Make all phantoms unused

This removes the necesity of erasing phantoms. This process
is done by the erasure of `unused`.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -80,8 +80,7 @@ class Compiler {
          new ShortcutImplicits,      // Allow implicit functions without creating closures
          new CrossCastAnd,           // Normalize selections involving intersection types.
          new Splitter) ::            // Expand selections involving union types into conditionals
-    List(new PhantomArgLift,         // Extracts the evaluation of phantom arguments placing them before the call.
-         new UnusedDecls,            // Removes all unused defs and vals decls (except for parameters)
+    List(new UnusedDecls,            // Removes all unused defs and vals decls (except for parameters)
          new VCInlineMethods,        // Inlines calls to value class methods
          new SeqLiterals,            // Express vararg arguments as arrays
          new InterceptedMethods,     // Special handling of `==`, `|=`, `getClass` methods

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -444,6 +444,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     else if (tpw isRef defn.DoubleClass) Literal(Constant(0d))
     else if (tpw isRef defn.ByteClass) Literal(Constant(0.toByte))
     else if (tpw isRef defn.ShortClass) Literal(Constant(0.toShort))
+    else if (tpw.isPhantom) Literal(Constant(null)).withType(tpw)
     else Literal(Constant(null)).select(defn.Any_asInstanceOf).appliedToType(tpe)
   }
 

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

@@ -1174,21 +1174,15 @@ class Definitions {
 
     val any = enterCompleteClassSymbol(cls, tpnme.Any, Protected | Final | NoInitsTrait, Nil)
     val nothing = enterCompleteClassSymbol(cls, tpnme.Nothing, Protected | Final | NoInitsTrait, List(any.typeRef))
-    enterMethod(cls, nme.assume_, ExprType(nothing.typeRef), Protected | Final | Method)
+    enterMethod(cls, nme.assume_, ExprType(nothing.typeRef), Protected | Final | Method | Unused)
 
     cls
   }
   lazy val Phantom_AnyClass = PhantomClass.unforcedDecls.find(_.name eq tpnme.Any).asClass
   lazy val Phantom_NothingClass = PhantomClass.unforcedDecls.find(_.name eq tpnme.Nothing).asClass
-  lazy val Phantom_assume = PhantomClass.unforcedDecls.find(_.name eq nme.assume_)
 
   /** If the symbol is of the class scala.Phantom.Any or scala.Phantom.Nothing */
   def isPhantomTerminalClass(sym: Symbol) = (sym eq Phantom_AnyClass) || (sym eq Phantom_NothingClass)
 
-
   lazy val ErasedPhantomType: TypeRef = ctx.requiredClassRef("dotty.runtime.ErasedPhantom")
-  def ErasedPhantomClass(implicit ctx: Context) = ErasedPhantomType.symbol.asClass
-
-  def ErasedPhantom_UNIT(implicit ctx: Context) = ErasedPhantomClass.linkedClass.requiredValue("UNIT")
-
 }

compiler/src/dotty/tools/dotc/core/PhantomErasure.scala

@@ -84,7 +84,7 @@ case class Signature(paramsSig: List[TypeName], resSig: TypeName) {
    *  to the parameter part of this signature.
    */
   def prepend(params: List[Type], isJava: Boolean)(implicit ctx: Context) =
-    Signature(params.collect { case p if !p.isPhantom => sigName(p, isJava) } ++ paramsSig, resSig)
+    Signature(params.map(p => sigName(p, isJava)) ++ paramsSig, resSig)
 
   /** A signature is under-defined if its paramsSig part contains at least one
    *  `tpnme.Uninstantiated`. Under-defined signatures arise when taking a signature

