Desugar module var and accessor in refchecks/lazyvals

Rather than leaving it until mixin.

The broader motivation is to simplify the mixin phase of the
compiler before we get rid of implementatation classes in
favour of using JDK8 default interface methods.

The current code in mixin is used for both lazy val and modules,
and puts the "slow path" code that uses the monitor into a
dedicated method (`moduleName$lzyCompute`). I tracked this
back to a3d4d17. I can't tell from that commit whether the
performance sensititivity was related to modules or lazy vals,
from the commit message I'd say the latter.

As the initialization code for a module is just a constructor call,
rather than an arbitraryly large chunk of code for a lazy initializer,
this commit opts to inline the `lzycompute` method.

During refchecks, mixin module accessors are added to classes, so
that mixed in and defined modules are translated uniformly.

Defer synthesis of the double checked locking idiom to the lazyvals
phase, which gets us a step closer to a unified translation of
modules and lazy vals.

I had to change the `atOwner` methods to to avoid using the
non-existent module class of a module accessor method as the
current owner. This fixes a latent bug. Without this change,
retypechecking of the module accessor method during erasure crashes
with an accessibility error selecting the module var.

src/compiler/scala/tools/nsc/transform/LazyVals.scala

@@ -54,6 +54,12 @@ abstract class LazyVals extends Transform with TypingTransformers with ast.TreeD
     private val lazyVals = perRunCaches.newMap[Symbol, Int]() withDefaultValue 0
 
     import symtab.Flags._
+    private def flattenThickets(stats: List[Tree]): List[Tree] = stats.flatMap(_ match {
+      case b @ Block(List(d1@DefDef(_, n1, _, _, _, _)), d2@DefDef(_, n2, _, _, _, _)) if b.tpe == null && n1.endsWith(nme.LAZY_SLOW_SUFFIX) =>
+        List(d1, d2)
+      case stat =>
+        List(stat)
+    })
 
     /** Perform the following transformations:
      *  - for a lazy accessor inside a method, make it check the initialization bitmap
@@ -72,20 +78,14 @@ abstract class LazyVals extends Transform with TypingTransformers with ast.TreeD
         case Block(_, _) =>
           val block1 = super.transform(tree)
           val Block(stats, expr) = block1
-          val stats1 = stats.flatMap(_ match {
-            case Block(List(d1@DefDef(_, n1, _, _, _, _)), d2@DefDef(_, n2, _, _, _, _)) if (nme.newLazyValSlowComputeName(n2) == n1) =>
-              List(d1, d2)
-            case stat =>
-              List(stat)
-          })
-          treeCopy.Block(block1, stats1, expr)
+          treeCopy.Block(block1, flattenThickets(stats), expr)
 
         case DefDef(_, _, _, _, _, rhs) => atOwner(tree.symbol) {
           val (res, slowPathDef) = if (!sym.owner.isClass && sym.isLazy) {
             val enclosingClassOrDummyOrMethod = {
               val enclMethod = sym.enclMethod
 
-              if (enclMethod != NoSymbol ) {
+              if (enclMethod != NoSymbol) {
                 val enclClass = sym.enclClass
                 if (enclClass != NoSymbol && enclMethod == enclClass.enclMethod)
                   enclClass
@@ -100,11 +100,26 @@ abstract class LazyVals extends Transform with TypingTransformers with ast.TreeD
             val (rhs1, sDef) = mkLazyDef(enclosingClassOrDummyOrMethod, transform(rhs), idx, sym)
             sym.resetFlag((if (lazyUnit(sym)) 0 else LAZY) | ACCESSOR)
             (rhs1, sDef)
-          } else
+          } else if (!sym.owner.isTrait && sym.isModule && sym.isMethod) {
+            rhs match {
+              case b @ Block((assign @ Assign(lhs, rhs)) :: Nil, expr) =>
+                val cond = Apply(Select(lhs, Object_eq), List(Literal(Constant(null))))
+                val moduleDefs = mkFastPathBody(sym.owner.enclClass, lhs.symbol, cond, transform(assign) :: Nil, Nil, transform(expr))
+                (localTyper.typedPos(tree.pos)(moduleDefs._1), localTyper.typedPos(tree.pos)(moduleDefs._2))
+              case rhs =>
+                global.reporter.error(tree.pos, "Unexpected tree on the RHS of a module accessor: " + rhs)
+                (rhs, EmptyTree)
+            }
+          } else {
             (transform(rhs), EmptyTree)
+          }
 
