Refactor VarState

 - Drop Frozen enum
 - Make VarState subclasses inner classes of companion object
 - Rename them
 - Give implicit parameter VarState of subCapture method a default value

Author: odersky

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

@@ -16,7 +16,7 @@ import config.Feature
 import collection.mutable
 import CCState.*
 import reporting.Message
-import CaptureSet.Frozen
+import CaptureSet.VarState
 
 /** Attachment key for capturing type trees */
 private val Captures: Key[CaptureSet] = Key()
@@ -251,7 +251,7 @@ extension (tp: Type)
    *  the two capture sets are combined.
    */
   def capturing(cs: CaptureSet)(using Context): Type =
-    if (cs.isAlwaysEmpty || cs.isConst && cs.subCaptures(tp.captureSet, Frozen.All).isOK)
+    if (cs.isAlwaysEmpty || cs.isConst && cs.subCaptures(tp.captureSet, VarState.Separate).isOK)
         && !cs.keepAlways
     then tp
     else tp match

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

@@ -13,6 +13,7 @@ import CCState.*
 import Periods.NoRunId
 import compiletime.uninitialized
 import StdNames.nme
+import CaptureSet.VarState
 
 /** A trait for references in CaptureSets. These can be NamedTypes, ThisTypes or ParamRefs,
  *  as well as three kinds of AnnotatedTypes representing readOnly, reach, and maybe capabilities.
@@ -134,8 +135,6 @@ trait CaptureRef extends TypeProxy, ValueType:
   final def invalidateCaches() =
     myCaptureSetRunId = NoRunId
 
-  import CaptureSet.{VarState, FrozenAllState}
-
   /**  x subsumes x
    *   x =:= y       ==>  x subsumes y
    *   x subsumes y  ==>  x subsumes y.f
@@ -150,7 +149,7 @@ trait CaptureRef extends TypeProxy, ValueType:
    *
    *   TODO: Move to CaptureSet
    */
-  final def subsumes(y: CaptureRef)(using ctx: Context, vs: VarState = FrozenAllState): Boolean =
+  final def subsumes(y: CaptureRef)(using ctx: Context, vs: VarState = VarState.Separate): Boolean =
 
     def subsumingRefs(x: Type, y: Type): Boolean = x match
       case x: CaptureRef => y match
@@ -166,7 +165,7 @@ trait CaptureRef extends TypeProxy, ValueType:
       case _ => false
 
     (this eq y)
-    || maxSubsumes(y, canAddHidden = vs.frozen == CaptureSet.Frozen.Vars)
+    || maxSubsumes(y, canAddHidden = !vs.isOpen)
     || y.match
         case y: TermRef if !y.isCap =>
             y.prefix.match
@@ -223,18 +222,11 @@ trait CaptureRef extends TypeProxy, ValueType:
    *                       the test again with canAddHidden = true as a last effort before we
    *                       fail a comparison.
    */
-  def maxSubsumes(y: CaptureRef, canAddHidden: Boolean)(using ctx: Context, vs: VarState = FrozenAllState): Boolean =
+  def maxSubsumes(y: CaptureRef, canAddHidden: Boolean)(using ctx: Context, vs: VarState = VarState.Separate): Boolean =
     this.match
       case Fresh.Cap(hidden) =>
-        if vs.ifNotSeen(this)(hidden.elems.exists(_.subsumes(y))) then true
-        else if !y.stripReadOnly.isCap && hidden.recordElemsState() then
-          if canAddHidden then
-            if vs != CaptureSet.FrozenUnrecordedHiddenState then
-              hidden.elems += y
-            true
-          else
-            false
-        else false
+        vs.ifNotSeen(this)(hidden.elems.exists(_.subsumes(y)))
+        || !y.stripReadOnly.isCap && canAddHidden && vs.addHidden(hidden, y)
       case _ =>
         this.isCap && canAddHidden
         || y.match