compiler/src/dotty/tools/dotc/core/Signature.scala

@@ -380,7 +380,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       else if (semiEraseVCs && isDerivedValueClass(sym)) eraseDerivedValueClassRef(tp)
       else if (sym == defn.ArrayClass) apply(tp.appliedTo(TypeBounds.empty)) // i966 shows that we can hit a raw Array type.
       else if (defn.isSyntheticFunctionClass(sym)) defn.erasedFunctionType(sym)
-      else if (defn.isPhantomTerminalClass(sym)) PhantomErasure.erasedPhantomType
+      else if (defn.isPhantomTerminalClass(sym)) defn.ErasedPhantomType
       else if (sym eq defn.PhantomClass) defn.ObjectType // To erase the definitions of Phantom.{assume, Any, Nothing}
       else eraseNormalClassRef(tp)
     case tp: AppliedType =>
@@ -401,10 +401,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
     case tp: MethodType =>
       def paramErasure(tpToErase: Type) =
         erasureFn(tp.isJavaMethod, semiEraseVCs, isConstructor, wildcardOK)(tpToErase)
-      val (names, formals0) =
-        if (tp.isUnusedMethod) (Nil, Nil)
-        else if (tp.paramInfos.exists(_.isPhantom)) tp.paramNames.zip(tp.paramInfos).filterNot(_._2.isPhantom).unzip
-        else (tp.paramNames, tp.paramInfos)
+      val (names, formals0) = if (tp.isUnusedMethod) (Nil, Nil) else (tp.paramNames, tp.paramInfos)
       val formals = formals0.mapConserve(paramErasure)
       eraseResult(tp.resultType) match {
         case rt: MethodType =>
@@ -525,8 +522,6 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
         }
         if (defn.isSyntheticFunctionClass(sym))
           sigName(defn.erasedFunctionType(sym))
-        else if (defn.isPhantomTerminalClass(tp.symbol))
-          sigName(PhantomErasure.erasedPhantomType)
         else
           normalizeClass(sym.asClass).fullName.asTypeName
       case tp: AppliedType =>

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -131,7 +131,7 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
     // Produce aligned accessors and constructor parameters. We have to adjust
     // for any outer parameters, which are last in the sequence of original
     // parameter accessors but come first in the constructor parameter list.
-    val accessors = cls.paramAccessors.filterNot(x => x.isSetter || x.info.resultType.classSymbol == defn.ErasedPhantomClass)
+    val accessors = cls.paramAccessors.filterNot(x => x.isSetter)
     val vparamsWithOuterLast = vparams match {
       case vparam :: rest if vparam.name == nme.OUTER => rest ::: vparam :: Nil
       case _ => vparams

compiler/src/dotty/tools/dotc/transform/Constructors.scala

@@ -28,7 +28,6 @@ import ValueClasses._
 import TypeUtils._
 import ExplicitOuter._
 import core.Mode
-import core.PhantomErasure
 import reporting.trace
 
 class Erasure extends Phase with DenotTransformer {
@@ -213,8 +212,6 @@ object Erasure {
           val tree1 =
             if (tree.tpe isRef defn.NullClass)
               adaptToType(tree, underlying)
-            else if (wasPhantom(underlying))
-              PhantomErasure.erasedParameterRef
             else if (!(tree.tpe <:< tycon)) {
               assert(!(tree.tpe.typeSymbol.isPrimitiveValueClass))
               val nullTree = Literal(Constant(null))
@@ -437,16 +434,9 @@ object Erasure {
         }
       }
 
-      if ((origSym eq defn.Phantom_assume) || (origSym.is(Flags.ParamAccessor) && wasPhantom(pt)))
-        PhantomErasure.erasedAssume
-      else recur(typed(tree.qualifier, AnySelectionProto))
+      recur(typed(tree.qualifier, AnySelectionProto))
     }
 
