Merge pull request #2399 from dotty-staging/implement-phantom-types-part-2

Erase arguments of phantom type

Author: nicolasstucki

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

@@ -69,7 +69,8 @@ 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 VCInlineMethods,     // Inlines calls to value class methods
+      List(new PhantomArgLift, // Extracts the evaluation of phantom arguments placing them before the call.
+           new VCInlineMethods,     // Inlines calls to value class methods
            new SeqLiterals,         // Express vararg arguments as arrays
            new InterceptedMethods,  // Special handling of `==`, `|=`, `getClass` methods
            new Getters,             // Replace non-private vals and vars with getter defs (fields are added later)

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

@@ -5,10 +5,13 @@ import dotty.tools.dotc.core.Contexts.Context
 import dotty.tools.dotc.core.Symbols.defn
 import dotty.tools.dotc.core.Types.Type
 
-/** Phantom erasure erases (minimal erasure):
+/** Phantom erasure erases:
  *
- *  - Parameters/arguments are erased to ErasedPhantom. The next step will remove the parameters
- *    from the method definitions and calls (implemented in branch implement-phantom-types-part-2).
+ *  - Parameters/arguments are removed from the function definition/call in `PhantomArgLift`.
+ *    If the evaluation of the phantom arguments may produce a side effect, these are evaluated and stored in
+ *    local `val`s and then the non phantoms are used in the Apply. Phantom `val`s are then erased to
+ *    `val ev$i: ErasedPhantom = myPhantom` intended to be optimized away by local optimizations. `myPhantom` could be
+ *    a reference to a phantom parameter, a call to Phantom assume or a call to a method that returns a phantom.
  *  - Definitions of `def`, `val`, `lazy val` and `var` returning a phantom type to return a ErasedPhantom. Where fields
  *    with ErasedPhantom type are not memoized (see transform/Memoize.scala).
  *  - Calls to Phantom.assume become calls to ErasedPhantom.UNIT. Intended to be optimized away by local optimizations.
@@ -21,4 +24,7 @@ object PhantomErasure {
   /** Returns the default erased tree for a call to Phantom.assume */
   def erasedAssume(implicit ctx: Context): Tree = ref(defn.ErasedPhantom_UNIT)
 
+  /** Returns the default erased tree for a phantom parameter ref */
+  def erasedParameterRef(implicit ctx: Context): Tree = ref(defn.ErasedPhantom_UNIT)
+
 }

compiler/src/dotty/tools/dotc/core/Signature.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.map(sigName(_, isJava))) ++ paramsSig, resSig)
+    Signature(params.collect { case p if !p.isPhantom => 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/TypeErasure.scala

@@ -400,12 +400,15 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
     case tp: MethodType =>
       def paramErasure(tpToErase: Type) =
         erasureFn(tp.isJava, semiEraseVCs, isConstructor, wildcardOK)(tpToErase)
-      val formals = tp.paramInfos.mapConserve(paramErasure)
+      val (names, formals0) =
+        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 {
         case rt: MethodType =>
-          tp.derivedLambdaType(tp.paramNames ++ rt.paramNames, formals ++ rt.paramInfos, rt.resultType)
+          tp.derivedLambdaType(names ++ rt.paramNames, formals ++ rt.paramInfos, rt.resultType)
         case rt =>
-          tp.derivedLambdaType(tp.paramNames, formals, rt)
+          tp.derivedLambdaType(names, formals, rt)
       }
     case tp: PolyType =>
       this(tp.resultType)

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

@@ -209,6 +209,8 @@ 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))
@@ -418,6 +420,7 @@ object Erasure {
 
     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 =
@@ -475,7 +478,8 @@ object Erasure {
                   .withType(defn.ArrayOf(defn.ObjectType))
                 args0 = bunchedArgs :: Nil
               }
-              val args1 = args0.zipWithConserve(mt.paramInfos)(typedExpr)
+              // 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)).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}")
@@ -536,6 +540,11 @@ object Erasure {
         vparamss1 = (tpd.ValDef(bunchedParam) :: Nil) :: Nil
         rhs1 = untpd.Block(paramDefs, rhs1)
       }
+      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,
@@ -619,4 +628,7 @@ 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/PhantomArgLift.scala