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

@@ -147,7 +147,7 @@ sealed abstract class CaptureSet extends Showable:
    *  capture set.
    */
   protected final def addNewElem(elem: CaptureRef)(using ctx: Context, vs: VarState): CompareResult =
-    if elem.isMaxCapability || vs.frozen != Frozen.None then
+    if elem.isMaxCapability || !vs.isOpen then
       addThisElem(elem)
     else
       addThisElem(elem).orElse:
@@ -168,7 +168,7 @@ sealed abstract class CaptureSet extends Showable:
   protected def addThisElem(elem: CaptureRef)(using Context, VarState): CompareResult
 
   protected def addHiddenElem(elem: CaptureRef)(using ctx: Context, vs: VarState): CompareResult =
-    if elems.exists(_.maxSubsumes(elem, canAddHidden = vs.frozen != Frozen.All))
+    if elems.exists(_.maxSubsumes(elem, canAddHidden = true))
     then CompareResult.OK
     else CompareResult.Fail(this :: Nil)
 
@@ -177,13 +177,13 @@ sealed abstract class CaptureSet extends Showable:
 
   /** If `cs` is a variable, add this capture set as one of its dependent sets */
   protected def addAsDependentTo(cs: CaptureSet)(using Context): this.type =
-    cs.addDependent(this)(using ctx, UnrecordedState)
+    cs.addDependent(this)(using ctx, VarState.Unrecorded)
     this
 
   /** {x} <:< this   where <:< is subcapturing, but treating all variables
    *                 as frozen.
    */
-  def accountsFor(x: CaptureRef)(using ctx: Context, vs: VarState = FrozenAllState): Boolean =
+  def accountsFor(x: CaptureRef)(using ctx: Context, vs: VarState = VarState.Separate): Boolean =
 
     /** Like `refs.exists(p)`, but testing fresh cap instances in refs last */
     def existsElem(refs: SimpleIdentitySet[CaptureRef], p: CaptureRef => Boolean): Boolean =
@@ -201,9 +201,9 @@ sealed abstract class CaptureSet extends Showable:
       existsElem(elems, _.subsumes(x))
       || !x.isMaxCapability
         && !x.derivesFrom(defn.Caps_CapSet)
-        && !(vs.frozen == Frozen.All && x.captureSetOfInfo.containsRootCapability)
-           // under Frozen.All, don't try to widen to cap since that might succeed with {cap} <: {cap}
-        && x.captureSetOfInfo.subCaptures(this, Frozen.All).isOK
+        && !(vs == VarState.Separate && x.captureSetOfInfo.containsRootCapability)
+           // in VarState.Separate, don't try to widen to cap since that might succeed with {cap} <: {cap}
+        && x.captureSetOfInfo.subCaptures(this, VarState.Separate).isOK
 
     comparer match
       case comparer: ExplainingTypeComparer => comparer.traceIndented(debugInfo)(test)
@@ -219,7 +219,7 @@ sealed abstract class CaptureSet extends Showable:
    */
   def mightAccountFor(x: CaptureRef)(using Context): Boolean =
     reporting.trace(i"$this mightAccountFor $x, ${x.captureSetOfInfo}?", show = true):
-      elems.exists(_.subsumes(x)(using ctx, FrozenUnrecordedHiddenState))
+      elems.exists(_.subsumes(x)(using ctx, VarState.ClosedUnrecorded))
       || !x.isMaxCapability
         && {
           val elems = x.captureSetOfInfo.elems
@@ -240,15 +240,11 @@ sealed abstract class CaptureSet extends Showable:
    *                  be added when making this test. An attempt to add either
    *                  will result in failure.
    */
-  final def subCaptures(that: CaptureSet, frozen: Frozen)(using Context): CompareResult =
-    val state = frozen match
-      case Frozen.None => VarState()
-      case Frozen.Vars => FrozenVarState()
-      case Frozen.All => FrozenAllState
-    subCaptures(that)(using ctx, state)
+  final def subCaptures(that: CaptureSet, vs: VarState)(using Context): CompareResult =
+    subCaptures(that)(using ctx, vs)
 
   /** The subcapturing test, using a given VarState */
-  private def subCaptures(that: CaptureSet)(using Context, VarState): CompareResult =
+  def subCaptures(that: CaptureSet)(using ctx: Context, vs: VarState = VarState()): CompareResult =
     val result = that.tryInclude(elems, this)
     if result.isOK then
       addDependent(that)
@@ -262,16 +258,16 @@ sealed abstract class CaptureSet extends Showable:
    *  in a frozen state.
    */
   def =:= (that: CaptureSet)(using Context): Boolean =
-       this.subCaptures(that, Frozen.All).isOK
-    && that.subCaptures(this, Frozen.All).isOK
+       this.subCaptures(that, VarState.Separate).isOK
+    && that.subCaptures(this, VarState.Separate).isOK
 
