Changes around reaches and uses (#23584)

Author: odersky

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

@@ -843,6 +843,7 @@ object Capabilities:
     case LambdaActual(restp: Type)
     case OverriddenType(member: Symbol)
     case DeepCS(ref: TypeRef)
+    case Parameter(param: Symbol)
     case Unknown
 
     def explanation(using Context): String = this match
@@ -876,6 +877,8 @@ object Capabilities:
         i" when instantiating upper bound of member overridden by $member"
       case DeepCS(ref: TypeRef) =>
         i" when computing deep capture set of $ref"
+      case Parameter(param) =>
+        i" of parameter $param of ${param.owner}"
       case Unknown =>
         ""
   end Origin
@@ -942,8 +945,8 @@ object Capabilities:
       CapToFresh(origin)(tp)
 
   /** Maps fresh to cap */
-  def freshToCap(tp: Type)(using Context): Type =
-    CapToFresh(Origin.Unknown).inverse(tp)
+  def freshToCap(param: Symbol, tp: Type)(using Context): Type =
+    CapToFresh(Origin.Parameter(param)).inverse(tp)
 
   /** Map top-level free existential variables one-to-one to Fresh instances */
   def resultToFresh(tp: Type, origin: Origin)(using Context): Type =

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

@@ -99,9 +99,17 @@ extension (tp: Type)
     case AnnotatedType(tp1, ann) if tp1.derivesFrom(defn.Caps_CapSet) && ann.symbol.isRetains =>
       ann.tree.retainedSet.retainedElementsRaw
     case tp =>
-      // Nothing is a special type to represent the empty set
-      if tp.isNothingType then Nil
-      else tp :: Nil // should be checked by wellformedness
+      tp.dealiasKeepAnnots match
+        case tp: TypeRef if tp.symbol == defn.Caps_CapSet =>
+          // This can happen in cases where we try to type an eta expansion `$x => f($x)`
+          // from a polymorphic target type using capture sets. In that case the parameter type
+          // of $x is not treated as inferred and is approximated to CapSet. An example is
+          // capset-problem.scala. We handle these cases by appromxating to the empty set.
+          Nil
+        case _ =>
+          // Nothing is a special type to represent the empty set
+          if tp.isNothingType then Nil
+          else tp :: Nil // should be checked by wellformedness
 
   /** A list of capabilities of a retained set. */
   def retainedElements(using Context): List[Capability] =
@@ -575,18 +583,30 @@ extension (sym: Symbol)
   def hasTrackedParts(using Context): Boolean =
     !CaptureSet.ofTypeDeeply(sym.info).isAlwaysEmpty
 
-  /** `sym` itself or its info is annotated @use or it is a type parameter with a matching
-   *  @use-annotated term parameter that contains `sym` in its deep capture set.
+  /** Until 3.7:
+   *    `sym` itself or its info is annotated @use or it is a type parameter with a matching
+   *    @use-annotated term parameter that contains `sym` in its deep capture set.
+   *  From 3.8:
+   *    `sym` is a capset parameter without a `@reserve` annotation that
+   *      - belongs to a class in a class, or
+   *      - belongs to a method where it appears in a the deep capture set of a following term parameter of the same method.
    */
   def isUseParam(using Context): Boolean =
     sym.hasAnnotation(defn.UseAnnot)
     || sym.info.hasAnnotation(defn.UseAnnot)
     || sym.is(TypeParam)
-        && sym.owner.rawParamss.nestedExists: param =>
-            param.is(TermParam) && param.hasAnnotation(defn.UseAnnot)
-            && param.info.deepCaptureSet.elems.exists:
-                case c: TypeRef => c.symbol == sym
-                case _ => false
+        && !sym.info.hasAnnotation(defn.ReserveAnnot)
+        && (sym.owner.isClass
+            || sym.owner.rawParamss.nestedExists: param =>
+                param.is(TermParam)
+                && (!ccConfig.allowUse || param.hasAnnotation(defn.UseAnnot))
+                && param.info.deepCaptureSet.elems.exists:
+                    case c: TypeRef => c.symbol == sym
+                    case _ => false
+            || {
+              //println(i"not is use param $sym")
+              false
+            })
 
   /** `sym` or its info is annotated with `@consume`. */
   def isConsumeParam(using Context): Boolean =

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

@@ -468,11 +468,11 @@ sealed abstract class CaptureSet extends Showable:
       case elem: FreshCap => elem.ccOwner.isContainedIn(rootLimit)
       case _ => false
 
