Introduce @capability annotations

This replaces the earlier @ability annotation. The mechanisms
are different, though. @ability was an annotation on `val`s whereas
`capability` is an annotation on classes.

Author: odersky

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

@@ -523,7 +523,9 @@ object CaptureSet:
         tp.captureSet
       case tp: TermParamRef =>
         tp.captureSet
-      case _: TypeRef | _: TypeParamRef =>
+      case _: TypeRef =>
+        if tp.classSymbol.hasAnnotation(defn.CapabilityAnnot) then universal else empty
+      case _: TypeParamRef =>
         empty
       case CapturingType(parent, refs, _) =>
         recur(parent) ++ refs

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

@@ -951,6 +951,7 @@ 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")
@@ -995,7 +996,6 @@ class Definitions {
   @tu lazy val VarargsAnnot: ClassSymbol = requiredClass("scala.annotation.varargs")
   @tu lazy val SinceAnnot: ClassSymbol = requiredClass("scala.annotation.since")
   @tu lazy val RetainsAnnot: ClassSymbol = requiredClass("scala.retains")
-  @tu lazy val AbilityAnnot: ClassSymbol = requiredClass("scala.annotation.ability")
 
   @tu lazy val JavaRepeatableAnnot: ClassSymbol = requiredClass("java.lang.annotation.Repeatable")
 

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

@@ -2767,7 +2767,6 @@ object Types {
     def canBeTracked(using Context) =
       ((prefix eq NoPrefix)
       || symbol.is(ParamAccessor) && (prefix eq symbol.owner.thisType)
-      || symbol.hasAnnotation(defn.AbilityAnnot)
       || isRootCapability
       ) && !symbol.is(Method)
 

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

@@ -406,8 +406,7 @@ class CheckCaptures extends Recheck:
     def checkNotGlobal(tree: Tree, tp: Type, allArgs: Tree*)(using Context): Unit =
       for ref <-tp.captureSet.elems do
         val isGlobal = ref match
-          case ref: TermRef =>
-            ref.isRootCapability || ref.prefix != NoPrefix && ref.symbol.hasAnnotation(defn.AbilityAnnot)
+          case ref: TermRef => ref.isRootCapability
           case _ => false
         if isGlobal then
           val what = if ref.isRootCapability then "universal" else "global"

library/src-bootstrapped/scala/annotation/ability.scala

@@ -0,0 +1,13 @@
+package scala.annotation
+
+/** Marks an annotated class as a capabulity.
+ *  If the annotation is present and -Ycc is set, any (possibly aliased
+ *  or refined) instance of the class type is implicitly augmented with
+ *  the universal capture set. Example
+ *
+ *    @capability class CanThrow[T]
+ *
+ *  THere, the capture set of any instance of `CanThrow` is assumed to be
+ *  `{*}`.
+ */
+final class capability extends StaticAnnotation

library/src/scala/annotation/capability.scala

@@ -34,8 +34,7 @@ object LazyNil extends LazyList[Nothing]:
 def map[A, B](xs: {*} LazyList[A], f: {*} A => B): {f, xs} LazyList[B] =
   xs.map(f)
 
-class CC
-type Cap = {*} CC
+@annotation.capability class Cap
 
 def test(cap1: Cap, cap2: Cap, cap3: Cap) =
   def f[T](x: LazyList[T]): LazyList[T] = if cap1 == cap1 then x else LazyNil

tests/disabled/pos/lazylist.scala

@@ -1,5 +1,4 @@
-class CC
-type Cap = {*} CC
+@annotation.capability class Cap
 
 def test(cap1: Cap, cap2: Cap) =
   def f() = if cap1 == cap1 then g else g

tests/neg-custom-args/captures/byname.scala

@@ -0,0 +1,28 @@
+import annotation.capability
+
+@capability class Cap
+def f1(c: Cap): {c} () => c.type = () => c // ok
+
+def f2: Int =
+  val g: {*} Boolean => Int = ???
+  val x = g(true)
+  x
+
+def f3: Int =
+  def g: {*} Boolean => Int = ???
+  def h = g
+  val x = g.apply(true)
+  x
+
+def foo() =
+  val x: Cap = ???
+  val y: Cap = x
+  val x2: {x} () => Cap = ???
+  val y2: {x} () => Cap = x2
+
+  val z1: {*} () => Cap = f1(x)
+  def h[X](a: X)(b: X) = a
+
+  val z2 =
+    if x == null then () => x else () => Cap()
+  x

tests/pos-custom-args/captures/capt-capability.scala

@@ -1,5 +1,5 @@
 import language.experimental.erasedDefinitions
-import annotation.ability
+import annotation.capability
 import java.io.IOException
 
 class CT[-E]  // variance is needed for correct rechecking inference

tests/pos-custom-args/captures/try3.scala

@@ -11,9 +11,6 @@ object A {
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.type
 
   1: v.
@@ -25,16 +22,10 @@ object A {
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.T
 
   val u = new B { type T = Int }
   u: u.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
-    x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.
@@ -55,8 +46,5 @@ object A {
     T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#
     T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#
     T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#
-    T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#
-    T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#
-    T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#
     T#T#T#T#T#T#T#T#T#T#T#T#T#T#T#T
 }