   /** The smallest capture set (via <:<) that is a superset of both
    *  `this` and `that`
    */
   def ++ (that: CaptureSet)(using Context): CaptureSet =
-    if this.subCaptures(that, Frozen.All).isOK then
+    if this.subCaptures(that, VarState.Separate).isOK then
       if that.isAlwaysEmpty && this.keepAlways then this else that
-    else if that.subCaptures(this, Frozen.All).isOK then this
+    else if that.subCaptures(this, VarState.Separate).isOK then this
     else if this.isConst && that.isConst then Const(this.elems ++ that.elems)
     else Union(this, that)
 
@@ -286,8 +282,8 @@ sealed abstract class CaptureSet extends Showable:
   /** The largest capture set (via <:<) that is a subset of both `this` and `that`
    */
   def **(that: CaptureSet)(using Context): CaptureSet =
-    if this.subCaptures(that, Frozen.Vars).isOK then this
-    else if that.subCaptures(this, Frozen.Vars).isOK then that
+    if this.subCaptures(that, VarState.Closed()).isOK then this
+    else if that.subCaptures(this, VarState.Closed()).isOK then that
     else if this.isConst && that.isConst then Const(elemIntersection(this, that))
     else Intersection(this, that)
 
@@ -556,7 +552,7 @@ object CaptureSet:
       else
         // id == 108 then assert(false, i"trying to add $elem to $this")
         assert(elem.isTrackableRef, elem)
-        assert(!this.isInstanceOf[HiddenSet] || summon[VarState] == FrozenAllState, summon[VarState])
+        assert(!this.isInstanceOf[HiddenSet] || summon[VarState] == VarState.Separate, summon[VarState])
         elems += elem
         if elem.isRootCapability then
           rootAddedHandler()
@@ -652,7 +648,8 @@ object CaptureSet:
           .showing(i"solve $this = $result", capt)
         //println(i"solving var $this $approx ${approx.isConst} deps = ${deps.toList}")
         val newElems = approx.elems -- elems
-        if tryInclude(newElems, empty)(using ctx, VarState()).isOK then
+        given VarState()
+        if tryInclude(newElems, empty).isOK then
           markSolved()
 
     /** Mark set as solved and propagate this info to all dependent sets */
@@ -939,10 +936,6 @@ object CaptureSet:
   /** A capture set variable used to record the references hidden by a Fresh.Cap instance */
   class HiddenSet(initialHidden: Refs = emptySet)(using @constructorOnly ictx: Context)
   extends Var(initialElems = initialHidden):
-    override def recordElemsState()(using VarState): Boolean =
-      varState.getElems(this) match
-        case None => varState.putHidden(this, elems)
-        case _ => true
 
     /** Apply function `f` to `elems` while setting `elems` to empty for the
      *  duration. This is used to escape infinite recursions if two Frash.Caps
@@ -990,7 +983,7 @@ object CaptureSet:
    */
   def subCapturesRange(arg1: TypeBounds, arg2: Type)(using Context): Boolean = arg1 match
     case TypeBounds(CapturingType(lo, loRefs), CapturingType(hi, hiRefs)) if lo =:= hi =>
-      given VarState = VarState()
+      given VarState()
       val cs2 = arg2.captureSet
       hiRefs.subCaptures(cs2).isOK && cs2.subCaptures(loRefs).isOK
     case _ =>
@@ -1048,19 +1041,11 @@ object CaptureSet:
       case _ => this
   end CompareResult
 
-  /** An enum indicating a Frozen degree for subCapturing tests */
-  enum Frozen:
-    case None // operations are performed in a regular VarState
-    case Vars // operations are performed in a FrozenVarState
-    case All  // operations are performed in FrozenAllState
-
   /** A VarState serves as a snapshot mechanism that can undo
    *  additions of elements or super sets if an operation fails
    */
   class VarState:
 
