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.

We also need to add an restriction related to invokespecial to Java
parents: to support a super call to one of these to implement a
super accessor, the interface must be listed as a direct parent
of the class.

The static method names has a trailing $ added to avoid duplicate
name and signature errors in classfiles.

Author: retronym

src/compiler/scala/tools/nsc/ast/TreeGen.scala

@@ -336,12 +336,13 @@ abstract class TreeGen extends scala.reflect.internal.TreeGen with TreeDSL {
     *       - are associating the RHS with a cloned symbol, but intend for the original
     *         method to remain and for recursive calls to target it.
     */
-  final def mkStatic(orig: DefDef, maybeClone: Symbol => Symbol): DefDef = {
+  final def mkStatic(orig: DefDef, newName: Name, maybeClone: Symbol => Symbol): DefDef = {
     assert(phase.erasedTypes, phase)
     assert(!orig.symbol.hasFlag(SYNCHRONIZED), orig.symbol.defString)
     val origSym = orig.symbol
     val origParams = orig.symbol.info.params
     val newSym = maybeClone(orig.symbol)
+    newSym.setName(newName)
     newSym.setFlag(STATIC)
     // Add an explicit self parameter
     val selfParamSym = newSym.newSyntheticValueParam(newSym.owner.typeConstructor, nme.SELF).setFlag(ARTIFACT)

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

@@ -11,10 +11,10 @@ package jvm
 
 import scala.annotation.switch
 import scala.reflect.internal.Flags
-
 import scala.tools.asm
 import GenBCode._
 import BackendReporting._
+import scala.tools.asm.Opcodes
 import scala.tools.asm.tree.MethodInsnNode
 import scala.tools.nsc.backend.jvm.BCodeHelpers.{InvokeStyle, TestOp}
 
@@ -637,15 +637,15 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
           val nativeKind = tpeTK(expr)
           genLoad(expr, nativeKind)
           val MethodNameAndType(mname, methodType) = srBoxesRuntimeBoxToMethods(nativeKind)
-          bc.invokestatic(srBoxesRunTimeRef.internalName, mname, methodType.descriptor, app.pos)
+          bc.invokestatic(srBoxesRunTimeRef.internalName, mname, methodType.descriptor, itf = false, app.pos)
           generatedType = boxResultType(fun.symbol)
 
         case Apply(fun, List(expr)) if currentRun.runDefinitions.isUnbox(fun.symbol) =>
           genLoad(expr)
           val boxType = unboxResultType(fun.symbol)
           generatedType = boxType
           val MethodNameAndType(mname, methodType) = srBoxesRuntimeUnboxToMethods(boxType)
-          bc.invokestatic(srBoxesRunTimeRef.internalName, mname, methodType.descriptor, app.pos)
+          bc.invokestatic(srBoxesRunTimeRef.internalName, mname, methodType.descriptor, itf = false, app.pos)
 
         case app @ Apply(fun, args) =>
           val sym = fun.symbol
@@ -1058,31 +1058,40 @@ 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)
-      }
+      val receiverBType = classBTypeFromSymbol(receiverClass)
+      val receiverName = receiverBType.internalName
 
       def needsInterfaceCall(sym: Symbol) = {
         sym.isTraitOrInterface ||
           sym.isJavaDefined && sym.isNonBottomSubClass(definitions.ClassfileAnnotationClass)
       }
 
-      val jname    = method.javaSimpleName.toString
-      val bmType   = methodBTypeFromSymbol(method)
-      val mdescr   = bmType.descriptor
+      val jname  = method.javaSimpleName.toString
+      val bmType = methodBTypeFromSymbol(method)
+      val mdescr = bmType.descriptor
 
+      val isInterface = receiverBType.isInterface.get
       import InvokeStyle._
-      style match {
-        case Static  =>                      bc.invokestatic   (receiverName, jname, mdescr, pos)
-        case Special =>                      bc.invokespecial  (receiverName, jname, mdescr, pos)
-        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)
+      if (style == Super) {
+        assert(receiverClass == methodOwner, s"for super call, expecting $receiverClass == $methodOwner")
+        if (receiverClass.isTrait && !receiverClass.isJavaDefined) {
+          val staticDesc = MethodBType(typeToBType(method.owner.info) :: bmType.argumentTypes, bmType.returnType).descriptor
+          val staticName = traitImplMethodName(method).toString
+          bc.invokestatic(receiverName, staticName, staticDesc, isInterface, pos)
+        } else {
+          if (receiverClass.isTraitOrInterface) {
+            // An earlier check in Mixin reports an error in this case, so it doesn't reach the backend
+            assert(cnode.interfaces.contains(receiverName), s"cannot invokespecial $receiverName.$jname, the interface is not a direct parent.")
+          }
+          bc.invokespecial(receiverName, jname, mdescr, isInterface, pos)
+        }
+      } else {
+        val opc = style match {
+          case Static => Opcodes.INVOKESTATIC
+          case Special => Opcodes.INVOKESPECIAL
+          case Virtual => if (isInterface) Opcodes.INVOKEINTERFACE else Opcodes.INVOKEVIRTUAL
+        }
+        bc.emitInvoke(opc, receiverName, jname, mdescr, isInterface, 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 +50,14 @@ abstract class BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
     }
   }
 