           val ddef1 = deriveDefDef(tree)(_ => if (LocalLazyValFinder.find(res)) typed(addBitmapDefs(sym, res)) else res)
-          if (slowPathDef != EmptyTree) Block(slowPathDef, ddef1) else ddef1
+          if (slowPathDef != EmptyTree) {
+            // The contents of this block are flattened into the enclosing statement sequence, see flattenThickets
+            // This is a poor man's version of dotty's Thicket: https://github.com/lampepfl/dotty/blob/d5280358d1/src/dotty/tools/dotc/ast/Trees.scala#L707
+            Block(slowPathDef, ddef1)
+          } else ddef1
         }
 
         case Template(_, _, body) => atOwner(currentOwner) {
@@ -135,7 +150,7 @@ abstract class LazyVals extends Transform with TypingTransformers with ast.TreeD
                 })
                 toAdd0
             } else List()
-          deriveTemplate(tree)(_ => innerClassBitmaps ++ stats)
+          deriveTemplate(tree)(_ => innerClassBitmaps ++ flattenThickets(stats))
         }
 
         case ValDef(_, _, _, _) if !sym.owner.isModule && !sym.owner.isClass =>

src/compiler/scala/tools/nsc/transform/Mixin.scala

@@ -861,16 +861,6 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
         typedPos(init.head.pos)(mkFastPathLazyBody(clazz, lzyVal, cond, syncBody, nulls, retVal))
       }
 
-      def mkInnerClassAccessorDoubleChecked(attrThis: Tree, rhs: Tree, moduleSym: Symbol, args: List[Tree]): Tree =
-        rhs match {
-          case Block(List(assign), returnTree) =>
-            val Assign(moduleVarRef, _) = assign
-            val cond                    = Apply(Select(moduleVarRef, Object_eq), List(NULL))
-            mkFastPathBody(clazz, moduleSym, cond, List(assign), List(NULL), returnTree, attrThis, args)
-          case _ =>
-            abort(s"Invalid getter $rhs for module in $clazz")
-        }
-
       def mkCheckedAccessor(clazz: Symbol, retVal: Tree, offset: Int, pos: Position, fieldSym: Symbol): Tree = {
         val sym = fieldSym.getterIn(fieldSym.owner)
         val bitmapSym = bitmapFor(clazz, offset, sym)
@@ -926,18 +916,6 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
               deriveDefDef(stat)(rhs => Block(List(rhs, localTyper.typed(mkSetFlag(clazz, fieldOffset(getter), getter, bitmapKind(getter)))), UNIT))
             else stat
           }
-          else if (sym.isModule && (!clazz.isTrait || clazz.isImplClass) && !sym.isBridge) {
-            deriveDefDef(stat)(rhs =>
-              typedPos(stat.pos)(
-                mkInnerClassAccessorDoubleChecked(
-                  // Martin to Hubert: I think this can be replaced by selfRef(tree.pos)
-                  // @PP: It does not seem so, it crashes for me trying to bootstrap.
-                  if (clazz.isImplClass) gen.mkAttributedIdent(stat.vparamss.head.head.symbol) else gen.mkAttributedThis(clazz),
-                  rhs, sym, stat.vparamss.head
-                )
-              )
-            )
-          }
           else stat
         }
         stats map {
@@ -1082,18 +1060,7 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
             })
           }
           else if (sym.isModule && !(sym hasFlag LIFTED | BRIDGE)) {
-            // add modules
-            val vsym = sym.owner.newModuleVarSymbol(sym)
-            addDef(position(sym), ValDef(vsym))
-
-            // !!! TODO - unravel the enormous duplication between this code and
-            // eliminateModuleDefs in RefChecks.
-            val rhs          = gen.newModule(sym, vsym.tpe)
-            val assignAndRet = gen.mkAssignAndReturn(vsym, rhs)
-            val attrThis     = gen.mkAttributedThis(clazz)
-            val rhs1         = mkInnerClassAccessorDoubleChecked(attrThis, assignAndRet, sym, List())
-
-            addDefDef(sym, rhs1)
+            // Moved to Refchecks
           }
           else if (!sym.isMethod) {
             // add fields

src/compiler/scala/tools/nsc/transform/TypingTransformers.scala

@@ -26,7 +26,7 @@ trait TypingTransformers {
 
     def atOwner[A](tree: Tree, owner: Symbol)(trans: => A): A = {
       val savedLocalTyper = localTyper
-      localTyper = localTyper.atOwner(tree, if (owner.isModule) owner.moduleClass else owner)
+      localTyper = localTyper.atOwner(tree, if (owner.isModuleNotMethod) owner.moduleClass else owner)
       val result = super.atOwner(owner)(trans)
       localTyper = savedLocalTyper
       result