-    def frozen: Frozen = Frozen.None
-
     /** A map from captureset variables to their elements at the time of the snapshot. */
     protected val elemsMap: util.EqHashMap[Var, Refs] = new util.EqHashMap
 
@@ -1089,11 +1074,19 @@ object CaptureSet:
      */
     def putDeps(v: Var, deps: Deps): Boolean = { depsMap(v) = deps; true }
 
-    /** Record hidden elements in elemsMap of hidden set `v`,
+    /** Does this state allow additions of elements to capture set variables? */
+    def isOpen = true
+
+    /** Add element to hidden set, recording it in elemsMap,
      *  return whether this was allowed. By default, recording is allowed
-     *  but the special state FrozenAllState overrides this.
+     *  but the special state VarState.Separate overrides this.
      */
-    def putHidden(v: HiddenSet, elems: Refs): Boolean = { elemsMap(v) = elems; true }
+    def addHidden(hidden: HiddenSet, elem: CaptureRef): Boolean =
+      elemsMap.get(hidden) match
+        case None => elemsMap(hidden) = hidden.elems
+        case _ =>
+      hidden.elems += elem
+      true
 
     /** Roll back global state to what was recorded in this VarState */
     def rollBack(): Unit =
@@ -1107,45 +1100,48 @@ object CaptureSet:
       if seen.add(ref) then
         try pred finally seen -= ref
       else false
-  end VarState
 
-  /** A class for states that do not allow to record elements or dependent sets.
-   *  In effect this means that no new elements or dependent sets can be added
-   *  in these states (since the previous state cannot be recorded in a snapshot)
-   *  On the other hand, these states do allow by default Fresh.Cap instances to
-   *  subsume arbitary types, which are then recorded in their hidden sets.
-   */
-  class FrozenVarState extends VarState:
-    override def frozen = Frozen.Vars
-    override def putElems(v: Var, refs: Refs) = false
-    override def putDeps(v: Var, deps: Deps) = false
-    override def putHidden(v: HiddenSet, elems: Refs): Boolean = { elemsMap(v) = elems; true }
+  object VarState:
 
-  @sharable
-  /** A frozen state that allows a Fresh.Cap instancce to subsume a
-   *  reference `r` only if `r` is already present in the hidden set of the instance.
-   *  No new references can be added.
-   */
-  object FrozenAllState extends FrozenVarState:
-    override def frozen = Frozen.All
-    override def putHidden(v: HiddenSet, elems: Refs): Boolean = false
+    /** A class for states that do not allow to record elements or dependent sets.
+     *  In effect this means that no new elements or dependent sets can be added
+     *  in these states (since the previous state cannot be recorded in a snapshot)
+     *  On the other hand, these states do allow by default Fresh.Cap instances to
+     *  subsume arbitary types, which are then recorded in their hidden sets.
+     */
+    class Closed extends VarState:
+      override def putElems(v: Var, refs: Refs) = false
+      override def putDeps(v: Var, deps: Deps) = false
+      override def isOpen = false
+
+    /** A closed state that allows a Fresh.Cap instance to subsume a
+     *  reference `r` only if `r` is already present in the hidden set of the instance.
+     *  No new references can be added.
+     */
+    @sharable
+    object Separate extends Closed:
+      override def addHidden(hidden: HiddenSet, elem: CaptureRef): Boolean = false
 
-  @sharable
-  /** A frozen state that allows a Fresh.Cap instancce to subsume a
-   *  reference `r` only if `r` is already present in the hidden set of the instance.
-   *  No new references can be added.
-   */
-  object FrozenUnrecordedHiddenState extends FrozenVarState
+    /** A special state that turns off recording of elements. Used only
+     *  in `addSub` to prevent cycles in recordings.
+     */
+    @sharable
+    private[CaptureSet] object Unrecorded extends VarState:
+      override def putElems(v: Var, refs: Refs) = true
+      override def putDeps(v: Var, deps: Deps) = true
+      override def rollBack(): Unit = ()
+      override def addHidden(hidden: HiddenSet, elem: CaptureRef): Boolean = true
+
+    /** A closed state that turns off recording of hidden elements (but allows
+     *  adding them). