Add path support for capture checking

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -525,8 +525,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   def makeRetaining(parent: Tree, refs: List[Tree], annotName: TypeName)(using Context): Annotated =
     Annotated(parent, New(scalaAnnotationDot(annotName), List(refs)))
 
-  def makeCapsOf(id: Ident)(using Context): Tree =
-    TypeApply(Select(scalaDot(nme.caps), nme.capsOf), id :: Nil)
+  def makeCapsOf(tp: RefTree)(using Context): Tree =
+    TypeApply(Select(scalaDot(nme.caps), nme.capsOf), tp :: Nil)
 
   def makeCapsBound()(using Context): Tree =
     makeRetaining(

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

@@ -194,7 +194,7 @@ extension (tp: Type)
       true
     case tp: TermRef =>
       ((tp.prefix eq NoPrefix)
-      || tp.symbol.is(ParamAccessor) && tp.prefix.isThisTypeOf(tp.symbol.owner)
+      || tp.symbol.isField && !tp.symbol.isStatic && tp.prefix.isTrackableRef
       || tp.isRootCapability
       ) && !tp.symbol.isOneOf(UnstableValueFlags)
     case tp: TypeRef =>

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

@@ -61,18 +61,19 @@ trait CaptureRef extends TypeProxy, ValueType:
     case tp: TermParamRef => tp.underlying.derivesFrom(defn.Caps_Exists)
     case _ => false
 
-  /** Normalize reference so that it can be compared with `eq` for equality */
-  final def normalizedRef(using Context): CaptureRef = this match
-    case tp @ AnnotatedType(parent: CaptureRef, annot) if tp.isTrackableRef =>
-      tp.derivedAnnotatedType(parent.normalizedRef, annot)
-    case tp: TermRef if tp.isTrackableRef =>
-      tp.symbol.termRef
-    case _ => this
+  // With the support of pathes, we don't need to normalize the `TermRef`s anymore.
+  // /** Normalize reference so that it can be compared with `eq` for equality */
+  // final def normalizedRef(using Context): CaptureRef = this match
+  //   case tp @ AnnotatedType(parent: CaptureRef, annot) if tp.isTrackableRef =>
+  //     tp.derivedAnnotatedType(parent.normalizedRef, annot)
+  //   case tp: TermRef if tp.isTrackableRef =>
+  //     tp.symbol.termRef
+  //   case _ => this
 
   /** The capture set consisting of exactly this reference */
   final def singletonCaptureSet(using Context): CaptureSet.Const =
     if mySingletonCaptureSet == null then
-      mySingletonCaptureSet = CaptureSet(this.normalizedRef)
+      mySingletonCaptureSet = CaptureSet(this)
     mySingletonCaptureSet.uncheckedNN
 
   /** The capture set of the type underlying this reference */
@@ -97,27 +98,45 @@ trait CaptureRef extends TypeProxy, ValueType:
    *   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
+   *   TODO: Document path cases
    */
   final def subsumes(y: CaptureRef)(using Context): Boolean =
+
+    def subsumingRefs(x: Type, y: Type): Boolean = x match
+      case x: CaptureRef => y match
+        case y: CaptureRef => x.subsumes(y)
+        case _ => false
+      case _ => false
+
+    def viaInfo(info: Type)(test: Type => Boolean): Boolean = info.match
+      case info: SingletonCaptureRef => test(info)
+      case info: AndType => test(info.tp1) || test(info.tp2)
+      case info: OrType => test(info.tp1) && test(info.tp2)
+      case _ => false
+
     (this eq y)
     || this.isRootCapability
     || y.match
         case y: TermRef =>
-          (y.prefix eq this)
-          || y.info.match
-              case y1: SingletonCaptureRef => this.subsumes(y1)
+            y.prefix.match
+              case ypre: CaptureRef =>
+                this.subsumes(ypre)
+                || this.match
+                    case x @ TermRef(xpre: CaptureRef, _) if x.symbol == y.symbol =>
+                      subsumingRefs(xpre, ypre) && subsumingRefs(ypre, xpre)
+                    case _ =>
+                      false
               case _ => false
+          || viaInfo(y.info)(subsumingRefs(this, _))
         case MaybeCapability(y1) => this.stripMaybe.subsumes(y1)
         case _ => false
     || this.match
         case ReachCapability(x1) => x1.subsumes(y.stripReach)
-        case x: TermRef =>
-          x.info match
-            case x1: SingletonCaptureRef => x1.subsumes(y)
-            case _ => false
+        case x: TermRef => viaInfo(x.info)(subsumingRefs(_, y))
         case x: TermParamRef => subsumesExistentially(x, y)
         case x: TypeRef => assumedContainsOf(x).contains(y)
         case _ => false
+  end subsumes
 
   def assumedContainsOf(x: TypeRef)(using Context): SimpleIdentitySet[CaptureRef] =
     CaptureSet.assumedContains.getOrElse(x, SimpleIdentitySet.empty)

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

@@ -374,7 +374,7 @@ object CaptureSet:
 
   def apply(elems: CaptureRef*)(using Context): CaptureSet.Const =
     if elems.isEmpty then empty
-    else Const(SimpleIdentitySet(elems.map(_.normalizedRef.ensuring(_.isTrackableRef))*))
+    else Const(SimpleIdentitySet(elems.map(_.ensuring(_.isTrackableRef))*))
 
   def apply(elems: Refs)(using Context): CaptureSet.Const =
     if elems.isEmpty then empty else Const(elems)
@@ -508,7 +508,11 @@ object CaptureSet:
         !noUniversal
       else elem match
         case elem: TermRef if level.isDefined =>
-          elem.symbol.ccLevel <= level
+          elem.prefix match
+            case prefix: CaptureRef =>
+              levelOK(prefix)
+            case _ =>
+              elem.symbol.ccLevel <= level
         case elem: ThisType if level.isDefined =>
           elem.cls.ccLevel.nextInner <= level
         case ReachCapability(elem1) =>

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

@@ -13,7 +13,7 @@ import Trees.*
 import typer.RefChecks.{checkAllOverrides, checkSelfAgainstParents, OverridingPairsChecker}
 import typer.Checking.{checkBounds, checkAppliedTypesIn}
 import typer.ErrorReporting.{Addenda, NothingToAdd, err}
-import typer.ProtoTypes.{AnySelectionProto, LhsProto}
+import typer.ProtoTypes.{LhsProto, WildcardSelectionProto}
 import util.{SimpleIdentitySet, EqHashMap, EqHashSet, SrcPos, Property}
 import transform.{Recheck, PreRecheck, CapturedVars}
 import Recheck.*
@@ -122,10 +122,6 @@ object CheckCaptures:
    *  This check is performed at Typer.
    */
   def checkWellformed(parent: Tree, ann: Tree)(using Context): Unit =
-    parent.tpe match
-      case _: SingletonType =>
-        report.error(em"Singleton type $parent cannot have capture set", parent.srcPos)
-      case _ =>
     def check(elem: Tree, pos: SrcPos): Unit = elem.tpe match
       case ref: CaptureRef =>
         if !ref.isTrackableRef then
@@ -187,6 +183,9 @@ object CheckCaptures:
   /** Attachment key for bodies of closures, provided they are values */
   val ClosureBodyValue = Property.Key[Unit]
 
+  /** A prototype that indicates selection with an immutable value */
+  class PathSelectionProto(val sym: Symbol, val pt: Type)(using Context) extends WildcardSelectionProto
+
 class CheckCaptures extends Recheck, SymTransformer:
   thisPhase =>
 
@@ -361,57 +360,67 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  the environment in which `sym` is defined.
      */
     def markFree(sym: Symbol, pos: SrcPos)(using Context): Unit =
-      if sym.exists then
-        val ref = sym.termRef
-        if ref.isTracked then
-          forallOuterEnvsUpTo(sym.enclosure): env =>
-            capt.println(i"Mark $sym with cs ${ref.captureSet} free in ${env.owner}")
-            checkElem(ref, env.captured, pos, provenance(env))
+      markFree(sym, sym.termRef, pos)
+
+    def markFree(sym: Symbol, ref: TermRef, pos: SrcPos)(using Context): Unit =
+      if sym.exists && ref.isTracked then
+        forallOuterEnvsUpTo(sym.enclosure): env =>
+          capt.println(i"Mark $sym with cs ${ref.captureSet} free in ${env.owner}")
+          checkElem(ref, env.captured, pos, provenance(env))
 
