Drop @capability annotations (#20396)

Replace with references that inherit trait `Capability`. 

Also: Handle tracked vals in classes.

As a next step, addCaptureRefinements should use the tracked logic for
all capturing parameters. Right now, we create a fresh capture set
variable instead, but that is too loose.

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -203,6 +203,23 @@ extension (tp: Type)
     case _ =>
       false
 
+  /** Tests whether the type derives from `caps.Capability`, which means
+   *  references of this type are maximal capabilities.
+   */
+  def derivesFromCapability(using Context): Boolean = tp.dealias match
+    case tp: (TypeRef | AppliedType) =>
+      val sym = tp.typeSymbol
+      if sym.isClass then sym.derivesFrom(defn.Caps_Capability)
+      else tp.superType.derivesFromCapability
+    case tp: TypeProxy =>
+      tp.superType.derivesFromCapability
+    case tp: AndType =>
+      tp.tp1.derivesFromCapability || tp.tp2.derivesFromCapability
+    case tp: OrType =>
+      tp.tp1.derivesFromCapability && tp.tp2.derivesFromCapability
+    case _ =>
+      false
+
   /** Drop @retains annotations everywhere */
   def dropAllRetains(using Context): Type = // TODO we should drop retains from inferred types before unpickling
     val tm = new TypeMap:
@@ -408,7 +425,7 @@ extension (sym: Symbol)
   /** The owner of the current level. Qualifying owners are
    *   - methods other than constructors and anonymous functions
    *   - anonymous functions, provided they either define a local
-   *     root of type caps.Cap, or they are the rhs of a val definition.
+   *     root of type caps.Capability, or they are the rhs of a val definition.
    *   - classes, if they are not staticOwners
    *   - _root_
    */

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -115,7 +115,7 @@ sealed abstract class CaptureSet extends Showable:
    *  capture set.
    */
   protected final def addNewElem(elem: CaptureRef)(using Context, VarState): CompareResult =
-    if elem.isRootCapability || summon[VarState] == FrozenState then
+    if elem.isMaxCapability || summon[VarState] == FrozenState then
       addThisElem(elem)
     else
       addThisElem(elem).orElse:
@@ -147,17 +147,26 @@ sealed abstract class CaptureSet extends Showable:
    *   this subsumes this.f
    *   x subsumes y  ==>  x* subsumes y, x subsumes y?
    *   x subsumes y  ==>  x* subsumes y*, x? subsumes y?
+   *   x: x1.type /\ x1 subsumes y  ==>  x subsumes y
    */
   extension (x: CaptureRef)
      private def subsumes(y: CaptureRef)(using Context): Boolean =
       (x eq y)
       || x.isRootCapability
       || y.match
-          case y: TermRef => y.prefix eq x
+          case y: TermRef =>
+            (y.prefix eq x)
+            || y.info.match
+                case y1: CaptureRef => x.subsumes(y1)
+                case _ => false
           case MaybeCapability(y1) => x.stripMaybe.subsumes(y1)
           case _ => false
       || x.match
           case ReachCapability(x1) => x1.subsumes(y.stripReach)
+          case x: TermRef =>
+            x.info match
+              case x1: CaptureRef => x1.subsumes(y)
+              case _ => false
           case _ => false
 
   /** {x} <:< this   where <:< is subcapturing, but treating all variables
@@ -167,11 +176,11 @@ sealed abstract class CaptureSet extends Showable:
     if comparer.isInstanceOf[ExplainingTypeComparer] then // !!! DEBUG
       reporting.trace.force(i"$this accountsFor $x, ${x.captureSetOfInfo}?", show = true):
         elems.exists(_.subsumes(x))
-        || !x.isRootCapability && x.captureSetOfInfo.subCaptures(this, frozen = true).isOK
+        || !x.isMaxCapability && x.captureSetOfInfo.subCaptures(this, frozen = true).isOK
     else
       reporting.trace(i"$this accountsFor $x, ${x.captureSetOfInfo}?", show = true):
         elems.exists(_.subsumes(x))
-        || !x.isRootCapability && x.captureSetOfInfo.subCaptures(this, frozen = true).isOK
+        || !x.isMaxCapability && x.captureSetOfInfo.subCaptures(this, frozen = true).isOK
 
