Implement unused (renamed to ghost terms)

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

@@ -81,6 +81,7 @@ class Compiler {
          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)
          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/Desugar.scala

@@ -258,7 +258,7 @@ object desugar {
   private def toDefParam(tparam: TypeDef): TypeDef =
     tparam.withMods(tparam.rawMods & EmptyFlags | Param)
   private def toDefParam(vparam: ValDef): ValDef =
-    vparam.withMods(vparam.rawMods & Implicit | Param)
+    vparam.withMods(vparam.rawMods & (Implicit | Unused) | Param)
 
   /** The expansion of a class definition. See inline comments for what is involved */
   def classDef(cdef: TypeDef)(implicit ctx: Context): Tree = {
@@ -825,7 +825,7 @@ object desugar {
 
   def makeImplicitFunction(formals: List[Type], body: Tree)(implicit ctx: Context): Tree = {
     val params = makeImplicitParameters(formals.map(TypeTree))
-    new ImplicitFunction(params, body)
+    new NonEmptyFunction(params, body, Modifiers(Implicit))
   }
 
   /** Add annotation to tree:

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

@@ -396,7 +396,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
    *               flags set.
    */
   private def refPurity(tree: Tree)(implicit ctx: Context): PurityLevel =
-    if (!tree.tpe.widen.isParameterless) SimplyPure
+    if (!tree.tpe.widen.isParameterless || tree.symbol.is(Unused)) SimplyPure
     else if (!tree.symbol.isStable) Impure
     else if (tree.symbol.is(Lazy)) Idempotent // TODO add Module flag, sinxce Module vals or not Lazy from the start.
     else SimplyPure

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

@@ -202,7 +202,8 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
       case tp: MethodType =>
         def valueParam(name: TermName, info: Type): TermSymbol = {
           val maybeImplicit = if (tp.isImplicitMethod) Implicit else EmptyFlags
-          ctx.newSymbol(sym, name, TermParam | maybeImplicit, info, coord = sym.coord)
+          val maybeUnused = if (tp.isUnusedMethod) Unused else EmptyFlags
+          ctx.newSymbol(sym, name, TermParam | maybeImplicit | maybeUnused, info, coord = sym.coord)
         }
         val params = (tp.paramNames, tp.paramInfos).zipped.map(valueParam)
         val (paramss, rtp) = valueParamss(tp.instantiate(params map (_.termRef)))

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

@@ -50,15 +50,14 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
    */
   case class InterpolatedString(id: TermName, segments: List[Tree]) extends TermTree
 
+  /** A function type */
   case class Function(args: List[Tree], body: Tree) extends Tree {
     override def isTerm = body.isTerm
     override def isType = body.isType
   }
 
-  /** An implicit function type */
-  class ImplicitFunction(args: List[Tree], body: Tree) extends Function(args, body) {
-    override def toString = s"ImplicitFunction($args, $body)"
-  }
+  /** A function type that should have non empty args */
+  class NonEmptyFunction(args: List[Tree], body: Tree, val mods: Modifiers) extends Function(args, body)
 
   /** A function created from a wildcard expression
    *  @param  placeHolderParams  a list of definitions of synthetic parameters
@@ -119,6 +118,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
 
     case class Implicit() extends Mod(Flags.ImplicitCommon)
 
+    case class Unused() extends Mod(Flags.Unused)
+
     case class Final() extends Mod(Flags.Final)
 
     case class Sealed() extends Mod(Flags.Sealed)

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

@@ -88,7 +88,7 @@ class Definitions {
     newClassSymbol(ScalaPackageClass, name, EmptyFlags, completer).entered
   }
 