-  /** Invoke `handler` if this set has (or later aquires) a root capability.
-   *  Excluded are Fresh instances unless their ccOwner is contained in `upto`.
+  /** Invoke `handler` if this set has (or later aquires) a bad root capability.
+   *  Fresh instances count as good as long as their ccOwner is outside `upto`.
    *  If `upto` is NoSymbol, all Fresh instances are admitted.
    */
-  def disallowRootCapability(upto: Symbol)(handler: () => Context ?=> Unit)(using Context): this.type =
+  def disallowBadRoots(upto: Symbol)(handler: () => Context ?=> Unit)(using Context): this.type =
     if elems.exists(isBadRoot(upto, _)) then handler()
     this
 
@@ -720,7 +720,7 @@ object CaptureSet:
     private def narrowClassifier(cls: ClassSymbol)(using Context): Unit =
       val newClassifier = leastClassifier(classifier, cls)
       if newClassifier == defn.NothingClass then
-        println(i"conflicting classifications for $this, was $classifier, now $cls")
+        capt.println(i"conflicting classifications for $this, was $classifier, now $cls")
       myClassifier = newClassifier
 
     override def adoptClassifier(cls: ClassSymbol)(using Context): Unit =
@@ -840,10 +840,10 @@ object CaptureSet:
       || isConst
       || varState.canRecord && { includeDep(cs); true }
 
-    override def disallowRootCapability(upto: Symbol)(handler: () => Context ?=> Unit)(using Context): this.type =
+    override def disallowBadRoots(upto: Symbol)(handler: () => Context ?=> Unit)(using Context): this.type =
       rootLimit = upto
       rootAddedHandler = handler
-      super.disallowRootCapability(upto)(handler)
+      super.disallowBadRoots(upto)(handler)
 
     override def ensureWellformed(handler: Capability => (Context) ?=> Unit)(using Context): this.type =
       newElemAddedHandler = handler
@@ -945,7 +945,7 @@ object CaptureSet:
    *  Test case: Without that tweak, logger.scala would not compile.
    */
   class RefiningVar(owner: Symbol)(using Context) extends Var(owner):
-    override def disallowRootCapability(upto: Symbol)(handler: () => Context ?=> Unit)(using Context) = this
+    override def disallowBadRoots(upto: Symbol)(handler: () => Context ?=> Unit)(using Context) = this
 
   /** A variable that is derived from some other variable via a map or filter. */
   abstract class DerivedVar(owner: Symbol, initialElems: Refs)(using @constructorOnly ctx: Context)
@@ -1306,7 +1306,7 @@ object CaptureSet:
                 |cannot be included in outer capture set $cs"""
           else if !elem.tryClassifyAs(cs.classifier) then
             i"""capability ${elem} is not classified as ${cs.classifier}, therefore it
-                |cannot be included in capture set $cs of ${cs.classifier} elements"""
+                |cannot be included in capture set $cs of ${cs.classifier.name} elements"""
           else if cs.isBadRoot(elem) then
             elem match
               case elem: FreshCap =>

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

@@ -120,11 +120,11 @@ object CheckCaptures:
         report.error(em"$elem: $tpe is not a legal element of a capture set", ann.srcPos)
     ann.retainedSet.retainedElementsRaw.foreach(check)
 
-  /** Under the sealed policy, report an error if some part of `tp` contains the
-   *  root capability in its capture set or if it refers to a type parameter that
-   *  could possibly be instantiated with cap in a way that's visible at the type.
+  /** Disallow bad roots anywhere in type `tp``.
+   *  @param  upto  controls up to which owner local fresh capabilities should be disallowed.
+   *                See disallowBadRoots for details.
    */
-  private def disallowRootCapabilitiesIn(tp: Type, upto: Symbol, what: String, have: String, addendum: String, pos: SrcPos)(using Context) =
+  private def disallowBadRootsIn(tp: Type, upto: Symbol, what: => String, have: => String, addendum: => String, pos: SrcPos)(using Context) =
     val check = new TypeTraverser:
 
       private val seen = new EqHashSet[TypeRef]
@@ -151,7 +151,7 @@ object CheckCaptures:
           case CapturingType(parent, refs) =>
             if variance >= 0 then
               val openScopes = openExistentialScopes
-              refs.disallowRootCapability(upto): () =>
+              refs.disallowBadRoots(upto): () =>
                 def part =
                   if t eq tp then ""
                   else
@@ -179,7 +179,10 @@ object CheckCaptures:
           case t =>
             traverseChildren(t)
     check.traverse(tp)
-  end disallowRootCapabilitiesIn
+  end disallowBadRootsIn
+
+  private def ownerStr(owner: Symbol)(using Context): String =
+    if owner.isAnonymousFunction then "enclosing function" else owner.show
 
   trait CheckerAPI:
     /** Complete symbol info of a val or a def */
@@ -490,7 +493,7 @@ class CheckCaptures extends Recheck, SymTransformer:
               case c1: CoreCapability =>
                 CaptureSet.ofType(c1.widen, followResult = true)
           capt.println(i"Widen reach $c to $underlying in ${env.owner}")
-          underlying.disallowRootCapability(NoSymbol): () =>
+          underlying.disallowBadRoots(NoSymbol): () =>
             report.error(em"Local capability $c${env.owner.qualString("in")} cannot have `cap` as underlying capture set", tree.srcPos)
           recur(underlying, env, lastEnv)
 
