O will be marked effectivelyFinal.
+          val effectivelyFinal = methodSym.isEffectivelyFinalOrNotOverridden && !methodSym.isDeferred
+
+          val info = MethodInlineInfo(
+            effectivelyFinal  = effectivelyFinal,
+            annotatedInline   = methodSym.hasAnnotation(ScalaInlineClass),
+            annotatedNoInline = methodSym.hasAnnotation(ScalaNoInlineClass))
+
+          if (isTraitMethodRequiringStaticImpl(methodSym)) {
+            val selfParam = methodSym.newSyntheticValueParam(methodSym.owner.typeConstructor, nme.SELF)
+            val staticMethodType = methodSym.info match {
+              case mt @ MethodType(params, res) => copyMethodType(mt, selfParam :: params, res)
+            }
+            val staticMethodSignature = name + methodBTypeFromMethodType(staticMethodType, isConstructor = false)
+            val staticMethodInfo = MethodInlineInfo(
+              effectivelyFinal  = true,
+              annotatedInline   = info.annotatedInline,
+              annotatedNoInline = info.annotatedNoInline)
+            if (methodSym.isMixinConstructor)
+              List((staticMethodSignature, staticMethodInfo))
+            else
+              List((signature, info), (staticMethodSignature, staticMethodInfo))
+          } else
+            List((signature, info))
+        }
+    }).toMap
+
+    InlineInfo(isEffectivelyFinal, sam, methodInlineInfos, warning)
+  }
+
   /**
    * For top-level objects without a companion class, the compiler generates a mirror class with
    * static forwarders (Java compat). There's no symbol for the mirror class, but we still need a

src/compiler/scala/tools/nsc/backend/jvm/opt/BytecodeUtils.scala

@@ -93,6 +93,15 @@ object BytecodeUtils {
     op == INVOKESPECIAL || op == INVOKESTATIC
   }
 
+  def isVirtualCall(instruction: AbstractInsnNode): Boolean = {
+    val op = instruction.getOpcode
+    op == INVOKEVIRTUAL || op == INVOKEINTERFACE
+  }
+
+  def isCall(instruction: AbstractInsnNode): Boolean = {
+    isNonVirtualCall(instruction) || isVirtualCall(instruction)
+  }
+
   def isExecutable(instruction: AbstractInsnNode): Boolean = instruction.getOpcode >= 0
 
   def isConstructor(methodNode: MethodNode): Boolean = {

src/compiler/scala/tools/nsc/backend/jvm/opt/CallGraph.scala

@@ -9,14 +9,14 @@ package opt
 
 import scala.collection.immutable.IntMap
 import scala.reflect.internal.util.{NoPosition, Position}
-import scala.tools.asm.{Opcodes, Type, Handle}
+import scala.tools.asm.{Handle, Opcodes, Type}
 import scala.tools.asm.tree._
 import scala.collection.{concurrent, mutable}
 import scala.collection.JavaConverters._
-import scala.tools.nsc.backend.jvm.BTypes.InternalName
+import scala.tools.nsc.backend.jvm.BTypes.{InternalName, MethodInlineInfo}
 import scala.tools.nsc.backend.jvm.BackendReporting._
 import scala.tools.nsc.backend.jvm.analysis._
-import ByteCodeRepository.{Source, CompilationUnit}
+import ByteCodeRepository.{CompilationUnit, Source}
 import BytecodeUtils._
 
 class CallGraph[BT <: BTypes](val btypes: BT) {
@@ -68,6 +68,7 @@ class CallGraph[BT <: BTypes](val btypes: BT) {
   }
 
   def containsCallsite(callsite: Callsite): Boolean = callsites(callsite.callsiteMethod) contains callsite.callsiteInstruction
+  def findCallSite(method: MethodNode, call: MethodInsnNode): Option[Callsite] = callsites.getOrElse(method, Map.empty).get(call)
 
   def removeClosureInstantiation(indy: InvokeDynamicInsnNode, methodNode: MethodNode): Option[ClosureInstantiation] = {
     val methodClosureInits = closureInstantiations(methodNode)
@@ -359,7 +360,7 @@ class CallGraph[BT <: BTypes](val btypes: BT) {
       "Invocation of" +
         s" ${callee.map(_.calleeDeclarationClass.internalName).getOrElse("?")}.${callsiteInstruction.name + callsiteInstruction.desc}" +
         s"@${callsiteMethod.instructions.indexOf(callsiteInstruction)}" +
-        s" in ${callsiteClass.internalName}.${callsiteMethod.name}"
+        s" in ${callsiteClass.internalName}.${callsiteMethod.name}${callsiteMethod.desc}"
   }
 
   final case class ClonedCallsite(callsite: Callsite, clonedWhenInlining: Callsite)
@@ -394,6 +395,7 @@ class CallGraph[BT <: BTypes](val btypes: BT) {
                           samParamTypes: IntMap[btypes.ClassBType],
                           calleeInfoWarning: Option[CalleeInfoWarning]) {
     override def toString = s"Callee($calleeDeclarationClass.${callee.name})"
+    def textifyCallee = AsmUtils.textify(callee)
   }
 
   /**

src/compiler/scala/tools/nsc/backend/jvm/opt/InlineInfoAttribute.scala

@@ -51,7 +51,6 @@ case class InlineInfoAttribute(inlineInfo: InlineInfo) extends Attribute(InlineI
     if (inlineInfo.isEffectivelyFinal)      flags |= 1
     //                                      flags |= 2 // no longer written
     if (inlineInfo.sam.isDefined)           flags |= 4
-    if (inlineInfo.lateInterfaces.nonEmpty) flags |= 8
     result.putByte(flags)
 
     for (samNameDesc <- inlineInfo.sam) {
@@ -79,9 +78,6 @@ case class InlineInfoAttribute(inlineInfo: InlineInfo) extends Attribute(InlineI
       result.putByte(inlineInfo)
     }
 
-    result.putShort(inlineInfo.lateInterfaces.length)
-    for (i <- inlineInfo.lateInterfaces) result.putShort(cw.newUTF8(i))
-
     result
   }
 
@@ -105,7 +101,6 @@ case class InlineInfoAttribute(inlineInfo: InlineInfo) extends Attribute(InlineI
       val isFinal           = (flags & 1) != 0
       val hasSelf           = (flags & 2) != 0
       val hasSam            = (flags & 4) != 0
-      val hasLateInterfaces = (flags & 8) != 0
 
       if (hasSelf) nextUTF8() // no longer used
 
@@ -128,13 +123,7 @@ case class InlineInfoAttribute(inlineInfo: InlineInfo) extends Attribute(InlineI
         (name + desc, MethodInlineInfo(isFinal, isInline, isNoInline))
       }).toMap
 
-      val lateInterfaces = if (!hasLateInterfaces) Nil else {
-        val numLateInterfaces = nextShort()
-        (0 until numLateInterfaces).map(_ => nextUTF8())
-      }
-
       val info = InlineInfo(isFinal, sam, infos, None)
-      info.lateInterfaces ++= lateInterfaces
       InlineInfoAttribute(info)
     } else {
       val msg = UnknownScalaInlineInfoVersion(cr.getClassName, version)
@@ -161,8 +150,6 @@ object InlineInfoAttribute {
    *   [u2]  name (reference)
    *   [u2]  descriptor (reference)
    *   [u1]  isFinal (<< 0), traitMethodWithStaticImplementation (<< 1), hasInlineAnnotation (<< 2), hasNoInlineAnnotation (<< 3)
-   * [u2]?   numLateInterfaces
-   *   [u2]  lateInterface (reference)
    */
   final val VERSION: Byte = 1