-  /** The trait FunctionN or ImplicitFunctionN, for some N
+  /** The trait FunctionN, ImplicitFunctionN, UnusedFunctionN or UnusedImplicitFunction, for some N
    *  @param  name   The name of the trait to be created
    *
    *  FunctionN traits follow this template:
@@ -106,6 +106,20 @@ class Definitions {
    *      trait ImplicitFunctionN[T0,...,T{N-1}, R] extends Object with FunctionN[T0,...,T{N-1}, R] {
    *        def apply(implicit $x0: T0, ..., $x{N_1}: T{N-1}): R
    *      }
+   *
+   *  UnusedFunctionN traits follow this template:
+   *
+   *      trait UnusedFunctionN[T0,...,T{N-1}, R] extends Object {
+   *        def apply(unused $x0: T0, ..., $x{N_1}: T{N-1}): R
+   *      }
+   *
+   *  UnusedImplicitFunctionN traits follow this template:
+   *
+   *      trait UnusedImplicitFunctionN[T0,...,T{N-1}, R] extends Object with UnusedFunctionN[T0,...,T{N-1}, R] {
+   *        def apply(unused implicit $x0: T0, ..., $x{N_1}: T{N-1}): R
+   *      }
+   *
+   *  UnusedFunctionN and UnusedImplicitFunctionN erase to Function0.
    */
   def newFunctionNTrait(name: TypeName): ClassSymbol = {
     val completer = new LazyType {
@@ -117,18 +131,12 @@ class Definitions {
         val argParamRefs = List.tabulate(arity) { i =>
           enterTypeParam(cls, paramNamePrefix ++ "T" ++ (i + 1).toString, Contravariant, decls).typeRef
         }
-        val resParam = enterTypeParam(cls, paramNamePrefix ++ "R", Covariant, decls)
-        val (methodType, parentTraits) =
-          if (name.firstPart.startsWith(str.ImplicitFunction)) {
-            val superTrait =
-              FunctionType(arity).appliedTo(argParamRefs ::: resParam.typeRef :: Nil)
-            (ImplicitMethodType, superTrait :: Nil)
-          }
-          else (MethodType, Nil)
-        val applyMeth =
-          decls.enter(
-            newMethod(cls, nme.apply,
-              methodType(argParamRefs, resParam.typeRef), Deferred))
+        val resParamRef = enterTypeParam(cls, paramNamePrefix ++ "R", Covariant, decls).typeRef
+        val methodType = MethodType.maker(isJava = false, name.isImplicitFunction, name.isUnusedFunction)
+        val parentTraits =
+          if (!name.isImplicitFunction) Nil
+          else FunctionType(arity, isUnused = name.isUnusedFunction).appliedTo(argParamRefs ::: resParamRef :: Nil) :: Nil
+        decls.enter(newMethod(cls, nme.apply, methodType(argParamRefs, resParamRef), Deferred))
         denot.info =
           ClassInfo(ScalaPackageClass.thisType, cls, ObjectType :: parentTraits, decls)
       }
@@ -748,14 +756,14 @@ class Definitions {
     sym.owner.linkedClass.typeRef
 
   object FunctionOf {
-    def apply(args: List[Type], resultType: Type, isImplicit: Boolean = false)(implicit ctx: Context) =
-      FunctionType(args.length, isImplicit).appliedTo(args ::: resultType :: Nil)
+    def apply(args: List[Type], resultType: Type, isImplicit: Boolean = false, isUnused: Boolean = false)(implicit ctx: Context) =
+      FunctionType(args.length, isImplicit, isUnused).appliedTo(args ::: resultType :: Nil)
     def unapply(ft: Type)(implicit ctx: Context) = {
       val tsym = ft.typeSymbol
       if (isFunctionClass(tsym)) {
         val targs = ft.dealias.argInfos
         if (targs.isEmpty) None
-        else Some(targs.init, targs.last, tsym.name.isImplicitFunction)
+        else Some(targs.init, targs.last, tsym.name.isImplicitFunction, tsym.name.isUnusedFunction)
       }
       else None
     }
@@ -819,20 +827,29 @@ class Definitions {
 
   lazy val TupleType = mkArityArray("scala.Tuple", MaxTupleArity, 2)
 
-  def FunctionClass(n: Int, isImplicit: Boolean = false)(implicit ctx: Context) =
-    if (isImplicit) {
+  def FunctionClass(n: Int, isImplicit: Boolean = false, isUnused: Boolean = false)(implicit ctx: Context) = {
+    if (isImplicit && isUnused) {
+      require(n > 0)
+      ctx.requiredClass("scala.UnusedImplicitFunction" + n.toString)
+    }
+    else if (isImplicit) {
       require(n > 0)
       ctx.requiredClass("scala.ImplicitFunction" + n.toString)
     }
+    else if (isUnused) {
+      require(n > 0)
+      ctx.requiredClass("scala.UnusedFunction" + n.toString)
+    }
     else if (n <= MaxImplementedFunctionArity) FunctionClassPerRun()(ctx)(n)
     else ctx.requiredClass("scala.Function" + n.toString)
+  }
 
     lazy val Function0_applyR = ImplementedFunctionType(0).symbol.requiredMethodRef(nme.apply)
     def Function0_apply(implicit ctx: Context) = Function0_applyR.symbol
 