     /** Make sure (projected) `cs` is a subset of the capture sets of all enclosing
      *  environments. At each stage, only include references from `cs` that are outside
      *  the environment's owner
      */
     def markFree(cs: CaptureSet, pos: SrcPos)(using Context): Unit =
+      // A captured reference with the symbol `sym` is visible from the environment
+      // if `sym` is not defined inside the owner of the environment.
+      inline def isVisibleFromEnv(sym: Symbol, env: Env) =
+        if env.kind == EnvKind.NestedInOwner then
+          !sym.isProperlyContainedIn(env.owner)
+        else
+          !sym.isContainedIn(env.owner)
+
+      def checkSubsetEnv(cs: CaptureSet, env: Env)(using Context): Unit =
+        // Only captured references that are visible from the environment
+        // should be included.
+        val included = cs.filter: c =>
+          c.stripReach match
+            case ref: NamedType =>
+              val refSym = ref.symbol
+              val refOwner = refSym.owner
+              val isVisible = isVisibleFromEnv(refOwner, env)
+              if isVisible && !ref.isRootCapability then
+                ref match
+                  case ref: TermRef if ref.prefix `ne` NoPrefix =>
+                    // If c is a path of a class defined outside the environment,
+                    // we check the capture set of its info.
+                    checkSubsetEnv(ref.captureSetOfInfo, env)
+                  case _ =>
+              if !isVisible
+                  && (c.isReach || ref.isType)
+                  && (!ccConfig.useSealed || refSym.is(Param))
+                  && refOwner == env.owner
+              then
+                if refSym.hasAnnotation(defn.UnboxAnnot) then
+                  capt.println(i"exempt: $ref in $refOwner")
+                else
+                  // Reach capabilities that go out of scope have to be approximated
+                  // by their underlying capture set, which cannot be universal.
+                  // Reach capabilities of @unboxed parameters are exempted.
+                  val cs = CaptureSet.ofInfo(c)
+                  cs.disallowRootCapability: () =>
+                    report.error(em"Local reach capability $c leaks into capture scope of ${env.ownerString}", pos)
+                  checkSubset(cs, env.captured, pos, provenance(env))
+              isVisible
+            case ref: ThisType => isVisibleFromEnv(ref.cls, env)
+            case _ => false
+        checkSubset(included, env.captured, pos, provenance(env))
+        capt.println(i"Include call or box capture $included from $cs in ${env.owner} --> ${env.captured}")
+
       if !cs.isAlwaysEmpty then
         forallOuterEnvsUpTo(ctx.owner.topLevelClass): env =>
-          // Whether a symbol is defined inside the owner of the environment?
-          inline def isContainedInEnv(sym: Symbol) =
-            if env.kind == EnvKind.NestedInOwner then
-              sym.isProperlyContainedIn(env.owner)
-            else
-              sym.isContainedIn(env.owner)
-          // A captured reference with the symbol `sym` is visible from the environment
-          // if `sym` is not defined inside the owner of the environment
-          inline def isVisibleFromEnv(sym: Symbol) = !isContainedInEnv(sym)
-          // Only captured references that are visible from the environment
-          // should be included.
-          val included = cs.filter: c =>
-            c.stripReach match
-              case ref: NamedType =>
-                val refSym = ref.symbol
-                val refOwner = refSym.owner
-                val isVisible = isVisibleFromEnv(refOwner)
-                if !isVisible
-                    && (c.isReach || ref.isType)
-                    && (!ccConfig.useSealed || refSym.is(Param))
-                    && refOwner == env.owner
-                then
-                  if refSym.hasAnnotation(defn.UnboxAnnot) then
-                    capt.println(i"exempt: $ref in $refOwner")
-                  else
-                    // Reach capabilities that go out of scope have to be approximated
-                    // by their underlying capture set, which cannot be universal.
-                    // Reach capabilities of @unboxed parameters are exempted.
-                    val cs = CaptureSet.ofInfo(c)
-                    cs.disallowRootCapability: () =>
-                      report.error(em"Local reach capability $c leaks into capture scope of ${env.ownerString}", pos)
-                    checkSubset(cs, env.captured, pos, provenance(env))
-                isVisible
-              case ref: ThisType => isVisibleFromEnv(ref.cls)
-              case _ => false
-          checkSubset(included, env.captured, pos, provenance(env))
-          capt.println(i"Include call or box capture $included from $cs in ${env.owner} --> ${env.captured}")
+          checkSubsetEnv(cs, env)
     end markFree
 
     /** Include references captured by the called method in the current environment stack */
@@ -457,11 +466,26 @@ class CheckCaptures extends Recheck, SymTransformer:
         if tree.symbol.info.isParameterless then
           // there won't be an apply; need to include call captures now
           includeCallCaptures(tree.symbol, tree.srcPos)
-      else
+      else if !tree.symbol.isStatic then
         //debugShowEnvs()
-        markFree(tree.symbol, tree.srcPos)
+        def addSelects(ref: TermRef, pt: Type): TermRef = pt match
+          case pt: PathSelectionProto if ref.isTracked =>
+            // if `ref` is not tracked then the selection could not give anything new
+            // class SerializationProxy in stdlib-cc/../LazyListIterable.scala has an example where this matters.
+            addSelects(ref.select(pt.sym).asInstanceOf[TermRef], pt.pt)
+          case _ => ref
+        val ref = tree.symbol.termRef
+        val pathRef = addSelects(ref, pt)
+        //if pathRef ne ref then
+        //  println(i"add selects $ref --> $pathRef")
+        markFree(tree.symbol, if false then ref else pathRef, tree.srcPos)
       super.recheckIdent(tree, pt)
 