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

@@ -1188,26 +1188,6 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
       // set NoType so it will be ignored.
       val cdef          = ClassDef(module.moduleClass, impl) setType NoType
 
-      // Create the module var unless the immediate owner is a class and
-      // the module var already exists there. See SI-5012, SI-6712.
-      def findOrCreateModuleVar() = {
-        val vsym = (
-          if (site.isTerm) NoSymbol
-          else site.info decl nme.moduleVarName(moduleName)
-        )
-        vsym orElse (site newModuleVarSymbol module)
-      }
-      def newInnerObject() = {
-        // Create the module var unless it is already in the module owner's scope.
-        // The lookup is on module.enclClass and not module.owner lest there be a
-        // nullary method between us and the class; see SI-5012.
-        val moduleVar = findOrCreateModuleVar()
-        val rhs       = gen.newModule(module, moduleVar.tpe)
-        val body      = if (site.isTrait) rhs else gen.mkAssignAndReturn(moduleVar, rhs)
-        val accessor  = DefDef(module, body.changeOwner(moduleVar -> module))
-
-        ValDef(moduleVar) :: accessor :: Nil
-      }
       def matchingInnerObject() = {
         val newFlags = (module.flags | STABLE) & ~MODULE
         val newInfo  = NullaryMethodType(module.moduleClass.tpe)
@@ -1219,10 +1199,45 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
         if (module.isStatic)
           if (module.isOverridingSymbol) matchingInnerObject() else Nil
         else
-          newInnerObject()
+          newInnerObject(site, module)
       )
       transformTrees(newTrees map localTyper.typedPos(moduleDef.pos))
     }
+    def newInnerObject(site: Symbol, module: Symbol): List[Tree] = {
+      // Create the module var unless the immediate owner is a class and
+      // the module var already exists there. See SI-5012, SI-6712.
+      def findOrCreateModuleVar() = {
+        val vsym = (
+          if (site.isTerm) NoSymbol
+          else site.info decl nme.moduleVarName(module.name.toTermName)
+          )
+        vsym orElse (site newModuleVarSymbol module)
+      }
+      // Create the module var unless it is already in the module owner's scope.
+      // The lookup is on module.enclClass and not module.owner lest there be a
+      // nullary method between us and the class; see SI-5012.
+      val moduleVar = findOrCreateModuleVar()
+      val rhs       = gen.newModule(module, moduleVar.tpe)
+      val body      = if (site.isTrait) rhs else {
+        if (moduleVar.enclClass == NoSymbol) gen.mkAssignAndReturn(moduleVar, rhs) // e.g. neg/t6666c.scala, will be an error later on.
+        else gen.mkAssignAndReturn(moduleVar, rhs) // will be wrapped in double checked lock idiom in lazyvals
+      }
+      val accessor  = if (module.owner == site) module else {
+        site.newMethod(module.name.toTermName, site.pos, STABLE | MODULE | MIXEDIN).setInfo(moduleVar.tpe).andAlso(self => if (module.isPrivate) self.expandName(module.owner))
+      }
+      val accessorDef = DefDef(accessor, body.changeOwner(moduleVar -> accessor))
+
+      ValDef(moduleVar) :: accessorDef :: Nil
+    }
+
+    def mixinModuleDefs(clazz: Symbol): List[Tree] = {
+      val res = for {
+        mixinClass <- clazz.mixinClasses.iterator
+        module     <- mixinClass.info.decls.iterator.filter(_.isModule)
+        newMember  <- newInnerObject(clazz, module)
+      } yield transform(localTyper.typedPos(clazz.pos)(newMember))
+      res.toList
+    }
 
     def transformStat(tree: Tree, index: Int): List[Tree] = tree match {
       case t if treeInfo.isSelfConstrCall(t) =>
@@ -1508,9 +1523,9 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
       }
 
       val doTransform =
+        sym.name == nme.apply &&
         sym.isSourceMethod &&
         sym.isCase &&
-        sym.name == nme.apply &&
         isClassTypeAccessible(tree) &&
         !tree.tpe.resultType.typeSymbol.primaryConstructor.isLessAccessibleThan(tree.symbol)
 