@@ -534,20 +537,26 @@ class CheckCaptures extends Recheck, SymTransformer:
           	// Under deferredReaches, don't propagate out of methods inside terms.
           	// The use set of these methods will be charged when that method is called.
 
-      recur(cs, curEnv, null)
-      useInfos += ((tree, cs, curEnv))
+      if !cs.isAlwaysEmpty then
+        recur(cs, curEnv, null)
+        useInfos += ((tree, cs, curEnv))
     end markFree
 
-    /** If capability `c` refers to a parameter that is not @use declared, report an error.
+    /** If capability `c` refers to a parameter that is not implicitly or explicitly
+     *  @use declared, report an error.
      */
     def checkUseDeclared(c: Capability, pos: SrcPos)(using Context): Unit =
       c.paramPathRoot match
         case ref: NamedType if !ref.symbol.isUseParam =>
           val what = if ref.isType then "Capture set parameter" else "Local reach capability"
-          val owner = ref.symbol.owner
+          def mitigation =
+            if ccConfig.allowUse
+            then i"\nTo allow this, the ${ref.symbol} should be declared with a @use annotation."
+            else if !ref.isType then i"\nYou could try to abstract the capabilities referred to by $c in a capset variable."
+            else ""
           report.error(
-            em"""$what $c leaks into capture scope${owner.qualString("of")}.
-                |To allow this, the ${ref.symbol} should be declared with a @use annotation""", pos)
+            em"$what $c leaks into capture scope${ref.symbol.owner.qualString("of")}.$mitigation",
+            pos)
         case _ =>
 
     /** Include references captured by the called method in the current environment stack */
@@ -570,16 +579,16 @@ class CheckCaptures extends Recheck, SymTransformer:
         case _ =>
           tp
 
-    /** Under the sealed policy, disallow the root capability in type arguments.
-     *  Type arguments come either from a TypeApply node or from an AppliedType
+    /** Type arguments come either from a TypeApply node or from an AppliedType
      *  which represents a trait parent in a template.
-     *  Also, if a corresponding formal type parameter is declared or implied @use,
-     *  charge the deep capture set of the argument to the environent.
+     *   - Disallow global cap and result caps in such arguments.
+     *   - If a corresponding formal type parameter is declared or implied @use,
+     *     charge the deep capture set of the argument to the environent.
      *  @param  fn   the type application, of type TypeApply or TypeTree
      *  @param  sym  the constructor symbol (could be a method or a val or a class)
      *  @param  args the type arguments
      */
-    def disallowCapInTypeArgs(fn: Tree, sym: Symbol, args: List[Tree])(using Context): Unit =
+    def markFreeTypeArgs(fn: Tree, sym: Symbol, args: List[Tree])(using Context): Unit =
       def isExempt = sym.isTypeTestOrCast || defn.capsErasedValueMethods.contains(sym)
       if !isExempt then
         val paramNames = atPhase(thisPhase.prev):
@@ -592,17 +601,17 @@ class CheckCaptures extends Recheck, SymTransformer:
 
         for case (arg: TypeTree, pname) <- args.lazyZip(paramNames) do
           def where = if sym.exists then i" in an argument of $sym" else ""
-          val (addendum, errTree) =
+          def addendum =
             if arg.isInferred
-            then (i"\nThis is often caused by a local capability$where\nleaking as part of its result.", fn)
-            else if arg.span.exists then ("", arg)
-            else ("", fn)
-          disallowRootCapabilitiesIn(arg.nuType, NoSymbol,
+            then i"\nThis is often caused by a local capability$where\nleaking as part of its result."
+            else ""
+          def errTree = if !arg.isInferred && arg.span.exists then arg else fn
+          disallowBadRootsIn(arg.nuType, NoSymbol,
             i"Type variable $pname of $sym", "be instantiated to", addendum, errTree.srcPos)
 
           val param = fn.symbol.paramNamed(pname)
           if param.isUseParam then markFree(arg.nuType.deepCaptureSet, errTree)
-    end disallowCapInTypeArgs
+    end markFreeTypeArgs
 