+    override def selectionProto(tree: Select, pt: Type)(using Context): Type =
+      val sym = tree.symbol
+      if !sym.isOneOf(UnstableValueFlags) && !sym.isStatic then PathSelectionProto(sym, pt)
+      else super.selectionProto(tree, pt)
+
     /** A specialized implementation of the selection rule.
      *
      *  E |- f: T{ m: R^Cr }^{f}
@@ -488,21 +512,25 @@ class CheckCaptures extends Recheck, SymTransformer:
         case _ => denot
 
       val selType = recheckSelection(tree, qualType, name, disambiguate)
-      val selCs = selType.widen.captureSet
-      if selCs.isAlwaysEmpty
-          || selType.widen.isBoxedCapturing
+      val selWiden = selType.widen
+
+      if pt == LhsProto
           || qualType.isBoxedCapturing
-          || pt == LhsProto
+          || selType.isTrackableRef
+          || selWiden.isBoxedCapturing
+          || selWiden.captureSet.isAlwaysEmpty
       then
         selType
       else
         val qualCs = qualType.captureSet
-        capt.println(i"pick one of $qualType, ${selType.widen}, $qualCs, $selCs in $tree")
+        val selCs = selType.captureSet
+        capt.println(i"pick one of $qualType, ${selType.widen}, $qualCs, $selCs ${selWiden.captureSet} in $tree")
+
         if qualCs.mightSubcapture(selCs)
             && !selCs.mightSubcapture(qualCs)
             && !pt.stripCapturing.isInstanceOf[SingletonType]
         then
-          selType.widen.stripCapturing.capturing(qualCs)
+          selWiden.stripCapturing.capturing(qualCs)
             .showing(i"alternate type for select $tree: $selType --> $result, $qualCs / $selCs", capt)
         else
           selType
@@ -1121,11 +1149,14 @@ class CheckCaptures extends Recheck, SymTransformer:
         (erefs /: erefs.elems): (erefs, eref) =>
           eref match
             case eref: ThisType if isPureContext(ctx.owner, eref.cls) =>
-              erefs ++ arefs.filter {
-                case aref: TermRef => eref.cls.isProperlyContainedIn(aref.symbol.owner)
+              def isOuterRef(aref: Type): Boolean = aref match
+                case aref: TermRef =>
+                  val owner = aref.symbol.owner
+                  if owner.isClass then isOuterRef(aref.prefix)
+                  else eref.cls.isProperlyContainedIn(owner)
                 case aref: ThisType => eref.cls.isProperlyContainedIn(aref.cls)
                 case _ => false
-              }
+              erefs ++ arefs.filter(isOuterRef)
             case _ =>
               erefs
 

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

@@ -518,6 +518,11 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
           info match
             case mt: MethodOrPoly =>
               val psyms = psymss.head
+              // TODO: the substitution does not work for param-dependent method types.
+              // For example, `(x: T, y: x.f.type) => Unit`. In this case, when we
+              // substitute `x.f.type`, `x` becomes a `TermParamRef`. But the new method
+              // type is still under initialization and `paramInfos` is still `null`,
+              // so the new `NamedType` will not have a denoation.
               mt.companion(mt.paramNames)(
                 mt1 =>
                   if !paramSignatureChanges && !mt.isParamDependent && prevLambdas.isEmpty then

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1548,21 +1548,21 @@ object Parsers {
       case _ => None
     }
 
-    /** CaptureRef  ::=  ident [`*` | `^`] | `this`
+    /** CaptureRef  ::=  SimpleRef { `.` id } [`*` | `^`]
      */
     def captureRef(): Tree =
-      if in.token == THIS then simpleRef()
-      else
-        val id = termIdent()
-        if isIdent(nme.raw.STAR) then
-          in.nextToken()
-          atSpan(startOffset(id)):
-            PostfixOp(id, Ident(nme.CC_REACH))
-        else if isIdent(nme.UPARROW) then
-          in.nextToken()
-          atSpan(startOffset(id)):
-            makeCapsOf(cpy.Ident(id)(id.name.toTypeName))
-        else id
+      val ref = dotSelectors(simpleRef())
+      if isIdent(nme.raw.STAR) then
+        in.nextToken()
+        atSpan(startOffset(ref)):
+          PostfixOp(ref, Ident(nme.CC_REACH))
+      else if isIdent(nme.UPARROW) then
+        in.nextToken()
+        atSpan(startOffset(ref)):
+          convertToTypeId(ref) match
+            case ref: RefTree => makeCapsOf(ref)
+            case ref => ref
+      else ref
 
     /**  CaptureSet ::=  `{` CaptureRef {`,` CaptureRef} `}`    -- under captureChecking
      */