   /** A more optimistic version of accountsFor, which does not take variable supersets
    *  of the `x` reference into account. A set might account for `x` if it accounts
@@ -183,7 +192,7 @@ sealed abstract class CaptureSet extends Showable:
   def mightAccountFor(x: CaptureRef)(using Context): Boolean =
     reporting.trace(i"$this mightAccountFor $x, ${x.captureSetOfInfo}?", show = true) {
       elems.exists(_.subsumes(x))
-      || !x.isRootCapability
+      || !x.isMaxCapability
         && {
           val elems = x.captureSetOfInfo.elems
           !elems.isEmpty && elems.forall(mightAccountFor)
@@ -383,7 +392,7 @@ object CaptureSet:
 
   def apply(elems: CaptureRef*)(using Context): CaptureSet.Const =
     if elems.isEmpty then empty
-    else Const(SimpleIdentitySet(elems.map(_.normalizedRef)*))
+    else Const(SimpleIdentitySet(elems.map(_.normalizedRef.ensuring(_.isTrackableRef))*))
 
   def apply(elems: Refs)(using Context): CaptureSet.Const =
     if elems.isEmpty then empty else Const(elems)
@@ -491,6 +500,7 @@ object CaptureSet:
         CompareResult.LevelError(this, elem)
       else
         //if id == 34 then assert(!elem.isUniversalRootCapability)
+        assert(elem.isTrackableRef, elem)
         elems += elem
         if elem.isRootCapability then
           rootAddedHandler()
@@ -1032,7 +1042,9 @@ object CaptureSet:
 
   /** The capture set of the type underlying CaptureRef */
   def ofInfo(ref: CaptureRef)(using Context): CaptureSet = ref match
-    case ref: TermRef if ref.isRootCapability => ref.singletonCaptureSet
+    case ref: (TermRef | TermParamRef) if ref.isMaxCapability =>
+      if ref.isTrackableRef then ref.singletonCaptureSet
+      else CaptureSet.universal
     case ReachCapability(ref1) => deepCaptureSet(ref1.widen)
       .showing(i"Deep capture set of $ref: ${ref1.widen} = $result", capt)
     case _ => ofType(ref.underlying, followResult = true)
@@ -1046,7 +1058,8 @@ object CaptureSet:
         case tp: TermParamRef =>
           tp.captureSet
         case tp: TypeRef =>
-          if tp.typeSymbol == defn.Caps_Cap then universal else empty
+          if tp.derivesFromCapability then universal // TODO: maybe return another value that indicates that the underltinf ref is maximal?
+          else empty
         case _: TypeParamRef =>
           empty
         case CapturingType(parent, refs) =>
@@ -1074,7 +1087,7 @@ object CaptureSet:
         case _ =>
           empty
     recur(tp)
-      .showing(i"capture set of $tp = $result", captDebug)
+      //.showing(i"capture set of $tp = $result", captDebug)
 
   private def deepCaptureSet(tp: Type)(using Context): CaptureSet =
     val collect = new TypeAccumulator[CaptureSet]:

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -157,15 +157,17 @@ object CheckCaptures:
                 case _ =>
           case AnnotatedType(_, ann) if ann.symbol == defn.UncheckedCapturesAnnot =>
             ()
-          case t =>
+          case CapturingType(parent, refs) =>
             if variance >= 0 then
-              t.captureSet.disallowRootCapability: () =>
+              refs.disallowRootCapability: () =>
                 def part = if t eq tp then "" else i"the part $t of "
                 report.error(
                   em"""$what cannot $have $tp since
                       |${part}that type captures the root capability `cap`.
                       |$addendum""",
                   pos)
+            traverse(parent)
+          case t =>
             traverseChildren(t)
     check.traverse(tp)
   end disallowRootCapabilitiesIn
@@ -537,8 +539,8 @@ class CheckCaptures extends Recheck, SymTransformer:
          */
         def addParamArgRefinements(core: Type, initCs: CaptureSet): (Type, CaptureSet) =
           var refined: Type = core
-          var allCaptures: CaptureSet = if setup.isCapabilityClassRef(core)
-            then CaptureSet.universal else initCs
+          var allCaptures: CaptureSet =
+            if core.derivesFromCapability then CaptureSet.universal else initCs
           for (getterName, argType) <- mt.paramNames.lazyZip(argTypes) do
             val getter = cls.info.member(getterName).suchThat(_.is(ParamAccessor)).symbol
             if getter.termRef.isTracked && !getter.is(Private) then
@@ -572,8 +574,10 @@ class CheckCaptures extends Recheck, SymTransformer:
         val TypeApply(fn, args) = tree
         val polyType = atPhase(thisPhase.prev):
           fn.tpe.widen.asInstanceOf[TypeLambda]
+        def isExempt(sym: Symbol) =
+          sym.isTypeTestOrCast || sym == defn.Compiletime_erasedValue
         for case (arg: TypeTree, formal, pname) <- args.lazyZip(polyType.paramRefs).lazyZip((polyType.paramNames)) do
-          if !tree.symbol.isTypeTestOrCast then
+          if !isExempt(tree.symbol) then
             def where = if fn.symbol.exists then i" in an argument of ${fn.symbol}" else ""
             disallowRootCapabilitiesIn(arg.knownType, NoSymbol,
               i"Sealed type variable $pname", "be instantiated to",
@@ -1004,7 +1008,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             if (ares1 eq ares) && (aargs1 eq aargs) then actual
             else reconstruct(aargs1, ares1)
 
-          (resTp, curEnv.captured)
+          (resTp, CaptureSet(curEnv.captured.elems))
       end adaptFun
 
       /** Adapt type function type `actual` to the expected type.
@@ -1026,14 +1030,14 @@ class CheckCaptures extends Recheck, SymTransformer:
             if ares1 eq ares then actual
             else reconstruct(ares1)
 
-          (resTp, curEnv.captured)
+          (resTp, CaptureSet(curEnv.captured.elems))
       end adaptTypeFun
 
       def adaptInfo(actual: Type, expected: Type, covariant: Boolean): String =
         val arrow = if covariant then "~~>" else "<~~"
         i"adapting $actual $arrow $expected"
 
-      def adapt(actual: Type, expected: Type, covariant: Boolean): Type = trace(adaptInfo(actual, expected, covariant), recheckr, show = true) {
+      def adapt(actual: Type, expected: Type, covariant: Boolean): Type = trace(adaptInfo(actual, expected, covariant), recheckr, show = true):
         if expected.isInstanceOf[WildcardType] then actual
         else
           // Decompose the actual type into the inner shape type, the capture set and the box status
@@ -1113,7 +1117,22 @@ class CheckCaptures extends Recheck, SymTransformer:
               adaptedType(!boxed)
           else
             adaptedType(boxed)
-      }
+      end adapt
+
+      /** If result derives from caps.Capability, yet is not a capturing type itself,
+       *  make its capture set explicit.
+       */
+      def makeCaptureSetExplicit(result: Type) = result match
+        case CapturingType(_, _) => result
+        case _ =>
+          if result.derivesFromCapability then
+            val cap: CaptureRef = actual match
+              case ref: CaptureRef if ref.isTracked =>
+                ref
+              case _ =>
+                defn.captureRoot.termRef // TODO: skolemize?
+            CapturingType(result, cap.singletonCaptureSet)
+          else result
 
       if expected == LhsProto || expected.isSingleton && actual.isSingleton then
         actual
@@ -1129,10 +1148,12 @@ class CheckCaptures extends Recheck, SymTransformer:
               case _ =>
           case _ =>
         val adapted = adapt(actualw.withReachCaptures(actual), expected, covariant = true)
-        if adapted ne actualw then
-          capt.println(i"adapt boxed $actual vs $expected ===> $adapted")
-          adapted
-        else actual
+        makeCaptureSetExplicit:
+          if adapted ne actualw then
+            capt.println(i"adapt boxed $actual vs $expected ===> $adapted")
+            adapted
+          else
+            actual
     end adaptBoxed
 