-  def FunctionType(n: Int, isImplicit: Boolean = false)(implicit ctx: Context): TypeRef =
-    if (n <= MaxImplementedFunctionArity && (!isImplicit || ctx.erasedTypes)) ImplementedFunctionType(n)
-    else FunctionClass(n, isImplicit).typeRef
+  def FunctionType(n: Int, isImplicit: Boolean = false, isUnused: Boolean = false)(implicit ctx: Context): TypeRef =
+    if (n <= MaxImplementedFunctionArity && (!isImplicit || ctx.erasedTypes) && !isUnused) ImplementedFunctionType(n)
+    else FunctionClass(n, isImplicit, isUnused).typeRef
 
   private lazy val TupleTypes: Set[TypeRef] = TupleType.toSet
 
@@ -857,14 +874,23 @@ class Definitions {
   /** Is a function class.
    *   - FunctionN for N >= 0
    *   - ImplicitFunctionN for N > 0
+   *   - UnusedFunctionN for N > 0
+   *   - UnusedImplicitFunctionN for N > 0
    */
   def isFunctionClass(cls: Symbol) = scalaClassName(cls).isFunction
 
   /** Is an implicit function class.
    *   - ImplicitFunctionN for N > 0
+   *   - UnusedImplicitFunctionN for N > 0
    */
   def isImplicitFunctionClass(cls: Symbol) = scalaClassName(cls).isImplicitFunction
 
+  /** Is an unused function class.
+   *   - UnusedFunctionN for N > 0
+   *   - UnusedImplicitFunctionN for N > 0
+   */
+  def isUnusedFunctionClass(cls: Symbol) = scalaClassName(cls).isUnusedFunction
+
   /** Is a class that will be erased to FunctionXXL
    *   - FunctionN for N >= 22
    *   - ImplicitFunctionN for N >= 22
@@ -889,11 +915,14 @@ class Definitions {
    *    - FunctionN for 22 > N >= 0 remains as FunctionN
    *    - ImplicitFunctionN for N > 22 becomes FunctionXXL
    *    - ImplicitFunctionN for 22 > N >= 0 becomes FunctionN
+   *    - UnusedFunctionN becomes Function0
+   *    - ImplicitUnusedFunctionN becomes Function0
    *    - anything else becomes a NoSymbol
    */
   def erasedFunctionClass(cls: Symbol): Symbol = {
     val arity = scalaClassName(cls).functionArity
-    if (arity > 22) FunctionXXLClass
+    if (cls.name.isUnusedFunction) FunctionClass(0)
+    else if (arity > 22) FunctionXXLClass
     else if (arity >= 0) FunctionClass(arity)
     else NoSymbol
   }
@@ -903,12 +932,15 @@ class Definitions {
    *    - FunctionN for 22 > N >= 0 remains as FunctionN
    *    - ImplicitFunctionN for N > 22 becomes FunctionXXL
    *    - ImplicitFunctionN for 22 > N >= 0 becomes FunctionN
+   *    - UnusedFunctionN becomes Function0
+   *    - ImplicitUnusedFunctionN becomes Function0
    *    - anything else becomes a NoType
    */
   def erasedFunctionType(cls: Symbol): Type = {
     val arity = scalaClassName(cls).functionArity
-    if (arity > 22) defn.FunctionXXLType
-    else if (arity >= 0) defn.FunctionType(arity)
+    if (cls.name.isUnusedFunction) FunctionType(0)
+    else if (arity > 22) FunctionXXLType
+    else if (arity >= 0) FunctionType(arity)
     else NoType
   }
 
@@ -976,7 +1008,7 @@ class Definitions {
   def isNonDepFunctionType(tp: Type)(implicit ctx: Context) = {
     val arity = functionArity(tp)
     val sym = tp.dealias.typeSymbol
-    arity >= 0 && isFunctionClass(sym) && tp.isRef(FunctionType(arity, sym.name.isImplicitFunction).typeSymbol)
+    arity >= 0 && isFunctionClass(sym) && tp.isRef(FunctionType(arity, sym.name.isImplicitFunction, sym.name.isUnusedFunction).typeSymbol)
   }
 
   /** Is `tp` a representation of a (possibly depenent) function type or an alias of such? */
@@ -1042,6 +1074,9 @@ class Definitions {
   def isImplicitFunctionType(tp: Type)(implicit ctx: Context): Boolean =
     asImplicitFunctionType(tp).exists
 
+  def isUnusedFunctionType(tp: Type)(implicit ctx: Context) =
+    isFunctionType(tp) && tp.dealias.typeSymbol.name.isUnusedFunction
+
   // ----- primitive value class machinery ------------------------------------------
 
   /** This class would also be obviated by the implicit function type design */

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

@@ -367,6 +367,9 @@ object Flags {
   /** Symbol is a Java enum */
   final val Enum = commonFlag(40, "<enum>")
 