+  def needsStaticImplMethod(sym: Symbol) = sym.hasAttachment[global.mixer.NeedStaticImpl.type]
+
+  final def traitImplMethodName(sym: Symbol): Name = {
+    val name = sym.javaSimpleName
+    if (sym.isMixinConstructor) name
+    else name.append(nme.NAME_JOIN_STRING)
+  }
+
   /**
    * 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 +239,6 @@ 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
    */
@@ -568,15 +525,7 @@ abstract class BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
     /**
      * The class internal name for a given class symbol.
      */
-    final def internalName(sym: Symbol): String = {
-      // For each java class, the scala compiler creates a class and a module (thus a module class).
-      // If the `sym` is a java module class, we use the java class instead. This ensures that the
-      // ClassBType is created from the main class (instead of the module class).
-      // The two symbols have the same name, so the resulting internalName is the same.
-      // Phase travel (exitingPickler) required for SI-6613 - linkedCoC is only reliable in early phases (nesting)
-      val classSym = if (sym.isJavaDefined && sym.isModuleClass) exitingPickler(sym.linkedClassOfClass) else sym
-      classBTypeFromSymbol(classSym).internalName
-    }
+    final def internalName(sym: Symbol): String = classBTypeFromSymbol(sym).internalName
   } // end of trait BCInnerClassGen
 
   trait BCAnnotGen extends BCInnerClassGen {

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

@@ -190,6 +190,7 @@ abstract class BCodeIdiomatic extends SubComponent {
         JavaStringBuilderClassName,
         INSTANCE_CONSTRUCTOR_NAME,
         "()V",
+        itf = false,
         pos
       )
     }
@@ -373,30 +374,27 @@ abstract class BCodeIdiomatic extends SubComponent {
     final def rem(tk: BType) { emitPrimitive(JCodeMethodN.remOpcodes, tk) } // can-multi-thread
 
     // can-multi-thread
-    final def invokespecial(owner: String, name: String, desc: String, pos: Position) {
-      addInvoke(Opcodes.INVOKESPECIAL, owner, name, desc, false, pos)
+    final def invokespecial(owner: String, name: String, desc: String, itf: Boolean, pos: Position): Unit = {
+      emitInvoke(Opcodes.INVOKESPECIAL, owner, name, desc, itf, pos)
     }
     // can-multi-thread
-    final def invokestatic(owner: String, name: String, desc: String, pos: Position) {
-      addInvoke(Opcodes.INVOKESTATIC, owner, name, desc, false, pos)
+    final def invokestatic(owner: String, name: String, desc: String, itf: Boolean, pos: Position): Unit = {
+      emitInvoke(Opcodes.INVOKESTATIC, owner, name, desc, itf, pos)
     }
     // can-multi-thread
-    final def invokeinterface(owner: String, name: String, desc: String, pos: Position) {
-      addInvoke(Opcodes.INVOKEINTERFACE, owner, name, desc, true, pos)
+    final def invokeinterface(owner: String, name: String, desc: String, pos: Position): Unit = {
+      emitInvoke(Opcodes.INVOKEINTERFACE, owner, name, desc, itf = true, pos)
     }
     // can-multi-thread
-    final def invokevirtual(owner: String, name: String, desc: String, pos: Position) {
-      addInvoke(Opcodes.INVOKEVIRTUAL, owner, name, desc, false, pos)
+    final def invokevirtual(owner: String, name: String, desc: String, pos: Position): Unit = {
+      emitInvoke(Opcodes.INVOKEVIRTUAL, owner, name, desc, itf = false, pos)
     }
 
-    private def addInvoke(opcode: Int, owner: String, name: String, desc: String, itf: Boolean, pos: Position) = {
+    def emitInvoke(opcode: Int, owner: String, name: String, desc: String, itf: Boolean, pos: Position): Unit = {
       val node = new MethodInsnNode(opcode, owner, name, desc, itf)
       jmethod.instructions.add(node)
       if (settings.optInlinerEnabled) callsitePositions(node) = pos
     }
-    final def invokedynamic(owner: String, name: String, desc: String) {
-      jmethod.visitMethodInsn(Opcodes.INVOKEDYNAMIC, owner, name, desc)
-    }
 
     // can-multi-thread
     final def goTo(label: asm.Label) { jmethod.visitJumpInsn(Opcodes.GOTO, label) }

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

@@ -488,7 +488,22 @@ 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 (needsStaticImplMethod(sym)) {
+            val staticDefDef = global.gen.mkStatic(dd, traitImplMethodName(sym), _.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)