Allow only reach capabilities of parameters in deep capture sets

Author: odersky

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

@@ -232,14 +232,11 @@ extension (tp: Type)
    */
   def deepCaptureSet(using Context): CaptureSet =
     val dcs = CaptureSet.ofTypeDeeply(tp.widen.stripCapturing)
-    def reachCanSubsumDcs =
-      dcs.isUniversal
-      || dcs.elems.forall(c => c.pathOwner.isContainedIn(tp.pathOwner))
     if dcs.isAlwaysEmpty then tp.captureSet
     else tp match
-      case tp @ ReachCapability(_) if reachCanSubsumDcs =>
+      case tp @ ReachCapability(_) if tp.isParamPath =>
         tp.singletonCaptureSet
-      case tp: SingletonCaptureRef if tp.isTrackableRef && reachCanSubsumDcs =>
+      case tp: SingletonCaptureRef if tp.isTrackableRef && tp.isParamPath =>
         tp.reach.singletonCaptureSet
       case _ =>
         tp.captureSet ++ dcs
@@ -297,6 +294,13 @@ extension (tp: Type)
     case tp1: ThisType => tp1.cls
     case _ => NoSymbol
 
+  final def isParamPath(using Context): Boolean = tp.dealias match
+    case tp1: NamedType =>
+      tp1.prefix match
+        case _: ThisType | NoPrefix => tp1.symbol.isOneOf(Param | ParamAccessor)
+        case prefix => prefix.isParamPath
+    case _ => false
+
   /** If this is a unboxed capturing type with nonempty capture set, its boxed version.
    *  Or, if type is a TypeBounds of capturing types, the version where the bounds are boxed.
    *  The identity for all other types.

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

@@ -1119,12 +1119,18 @@ object CaptureSet:
    *  replaces caps with reach capabilties.
    */
   def ofTypeDeeply(tp: Type)(using Context): CaptureSet =
+    object ReachesMap extends IdempotentCaptRefMap:
+      def apply(t: Type): Type = t match
+        case ReachCapability(ref) if !ref.isParamPath =>
+          defn.Caps_CapSet.typeRef.capturing(ref.deepCaptureSet)
+        case _ =>
+          t
     val collect = new TypeAccumulator[CaptureSet]:
       def apply(cs: CaptureSet, t: Type) =
         if variance <= 0 then cs
         else t.dealias match
           case t @ CapturingType(p, cs1) =>
-            this(cs, p) ++ cs1
+            this(cs, p) ++ cs1.map(ReachesMap)
           case t @ AnnotatedType(parent, ann) =>
             this(cs, parent)
           case t @ FunctionOrMethod(args, res @ Existential(_, _))

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

@@ -751,7 +751,8 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
             report.warning(em"redundant capture: $dom already accounts for $ref", pos)
 
         if ref.captureSetOfInfo.elems.isEmpty && !ref.derivesFrom(defn.Caps_Capability) then
-          report.error(em"$ref cannot be tracked since its capture set is empty", pos)
+          val deepStr = if ref.isReach then " deep" else ""
+          report.error(em"$ref cannot be tracked since its$deepStr capture set is empty", pos)
         check(parent.captureSet, parent)
 
         val others =

tests/neg-custom-args/captures/delayedRunops.check

@@ -3,3 +3,13 @@
    |             ^^^^
    |             reference ops* is not included in the allowed capture set {}
    |             of an enclosing function literal with expected type () -> Unit
+-- Error: tests/neg-custom-args/captures/delayedRunops.scala:21:13 -----------------------------------------------------
+21 |      runOps(ops1)  // error
+   |             ^^^^
+   |             reference (caps.cap : caps.Capability) is not included in the allowed capture set {}
+   |             of an enclosing function literal with expected type () -> Unit
+-- Error: tests/neg-custom-args/captures/delayedRunops.scala:27:13 -----------------------------------------------------
+27 |      runOps(ops1)  // error
+   |             ^^^^
+   |             reference ops* is not included in the allowed capture set {}
+   |             of an enclosing function literal with expected type () -> Unit

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

@@ -13,3 +13,15 @@ import language.experimental.captureChecking
     () =>
       val ops1 = ops
       runOps(ops1)  // error
+
+  // unsound: impure operation pretended pure
+  def delayedRunOps2(ops: List[() => Unit]): () ->{} Unit =
+    () =>
+      val ops1: List[() => Unit] = ops
+      runOps(ops1)  // error
+
+  // unsound: impure operation pretended pure
+  def delayedRunOps3(ops: List[() => Unit]): () ->{} Unit =
+    () =>
+      val ops1: List[() ->{ops*} Unit] = ops
+      runOps(ops1)  // error

tests/neg-custom-args/captures/delayedRunops2.scala

@@ -8,3 +8,15 @@ def app[T, U](x: T, op: T => U): () ->{op} U =
 
 def unsafeRunOps(ops: List[() => Unit]): () ->{} Unit =
   app[List[() ->{ops*} Unit], Unit](ops, runOps) // error
+
+def app2[T, U](x: T, op: T => U): () ->{op} U =
+  () =>
+    def y: T = x
+    op(y)
+
+def unsafeRunOps2(ops: List[() => Unit]): () -> Unit =
+  app2[List[() => Unit], Unit](ops, runOps: List[() => Unit] -> Unit) // error
+
+
+
+

tests/neg-custom-args/captures/reaches.check

@@ -47,8 +47,8 @@
 -- Error: tests/neg-custom-args/captures/reaches.scala:60:31 -----------------------------------------------------------
 60 |  val leaked = usingFile[File^{id*}]: f => // error
    |                               ^^^
-   |                               id* cannot be tracked since its capture set is empty
+   |                               id* cannot be tracked since its deep capture set is empty
 -- Error: tests/neg-custom-args/captures/reaches.scala:61:18 -----------------------------------------------------------
 61 |    val f1: File^{id*} = id(f) // error, since now id(f): File^ // error
    |                  ^^^
-   |                  id* cannot be tracked since its capture set is empty
+   |                  id* cannot be tracked since its deep capture set is empty

tests/neg-custom-args/captures/wf-reach-1.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg-custom-args/captures/wf-reach-1.scala:2:17 ---------------------------------------------------------
+2 |  val y: Object^{x*} = ??? // error
+  |                 ^^
+  |                 x* cannot be tracked since its deep capture set is empty

tests/neg-custom-args/captures/wf-reach-1.scala

@@ -0,0 +1,2 @@
+def test(x: List[() -> Unit]) =
+  val y: Object^{x*} = ??? // error

tests/neg/leak-problem.scala

@@ -15,8 +15,7 @@ def useBoxedAsync(x: Box[Async^]): Unit =
 
 def useBoxedAsync1(x: Box[Async^]): Unit = x.get.read() // now ok
 
-def test(): Unit =
-  val xs: Box[Async^] = ???
+def test(xs: Box[Async^]): Unit =
   val xsLambda = () => useBoxedAsync(xs)
   val _: () ->{xs*} Unit = xsLambda
   val _: () -> Unit = xsLambda // error
@@ -27,8 +26,8 @@ def test(): Unit =
     t1.read()
 
   val xsLambda2 = () => useBoxedAsync2(xs)
-  val _: () ->{xs*} Unit = xsLambda
-  val _: () -> Unit = xsLambda // error
+  val _: () ->{xs*} Unit = xsLambda2
+  val _: () -> Unit = xsLambda2 // error
 
   val f: Box[Async^] => Unit = (x:  Box[Async^]) => useBoxedAsync(x)