@@ -0,0 +1,69 @@
+package dotty.tools.dotc.transform
+
+import dotty.tools.dotc.ast.tpd
+import dotty.tools.dotc.core.Contexts._
+import dotty.tools.dotc.core.NameKinds._
+import dotty.tools.dotc.core.Types._
+import dotty.tools.dotc.transform.TreeTransforms.{MiniPhaseTransform, TransformerInfo}
+import dotty.tools.dotc.typer.EtaExpansion
+
+import scala.collection.mutable.ListBuffer
+
+/** This phase extracts the arguments of phantom type before the application to avoid losing any
+ *  effects in the argument tree. This trivializes the removal of parameter in the Erasure phase.
+ *
+ *  `f(x1,...)(y1,...)...(...)` with at least one phantom argument
+ *
+ *    -->
+ *
+ *  `val ev$f = f` // if `f` is some expression that needs evaluation
+ *  `val ev$x1 = x1`
+  *  ...
+ *  `val ev$y1 = y1`
+  *  ...
+ *  `ev$f(ev$x1,...)(ev$y1,...)...(...)`
+ *
+ */
+class PhantomArgLift extends MiniPhaseTransform {
+  import tpd._
+
+  override def phaseName: String = "phantomArgLift"
+
+  /** Check what the phase achieves, to be called at any point after it is finished. */
+  override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = tree match {
+    case tree: Apply =>
+      tree.args.foreach { arg =>
+        assert(!arg.tpe.isPhantom || isPureExpr(arg))
+      }
+    case _ =>
+  }
+
+  /* Tree transform */
+
+  override def transformApply(tree: Apply)(implicit ctx: Context, info: TransformerInfo): Tree = tree.tpe.widen match {
+    case _: MethodType => tree // Do the transformation higher in the tree if needed
+    case _ =>
+      if (!hasImpurePhantomArgs(tree)) tree
+      else {
+        val buffer = ListBuffer.empty[Tree]
+        val app = EtaExpansion.liftApp(buffer, tree)
+        if (buffer.isEmpty) app
+        else Block(buffer.result(), app)
+      }
+  }
+
+  /* private methods */
+
+  /** Returns true if at least on of the arguments is an impure phantom.
+   *  Inner applies are also checked in case of multiple parameter list.
+   */
+  private def hasImpurePhantomArgs(tree: Apply)(implicit ctx: Context): Boolean = {
+    tree.args.exists(arg => arg.tpe.isPhantom && !isPureExpr(arg)) || {
+      tree.fun match {
+        case fun: Apply => hasImpurePhantomArgs(fun)
+        case _ => false
+      }
+    }
+  }
+
+}

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

@@ -44,7 +44,7 @@ class VCInlineMethods extends MiniPhaseTransform with IdentityDenotTransformer {
   override def phaseName: String = "vcInlineMethods"
 
   override def runsAfter: Set[Class[_ <: Phase]] =
-    Set(classOf[ExtensionMethods], classOf[PatternMatcher])
+    Set(classOf[ExtensionMethods], classOf[PatternMatcher], classOf[PhantomArgLift])
 
   /** Replace a value class method call by a call to the corresponding extension method.
    *

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -165,6 +165,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("../tests/neg/customArgs/xfatalWarnings.scala", defaultOptions.and("-Xfatal-warnings")) +
     compileFile("../tests/neg/customArgs/pureStatement.scala", defaultOptions.and("-Xfatal-warnings")) +
     compileFile("../tests/neg/customArgs/phantom-overload.scala", allowDoubleBindings) +
+    compileFile("../tests/neg/customArgs/phantom-overload-2.scala", allowDoubleBindings) +
     compileFile("../tests/neg/tailcall/t1672b.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/t3275.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/t6574.scala", defaultOptions) +

tests/neg/customArgs/phantom-overload-2.scala

@@ -0,0 +1,21 @@
+
+class phantomOverload2 {
+  import Boo._
+
+  def foo1() = ???
+  def foo1(x: A) = ??? // error
+  def foo1(x1: B)(x2: N) = ??? // error
+
+  def foo2(x1: Int, x2: A) = ???
+  def foo2(x1: A)(x2: Int) = ??? // error
+  def foo2(x1: N)(x2: A)(x3: Int) = ??? // error
+
+  def foo3(x1: Int, x2: A) = ???
+  def foo3(x1: Int, x2: A)(x3: A) = ??? // error
+}
+
+object Boo extends Phantom {
+  type A <: this.Any
+  type B <: this.Any
+  type N = this.Nothing
+}

tests/pos/phantom-in-value-class.scala

@@ -0,0 +1,16 @@
+
+object PhantomInValueClass {
+  import BooUtil._
+  new VC("ghi").foo(boo)
+}
+
+object BooUtil extends Phantom {
+
+  type Boo <: this.Any
+  def boo: Boo = assume
+
+  class VC[T](val x: T) extends AnyVal {
+    def foo(b: Boo) = println(x)
+  }
+
+}