New trait encoding: use default methods, jettison impl classes

Until now, concrete methods in traits were encoded with
"trait implementation classes".

  - Such a trait would compile to two class files
    - the trait interface, a Java interface, and
    - the implementation class, containing "trait implementation methods"
  - trait implementation methods are static methods has an explicit self
    parameter.
  - some methods don't require addition of an interface method, such as
    private methods. Calls to these directly call the implementation method
  - classes that mixin a trait install "trait forwarders", which implement
    the abstract method in the interface by forwarding to the trait
    implementation method.

The new encoding:
  - no longer emits trait implementation classes or trait implementation
    methods.
  - instead, concrete methods are simply retained in the interface, as JVM 8
    default interface methods (the JVM spec changes in
    [JSR-335](http://download.oracle.com/otndocs/jcp/lambda-0_9_3-fr-eval-spec/index.html)
    pave the way)
  - use `invokespecial` to call private or particular super implementations
    of a method (rather `invokestatic`)
    - in cases when we `invokespecial` to a method in an indirect ancestor, we add
    that ancestor redundantly as a direct parent. We are investigating alternatives
    approaches here.
  - we still emit trait fowrarders, although we are
    [investigating](https://github.com/scala/scala-dev/issues/98) ways to only do
    this when the JVM would be unable to resolve the correct method using its rules
    for default method resolution.

Here's an example:

```
trait T {
  println("T")
  def m1 = m2
  private def m2 = "m2"
}
trait U extends T {
  println("T")
  override def m1 = super[T].m1
}
class C extends U {
  println("C")
  def test = m1
}
```

The old and new encodings are displayed and diffed here: https://gist.github.com/retronym/f174d23f859f0e053580

Some notes in the implementation:

  - No need to filter members from class decls at all in AddInterfaces
    (although we do have to trigger side effecting info transformers)
  - We can now emit an EnclosingMethod attribute for classes nested
    in private trait methods
  - Created a factory method for an AST shape that is used in
    a number of places to symbolically bind to a particular
    super method without needed to specify the qualifier of
    the `Super` tree (which is too limiting, as it only allows
    you to refer to direct parents.)
    - I also found a similar tree shape created in Delambdafy,
      that is better expressed with an existing tree creation
      factory method, mkSuperInit.

1 files changed, 5 insertions, 10 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala b/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
index 3c2ee89b05..85563be428 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
@@ -172,7 +172,6 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
      */
     def primitiveOrClassToBType(sym: Symbol): BType = {
       assertClassNotArray(sym)
-      assert(!sym.isImplClass, sym)
       primitiveTypeToBType.getOrElse(sym, classBTypeFromSymbol(sym))
     }
 
@@ -337,7 +336,7 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
     // Check for hasAnnotationFlag for SI-9393: the classfile / java source parsers add
     // scala.annotation.Annotation as superclass to java annotations. In reality, java
     // annotation classfiles have superclass Object (like any interface classfile).
-    val superClassSym = if (classSym.isImplClass || classSym.hasJavaAnnotationFlag) ObjectClass else {
+    val superClassSym = if (classSym.hasJavaAnnotationFlag) ObjectClass else {
       val sc = classSym.superClass
       // SI-9393: Java annotation classes don't have the ABSTRACT/INTERFACE flag, so they appear
       // (wrongly) as superclasses. Fix this for BTypes: the java annotation will appear as interface
@@ -603,11 +602,7 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
    */
   final def isTopLevelModuleClass(sym: Symbol): Boolean = exitingPickler {
     // phase travel to pickler required for isNestedClass (looks at owner)
-    val r = sym.isModuleClass && !sym.isNestedClass
-    // The mixin phase adds the `lateMODULE` flag to trait implementation classes. Since the flag
-    // is late, it should not be visible here inside the time travel. We check this.
-    if (r) assert(!sym.isImplClass, s"isModuleClass should be false for impl class $sym")
-    r
+    sym.isModuleClass && !sym.isNestedClass
   }
 
   /**
@@ -684,7 +679,7 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
 
     val finalFlag = (
       (((sym.rawflags & symtab.Flags.FINAL) != 0) || isTopLevelModuleClass(sym))
-        && !sym.enclClass.isInterface
+        && !sym.enclClass.isTrait
         && !sym.isClassConstructor
         && !sym.isMutable // lazy vals and vars both
       )
@@ -697,12 +692,12 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
     GenBCode.mkFlags(
       if (privateFlag) ACC_PRIVATE else ACC_PUBLIC,
       if ((sym.isDeferred && !sym.hasFlag(symtab.Flags.JAVA_DEFAULTMETHOD))|| sym.hasAbstractFlag) ACC_ABSTRACT else 0,
-      if (sym.isInterface) ACC_INTERFACE else 0,
+      if (sym.isTraitOrInterface) ACC_INTERFACE else 0,
       if (finalFlag && !sym.hasAbstractFlag) ACC_FINAL else 0,
       if (sym.isStaticMember) ACC_STATIC else 0,
       if (sym.isBridge) ACC_BRIDGE | ACC_SYNTHETIC else 0,
       if (sym.isArtifact) ACC_SYNTHETIC else 0,
-      if (sym.isClass && !sym.isInterface) ACC_SUPER else 0,
+      if (sym.isClass && !sym.isTraitOrInterface) ACC_SUPER else 0,
       if (sym.hasJavaEnumFlag) ACC_ENUM else 0,
       if (sym.isVarargsMethod) ACC_VARARGS else 0,
       if (sym.hasFlag(symtab.Flags.SYNCHRONIZED)) ACC_SYNCHRONIZED else 0,