-    override def typedIdent(tree: untpd.Ident, pt: Type)(implicit ctx: Context): tpd.Tree =
-      if (tree.symbol eq defn.Phantom_assume) PhantomErasure.erasedAssume
-      else if (tree.symbol.is(Flags.Param) && wasPhantom(tree.typeOpt)) PhantomErasure.erasedParameterRef
-      else super.typedIdent(tree, pt)
-
     override def typedThis(tree: untpd.This)(implicit ctx: Context): Tree =
       if (tree.symbol == ctx.owner.lexicallyEnclosingClass || tree.symbol.isStaticOwner) promote(tree)
       else {
@@ -507,11 +497,9 @@ object Erasure {
                   .withType(defn.ArrayOf(defn.ObjectType))
                 args0 = bunchedArgs :: Nil
               }
-              // Arguments are phantom if an only if the parameters are phantom, guaranteed by the separation of type lattices
-              val args1 = args0.filterConserve(arg => !wasPhantom(arg.typeOpt))
-              assert(args1 hasSameLengthAs mt.paramInfos)
-              val args2 = args1.zipWithConserve(mt.paramInfos)(typedExpr)
-              untpd.cpy.Apply(tree)(fun1, args2) withType mt.resultType
+              assert(args0 hasSameLengthAs mt.paramInfos)
+              val args1 = args0.zipWithConserve(mt.paramInfos)(typedExpr)
+              untpd.cpy.Apply(tree)(fun1, args1) withType mt.resultType
             case _ =>
               throw new MatchError(i"tree $tree has unexpected type of function ${fun1.tpe.widen}, was ${fun.typeOpt.widen}")
           }
@@ -572,11 +560,6 @@ object Erasure {
         rhs1 = untpd.Block(paramDefs, rhs1)
       }
       vparamss1 = vparamss1.mapConserve(_.filterConserve(!_.symbol.is(Flags.Unused)))
-      vparamss1 = vparamss1.mapConserve(_.filterConserve(vparam => !wasPhantom(vparam.tpe)))
-      if (sym.is(Flags.ParamAccessor) && wasPhantom(ddef.tpt.tpe)) {
-        sym.resetFlag(Flags.ParamAccessor)
-        rhs1 = PhantomErasure.erasedParameterRef
-      }
       val ddef1 = untpd.cpy.DefDef(ddef)(
         tparams = Nil,
         vparamss = vparamss1,
@@ -691,7 +674,4 @@ object Erasure {
 
   def takesBridges(sym: Symbol)(implicit ctx: Context) =
     sym.isClass && !sym.is(Flags.Trait | Flags.Package)
-
-  private def wasPhantom(tp: Type)(implicit ctx: Context): Boolean =
-    tp.widenDealias.classSymbol eq defn.ErasedPhantomClass
 }

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -177,14 +177,6 @@ class FirstTransform extends MiniPhase with InfoTransformer { thisPhase =>
     } else ddef
   }
 
-  override def transformValDef(vdef: tpd.ValDef)(implicit ctx: Context): tpd.Tree = {
-    if (vdef.tpt.tpe.isPhantom) {
-      if (vdef.symbol.is(Mutable)) ctx.error("var fields cannot have Phantom types", vdef.pos)
-      else if (vdef.symbol.hasAnnotation(defn.VolatileAnnot)) ctx.error("Phantom fields cannot be @volatile", vdef.pos)
-    }
-    vdef
-  }
-
   override def transformStats(trees: List[Tree])(implicit ctx: Context): List[Tree] =
     ast.Trees.flatten(reorderAndComplete(trees)(ctx.withPhase(thisPhase.next)))
 

compiler/src/dotty/tools/dotc/transform/FirstTransform.scala

@@ -107,7 +107,6 @@ import Decorators._
       if (sym eq defn.NothingClass) Throw(Literal(Constant(null)))
       else if (sym eq defn.NullClass) Literal(Constant(null))
       else if (sym eq defn.BoxedUnitClass) ref(defn.BoxedUnit_UNIT)
-      else if (sym eq defn.ErasedPhantomClass) ref(defn.ErasedPhantom_UNIT)
       else {
         assert(false, sym + " has no erased bottom tree")
         EmptyTree
@@ -120,11 +119,8 @@ import Decorators._
       def adaptToField(tree: Tree): Tree =
         if (tree.isEmpty) tree else tree.ensureConforms(field.info.widen)
 
-      def isErasableBottomField(cls: Symbol): Boolean = {
-        // TODO: For Scala.js, return false if this field is in a js.Object unless it is an ErasedPhantomClass.
-        !field.isVolatile &&
-        ((cls eq defn.NothingClass) || (cls eq defn.NullClass) || (cls eq defn.BoxedUnitClass) || (cls eq defn.ErasedPhantomClass))
-      }
+      def isErasableBottomField(cls: Symbol): Boolean =
+        !field.isVolatile && ((cls eq defn.NothingClass) || (cls eq defn.NullClass) || (cls eq defn.BoxedUnitClass))
 
       if (sym.isGetter) {
         var rhs = tree.rhs.changeOwnerAfter(sym, field, thisPhase)