     /** Rechecking idents involves:
      *   - adding call captures for idents referring to methods
@@ -870,7 +879,7 @@ class CheckCaptures extends Recheck, SymTransformer:
         case fun @ Select(qual, nme.apply) => qual.symbol.orElse(fun.symbol)
         case fun => fun.symbol
       def methDescr = if meth.exists then i"$meth's type " else ""
-      disallowCapInTypeArgs(tree.fun, meth, tree.args)
+      markFreeTypeArgs(tree.fun, meth, tree.args)
       val funType = super.recheckTypeApply(tree, pt)
       val res = resultToFresh(funType, Origin.ResultInstance(funType, meth))
       includeCallCaptures(tree.symbol, res, tree)
@@ -919,7 +928,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             assert(params.hasSameLengthAs(argTypes), i"$mdef vs $pt, ${params}")
             for (argType, param) <- argTypes.lazyZip(params) do
               val paramTpt = param.asInstanceOf[ValDef].tpt
-              val paramType = freshToCap(paramTpt.nuType)
+              val paramType = freshToCap(param.symbol, paramTpt.nuType)
               checkConformsExpr(argType, paramType, param)
                 .showing(i"compared expected closure formal $argType against $param with ${paramTpt.nuType}", capt)
             if !pt.isInstanceOf[RefinedType]
@@ -986,7 +995,7 @@ class CheckCaptures extends Recheck, SymTransformer:
                 i"\n\nNote that $sym does not count as local since it is captured by $enclStr"
               case _ =>
                 ""
-            disallowRootCapabilitiesIn(
+            disallowBadRootsIn(
               tree.tpt.nuType, NoSymbol, i"Mutable $sym", "have type", addendum, sym.srcPos)
           checkInferredResult(super.recheckValDef(tree, sym), tree)
       finally
@@ -1169,7 +1178,7 @@ class CheckCaptures extends Recheck, SymTransformer:
         for case tpt: TypeTree <- impl.parents do
           tpt.tpe match
             case AppliedType(fn, args) =>
-              disallowCapInTypeArgs(tpt, fn.typeSymbol, args.map(TypeTree(_)))
+              markFreeTypeArgs(tpt, fn.typeSymbol, args.map(TypeTree(_)))
             case _ =>
         ccState.inNestedLevelUnless(cls.is(Module)):
           super.recheckClassDef(tree, impl, cls)
@@ -1202,7 +1211,7 @@ class CheckCaptures extends Recheck, SymTransformer:
         recheck(tree.expr, pt)
       val tp = recheckTryRest(bodyType, tree.cases, tree.finalizer, pt)
       if Feature.enabled(Feature.saferExceptions) then
-        disallowRootCapabilitiesIn(tp, ctx.owner,
+        disallowBadRootsIn(tp, ctx.owner,
           "The result of `try`", "have type",
           "\nThis is often caused by a locally generated exception capability leaking as part of its result.",
           tree.srcPos)
@@ -1747,7 +1756,7 @@ class CheckCaptures extends Recheck, SymTransformer:
 
         override def checkInheritedTraitParameters: Boolean = false
 
-        /** Check that overrides don't change the @use or @consume status of their parameters */
+        /** Check that overrides don't change the @use, @consume, or @reserve status of their parameters */
         override def additionalChecks(member: Symbol, other: Symbol)(using Context): Unit =
           for
             (params1, params2) <- member.rawParamss.lazyZip(other.rawParamss)
@@ -1764,6 +1773,7 @@ class CheckCaptures extends Recheck, SymTransformer:
 
             checkAnnot(defn.UseAnnot)
             checkAnnot(defn.ConsumeAnnot)
+            checkAnnot(defn.ReserveAnnot)
       end OverridingPairsCheckerCC
 
       def traverse(t: Tree)(using Context) =
@@ -1946,7 +1956,7 @@ class CheckCaptures extends Recheck, SymTransformer:
               if !(pos.span.isSynthetic && ctx.reporter.errorsReported)
                 && !arg.typeSymbol.name.is(WildcardParamName)
               then
-                CheckCaptures.disallowRootCapabilitiesIn(arg, NoSymbol,
+                CheckCaptures.disallowBadRootsIn(arg, NoSymbol,
                   "Array", "have element type", "",
                   pos)
               traverseChildren(t)
@@ -1991,7 +2001,13 @@ class CheckCaptures extends Recheck, SymTransformer:
               case c: DerivedCapability =>
                 checkElem(c.underlying)
               case c: FreshCap =>
-                check(c.hiddenSet)
+                c.origin match
+                  case Origin.Parameter(param) =>
+                    report.error(
+                      em"Local $c created in type of $param leaks into capture scope${param.owner.qualString("of")}",
+                      tree.srcPos)
+                  case _ =>
+                    check(c.hiddenSet)
               case _ =>
 
         check(uses)