     /** Check overrides again, taking capture sets into account.
@@ -1305,7 +1326,7 @@ class CheckCaptures extends Recheck, SymTransformer:
                 case ref: TermParamRef
                 if !allowed.contains(ref) && !seen.contains(ref) =>
                   seen += ref
-                  if ref.underlying.isRef(defn.Caps_Cap) then
+                  if ref.underlying.isRef(defn.Caps_Capability) then
                     report.error(i"escaping local reference $ref", tree.srcPos)
                   else
                     val widened = ref.captureSetOfInfo

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -23,7 +23,6 @@ trait SetupAPI:
   def setupUnit(tree: Tree, recheckDef: DefRecheck)(using Context): Unit
   def isPreCC(sym: Symbol)(using Context): Boolean
   def postCheck()(using Context): Unit
-  def isCapabilityClassRef(tp: Type)(using Context): Boolean
 
 object Setup:
 
@@ -58,8 +57,21 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
   private val toBeUpdated = new mutable.HashSet[Symbol]
 
   private def newFlagsFor(symd: SymDenotation)(using Context): FlagSet =
-    if symd.isAllOf(PrivateParamAccessor) && symd.owner.is(CaptureChecked) && !symd.hasAnnotation(defn.ConstructorOnlyAnnot)
-    then symd.flags &~ Private | Recheck.ResetPrivate
+
+    object containsCovarRetains extends TypeAccumulator[Boolean]:
+      def apply(x: Boolean, tp: Type): Boolean =
+        if x then true
+        else if tp.derivesFromCapability && variance >= 0 then true
+        else tp match
+          case AnnotatedType(_, ann) if ann.symbol.isRetains && variance >= 0 => true
+          case _ => foldOver(x, tp)
+      def apply(tp: Type): Boolean = apply(false, tp)
+
+    if symd.isAllOf(PrivateParamAccessor)
+        && symd.owner.is(CaptureChecked)
+        && !symd.hasAnnotation(defn.ConstructorOnlyAnnot)
+        && containsCovarRetains(symd.symbol.originDenotation.info)
+    then symd.flags &~ Private
     else symd.flags
 
   def isPreCC(sym: Symbol)(using Context): Boolean =
@@ -68,31 +80,6 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
     && !sym.owner.is(CaptureChecked)
     && !defn.isFunctionSymbol(sym.owner)
 