+  /** Labeled with `unused` modifier (unused value)  */
+  final val Unused = termFlag(42, "unused")
+
   // Flags following this one are not pickled
 
   /** Symbol is not a member of its owner */
@@ -438,7 +441,7 @@ object Flags {
   /** Flags representing source modifiers */
   final val SourceModifierFlags =
     commonFlags(Private, Protected, Abstract, Final, Inline,
-     Sealed, Case, Implicit, Override, AbsOverride, Lazy, JavaStatic)
+     Sealed, Case, Implicit, Override, AbsOverride, Lazy, JavaStatic, Unused)
 
   /** Flags representing modifiers that can appear in trees */
   final val ModifierFlags =
@@ -512,7 +515,7 @@ object Flags {
   /** Flags that can apply to a module val */
   final val RetainedModuleValFlags: FlagSet = RetainedModuleValAndClassFlags |
     Override | Final | Method | Implicit | Lazy |
-    Accessor | AbsOverride | Stable | Captured | Synchronized | Inline
+    Accessor | AbsOverride | Stable | Captured | Synchronized | Inline | Unused
 
   /** Flags that can apply to a module class */
   final val RetainedModuleClassFlags: FlagSet = RetainedModuleValAndClassFlags | ImplClass | Enum

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

@@ -93,4 +93,5 @@ object Mode {
 
   /** We are in the IDE */
   val Interactive = newMode(20, "Interactive")
+
 }

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

@@ -174,31 +174,53 @@ object NameOps {
      */
     def functionArity: Int =
       functionArityFor(str.Function) max {
-        val n = functionArityFor(str.ImplicitFunction)
+        val n =
+          functionArityFor(str.ImplicitFunction) max
+          functionArityFor(str.UnusedFunction) max
+          functionArityFor(str.UnusedImplicitFunction)
         if (n == 0) -1 else n
       }
 
     /** Is a function name
      *    - FunctionN for N >= 0
      *    - ImplicitFunctionN for N >= 1
+     *    - UnusedFunctionN for N >= 1
+     *    - UnusedImplicitFunctionN for N >= 1
      *    - false otherwise
      */
     def isFunction: Boolean = functionArity >= 0
 
     /** Is a implicit function name
      *    - ImplicitFunctionN for N >= 1
+     *    - UnusedImplicitFunctionN for N >= 1
      *    - false otherwise
      */
-    def isImplicitFunction: Boolean = functionArityFor(str.ImplicitFunction) >= 1
+    def isImplicitFunction: Boolean = {
+      functionArityFor(str.ImplicitFunction) >= 1 ||
+      functionArityFor(str.UnusedImplicitFunction) >= 1
+    }
+
+    /** Is a implicit function name
+      *    - UnusedFunctionN for N >= 1
+      *    - UnusedImplicitFunctionN for N >= 1
+      *    - false otherwise
+      */
+    def isUnusedFunction: Boolean = {
+      functionArityFor(str.UnusedFunction) >= 1 ||
+      functionArityFor(str.UnusedImplicitFunction) >= 1
+    }
 
     /** Is a synthetic function name
      *    - FunctionN for N > 22
      *    - ImplicitFunctionN for N >= 1
+     *    - UnusedFunctionN for N >= 1
+     *    - UnusedImplicitFunctionN for N >= 1
      *    - false otherwise
      */
     def isSyntheticFunction: Boolean = {
       functionArityFor(str.Function) > MaxImplementedFunctionArity ||
-        functionArityFor(str.ImplicitFunction) >= 1
+      functionArityFor(str.ImplicitFunction) >= 1 ||
+      isUnusedFunction
     }
 
     /** Parsed function arity for function with some specific prefix */

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

@@ -36,7 +36,9 @@ object StdNames {
     final val MODULE_INSTANCE_FIELD      = "MODULE$"
 
     final val Function                   = "Function"
+    final val UnusedFunction             = "UnusedFunction"
     final val ImplicitFunction           = "ImplicitFunction"
+    final val UnusedImplicitFunction     = "UnusedImplicitFunction"
     final val AbstractFunction           = "AbstractFunction"
     final val Tuple                      = "Tuple"
     final val Product                    = "Product"

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

@@ -402,7 +402,8 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       def paramErasure(tpToErase: Type) =
         erasureFn(tp.isJavaMethod, semiEraseVCs, isConstructor, wildcardOK)(tpToErase)
       val (names, formals0) =
-        if (tp.paramInfos.exists(_.isPhantom)) tp.paramNames.zip(tp.paramInfos).filterNot(_._2.isPhantom).unzip
+        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 formals = formals0.mapConserve(paramErasure)
       eraseResult(tp.resultType) match {