@@ -1651,11 +1666,12 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
             // SI-7870 default getters for constructors live in the companion module
             checkOverloadedRestrictions(currentOwner, currentOwner.companionModule)
             val bridges = addVarargBridges(currentOwner)
+            val moduleDesugared = if (currentOwner.isTrait) Nil else mixinModuleDefs(currentOwner)
             checkAllOverrides(currentOwner)
             checkAnyValSubclass(currentOwner)
             if (currentOwner.isDerivedValueClass)
               currentOwner.primaryConstructor makeNotPrivate NoSymbol // SI-6601, must be done *after* pickler!
-            if (bridges.nonEmpty) deriveTemplate(tree)(_ ::: bridges) else tree
+            if (bridges.nonEmpty || moduleDesugared.nonEmpty) deriveTemplate(tree)(_ ::: bridges ::: moduleDesugared) else tree
 
           case dc@TypeTreeWithDeferredRefCheck() => abort("adapt should have turned dc: TypeTreeWithDeferredRefCheck into tpt: TypeTree, with tpt.original == dc")
           case tpt@TypeTree() =>

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

@@ -387,7 +387,7 @@ abstract class SuperAccessors extends transform.Transform with transform.TypingT
       val savedValid = validCurrentOwner
       if (owner.isClass) validCurrentOwner = true
       val savedLocalTyper = localTyper
-      localTyper = localTyper.atOwner(tree, if (owner.isModule) owner.moduleClass else owner)
+      localTyper = localTyper.atOwner(tree, if (owner.isModuleNotMethod) owner.moduleClass else owner)
       typers = typers updated (owner, localTyper)
       val result = super.atOwner(tree, owner)(trans)
       localTyper = savedLocalTyper

src/reflect/scala/reflect/internal/StdNames.scala

@@ -876,7 +876,7 @@ trait StdNames {
     val toCharacter: NameType = "toCharacter"
     val toInteger: NameType   = "toInteger"
 
-    def newLazyValSlowComputeName(lzyValName: Name) = lzyValName append LAZY_SLOW_SUFFIX
+    def newLazyValSlowComputeName(lzyValName: Name) = (lzyValName stripSuffix MODULE_VAR_SUFFIX append LAZY_SLOW_SUFFIX).toTermName
 
     // ASCII names for operators
     val ADD       = encode("+")

src/reflect/scala/reflect/internal/Symbols.scala

@@ -323,7 +323,7 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     def newModuleVarSymbol(accessor: Symbol): TermSymbol = {
       val newName  = nme.moduleVarName(accessor.name.toTermName)
       val newFlags = MODULEVAR | ( if (this.isClass) PrivateLocal | SYNTHETIC else 0 )
-      val newInfo  = accessor.tpe.finalResultType
+      val newInfo  = thisType.memberType(accessor).finalResultType
       val mval     = newVariable(newName, accessor.pos.focus, newFlags.toLong) addAnnotation VolatileAttr
 
       if (this.isClass)

src/reflect/scala/reflect/internal/transform/Erasure.scala

@@ -123,7 +123,7 @@ trait Erasure {
       case tref @ TypeRef(pre, sym, args) =>
         if (sym == ArrayClass)
           if (unboundedGenericArrayLevel(tp) == 1) ObjectTpe
-          else if (args.head.typeSymbol.isBottomClass) arrayType(ObjectTpe)
+          else if (args.head.typeSymbol.isBottomClass)  arrayType(ObjectTpe)
           else typeRef(apply(pre), sym, args map applyInArray)
         else if (sym == AnyClass || sym == AnyValClass || sym == SingletonClass) ObjectTpe
         else if (sym == UnitClass) BoxedUnitTpe

