Drop @capability annotations

Replace with references that inherit trait `Capability`.

Author: odersky

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

@@ -203,6 +203,23 @@ extension (tp: Type)
     case _ =>
       false
 
+  /** Does type derive from caps.Capability?, which means it 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

@@ -1051,7 +1051,8 @@ 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) =>

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

@@ -537,8 +537,8 @@ 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 +572,10 @@ 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",
@@ -1305,7 +1307,7 @@ 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

@@ -77,9 +77,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
   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)
+      def checkSym: Boolean = sym.info.parents.exists(hasUniversalCapability)
       sym.isClass && capabilityClassMap.getOrElseUpdate(sym, checkSym)
     case _ => false
 
@@ -594,7 +592,7 @@ 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`.

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

@@ -3014,7 +3014,8 @@ object Types extends TypeUtils {
       name == nme.CAPTURE_ROOT && symbol == defn.captureRoot
 
     override def isMaxCapability(using Context): Boolean =
-      widen.derivesFrom(defn.Caps_Cap) && symbol.isStableMember
+      import cc.*
+      this.derivesFromCapability && symbol.isStableMember
 
     override def normalizedRef(using Context): CaptureRef =
       if isTrackableRef then symbol.termRef else this
@@ -4815,7 +4816,9 @@ object Types extends TypeUtils {
     def kindString: String = "Term"
     def copyBoundType(bt: BT): Type = bt.paramRefs(paramNum)
     override def isTrackableRef(using Context) = true
-    override def isMaxCapability(using Context) = widen.derivesFrom(defn.Caps_Cap)
+    override def isMaxCapability(using Context) =
+      import cc.*
+      this.derivesFromCapability
   }
 
   private final class TermParamRefImpl(binder: TermLambda, paramNum: Int) extends TermParamRef(binder, paramNum)

library/src/scala/CanThrow.scala

@@ -6,9 +6,9 @@ import annotation.{implicitNotFound, experimental, capability}
  *  experimental.saferExceptions feature, a `throw Ex()` expression will require
  *  a given of class `CanThrow[Ex]` to be available.
  */
-@experimental @capability
+@experimental
 @implicitNotFound("The capability to throw exception ${E} is missing.\nThe capability can be provided by one of the following:\n - Adding a using clause `(using CanThrow[${E}])` to the definition of the enclosing method\n - Adding `throws ${E}` clause after the result type of the enclosing method\n - Wrapping this piece of code with a `try` block that catches ${E}")
-erased class CanThrow[-E <: Exception]
+erased class CanThrow[-E <: Exception] extends caps.Capability
 
 @experimental
 object unsafeExceptions:

library/src/scala/annotation/capability.scala

@@ -11,4 +11,6 @@ import annotation.experimental
  *  THere, the capture set of any instance of `CanThrow` is assumed to be
  *  `{*}`.
  */
-@experimental final class capability extends StaticAnnotation
+@experimental
+@deprecated("To make a class a capability, let it derive from the `Capability` trait instead")
+final class capability extends StaticAnnotation

library/src/scala/caps.scala

@@ -4,14 +4,17 @@ import annotation.experimental
 
 @experimental object caps:
 
-  class Cap // should be @erased
+  trait Capability // should be @erased
+
+  /** The universal capture reference */
+  val cap: Capability = new Capability() {}
 
   /** The universal capture reference (deprecated) */
   @deprecated("Use `cap` instead")
-  val `*`: Cap = cap
+  val `*`: Capability = cap
 
-  /** The universal capture reference */
-  val cap: Cap = Cap()
+  @deprecated("Use `Capability` instead")
+  type Cap = Capability
 
   /** Reach capabilities x* which appear as terms in @retains annotations are encoded
    *  as `caps.reachCapability(x)`. When converted to CaptureRef types in capture sets

tests/disabled/pos/lazylist.scala

@@ -34,7 +34,7 @@ object LazyNil extends LazyList[Nothing]:
 def map[A, B](xs: {*} LazyList[A], f: {*} A => B): {f, xs} LazyList[B] =
   xs.map(f)
 
-@annotation.capability class Cap
+class Cap extends caps.Capability
 
 def test(cap1: Cap, cap2: Cap, cap3: Cap) =
   def f[T](x: LazyList[T]): LazyList[T] = if cap1 == cap1 then x else LazyNil