-  private val capabilityClassMap = new util.HashMap[Symbol, Boolean]
-
-  /** Check if the class is capability, which means:
-   *  1. the class has a capability annotation,
-   *  2. or at least one of its parent type has universal capability.
-   */
-  def isCapabilityClassRef(tp: Type)(using Context): Boolean = tp.dealiasKeepAnnots match
-    case _: TypeRef | _: AppliedType =>
-      val sym = tp.classSymbol
-      def checkSym: Boolean =
-        sym.hasAnnotation(defn.CapabilityAnnot)
-        || sym.info.parents.exists(hasUniversalCapability)
-      sym.isClass && capabilityClassMap.getOrElseUpdate(sym, checkSym)
-    case _ => false
-
-  private def hasUniversalCapability(tp: Type)(using Context): Boolean = tp.dealiasKeepAnnots match
-    case CapturingType(parent, refs) =>
-      refs.isUniversal || hasUniversalCapability(parent)
-    case AnnotatedType(parent, ann) =>
-      if ann.symbol.isRetains then
-        try ann.tree.toCaptureSet.isUniversal || hasUniversalCapability(parent)
-        catch case ex: IllegalCaptureRef => false
-      else hasUniversalCapability(parent)
-    case tp => isCapabilityClassRef(tp)
-
   private def fluidify(using Context) = new TypeMap with IdempotentCaptRefMap:
     def apply(t: Type): Type = t match
       case t: MethodType =>
@@ -196,7 +183,9 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
             case cls: ClassSymbol
             if !defn.isFunctionClass(cls) && cls.is(CaptureChecked) =>
               cls.paramGetters.foldLeft(tp) { (core, getter) =>
-                if atPhase(thisPhase.next)(getter.termRef.isTracked) then
+                if atPhase(thisPhase.next)(getter.termRef.isTracked)
+                    && !getter.is(Tracked)
+                then
                   val getterType =
                     mapInferred(refine = false)(tp.memberInfo(getter)).strippedDealias
                   RefinedType(core, getter.name,
@@ -317,10 +306,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
           case t: TypeVar =>
             this(t.underlying)
           case t =>
-            // Map references to capability classes C to C^
-            if isCapabilityClassRef(t)
-            then CapturingType(t, defn.expandedUniversalSet, boxed = false)
-            else recur(t)
+            recur(t)
     end expandAliases
 
     val tp1 = expandAliases(tp) // TODO: Do we still need to follow aliases?
@@ -592,7 +578,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
         if sym.isClass then
           !sym.isPureClass
         else
-          sym != defn.Caps_Cap && instanceCanBeImpure(tp.superType)
+          sym != defn.Caps_Capability && instanceCanBeImpure(tp.superType)
       case tp: (RefinedOrRecType | MatchType) =>
         instanceCanBeImpure(tp.underlying)
       case tp: AndType =>

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

@@ -991,7 +991,7 @@ class Definitions {
 
   @tu lazy val CapsModule: Symbol = requiredModule("scala.caps")
     @tu lazy val captureRoot: TermSymbol = CapsModule.requiredValue("cap")
-    @tu lazy val Caps_Cap: TypeSymbol = CapsModule.requiredType("Cap")
+    @tu lazy val Caps_Capability: TypeSymbol = CapsModule.requiredType("Capability")
     @tu lazy val Caps_reachCapability: TermSymbol = CapsModule.requiredMethod("reachCapability")
     @tu lazy val CapsUnsafeModule: Symbol = requiredModule("scala.caps.unsafe")
     @tu lazy val Caps_unsafeAssumePure: Symbol = CapsUnsafeModule.requiredMethod("unsafeAssumePure")
@@ -1014,7 +1014,6 @@ class Definitions {
   @tu lazy val BeanPropertyAnnot: ClassSymbol = requiredClass("scala.beans.BeanProperty")
   @tu lazy val BooleanBeanPropertyAnnot: ClassSymbol = requiredClass("scala.beans.BooleanBeanProperty")
   @tu lazy val BodyAnnot: ClassSymbol = requiredClass("scala.annotation.internal.Body")
-  @tu lazy val CapabilityAnnot: ClassSymbol = requiredClass("scala.annotation.capability")
   @tu lazy val ChildAnnot: ClassSymbol = requiredClass("scala.annotation.internal.Child")
   @tu lazy val ContextResultCountAnnot: ClassSymbol = requiredClass("scala.annotation.internal.ContextResultCount")
   @tu lazy val ProvisionalSuperClassAnnot: ClassSymbol = requiredClass("scala.annotation.internal.ProvisionalSuperClass")
@@ -2033,7 +2032,7 @@ class Definitions {
    */
   @tu lazy val ccExperimental: Set[Symbol] = Set(
     CapsModule, CapsModule.moduleClass, PureClass,
-    CapabilityAnnot, RequiresCapabilityAnnot,
+    RequiresCapabilityAnnot,
     RetainsAnnot, RetainsCapAnnot, RetainsByNameAnnot)
 
   /** Experimental language features defined in `scala.runtime.stdLibPatches.language.experimental`.