test/files/run/delambdafy_t6028.check

@@ -38,10 +38,24 @@ package <empty> {
       <synthetic> <stable> <artifact> def $outer(): T = MethodLocalObject$2.this.$outer;
       <synthetic> <stable> <artifact> def $outer(): T = MethodLocalObject$2.this.$outer
     };
-    final <stable> private[this] def MethodLocalObject$1(barParam$1: String, MethodLocalObject$module$1: runtime.VolatileObjectRef): T#MethodLocalObject$2.type = {
-      MethodLocalObject$module$1.elem = new T#MethodLocalObject$2.type(T.this, barParam$1);
+    final <stable> private[this] def MethodLocalObject$lzycompute$1(barParam$1: String, MethodLocalObject$module$1: runtime.VolatileObjectRef): T#MethodLocalObject$2.type = {
+      {
+        T.this.synchronized({
+          if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
+            {
+              MethodLocalObject$module$1.elem = new T#MethodLocalObject$2.type(T.this, barParam$1);
+              ()
+            };
+          scala.runtime.BoxedUnit.UNIT
+        });
+        ()
+      };
       MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]()
     };
+    final <stable> private[this] def MethodLocalObject$1(barParam$1: String, MethodLocalObject$module$1: runtime.VolatileObjectRef): T#MethodLocalObject$2.type = if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
+      T.this.MethodLocalObject$lzycompute$1(barParam$1, MethodLocalObject$module$1)
+    else
+      MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]();
     abstract trait MethodLocalTrait$1$class extends Object with T#MethodLocalTrait$1 {
       def /*MethodLocalTrait$1$class*/$init$(barParam$1: String): Unit = {
         ()

test/files/run/t6028.check

@@ -50,10 +50,24 @@ package <empty> {
       <synthetic> <stable> <artifact> def $outer(): T = MethodLocalObject$2.this.$outer;
       <synthetic> <stable> <artifact> def $outer(): T = MethodLocalObject$2.this.$outer
     };
-    final <stable> private[this] def MethodLocalObject$1(barParam$1: Int, MethodLocalObject$module$1: runtime.VolatileObjectRef): T#MethodLocalObject$2.type = {
-      MethodLocalObject$module$1.elem = new T#MethodLocalObject$2.type(T.this, barParam$1);
+    final <stable> private[this] def MethodLocalObject$lzycompute$1(barParam$1: Int, MethodLocalObject$module$1: runtime.VolatileObjectRef): T#MethodLocalObject$2.type = {
+      {
+        T.this.synchronized({
+          if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
+            {
+              MethodLocalObject$module$1.elem = new T#MethodLocalObject$2.type(T.this, barParam$1);
+              ()
+            };
+          scala.runtime.BoxedUnit.UNIT
+        });
+        ()
+      };
       MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]()
     };
+    final <stable> private[this] def MethodLocalObject$1(barParam$1: Int, MethodLocalObject$module$1: runtime.VolatileObjectRef): T#MethodLocalObject$2.type = if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
+      T.this.MethodLocalObject$lzycompute$1(barParam$1, MethodLocalObject$module$1)
+    else
+      MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]();
     abstract trait MethodLocalTrait$1$class extends Object with T#MethodLocalTrait$1 {
       def /*MethodLocalTrait$1$class*/$init$(barParam$1: Int): Unit = {
         ()

test/files/run/trait-defaults-modules.scala

@@ -0,0 +1,20 @@
+trait T1 { def a: Any }
+
+trait T2 extends T1 { object a; object b; private object c; def usec: Any = c}
+trait T3 extends T2
+
+class C1 extends T1 { object a; object b }
+class C2 extends C1
+class C3 extends T2
+class C4 extends T3
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val (c1, c2, c3, c4) = (new C1, new C2, new C3, new C4)
+    c1.a; c1.b; (c1: T1).a
+    c2.a; c2.b; (c2: T1).a
+    c3.a; c3.b; (c3: T1).a; c3.usec
+    c4.a; c4.b; (c4: T1).a; c4.usec
+  }
+
+}

test/files/run/trait-defaults-modules2/T_1.scala

@@ -0,0 +1,4 @@
+trait T {
+  private object O
+  def useO: Any = O
+}

test/files/run/trait-defaults-modules2/Test_2.scala

@@ -0,0 +1,5 @@
+object Test extends T {
+  def main(args: Array[String]): Unit = {
+    useO
+  }
+}

test/files/run/trait-defaults-modules3.scala

@@ -0,0 +1,8 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    object O
+    val x = O
+    val y = O
+    assert(x eq y)
+  }
+}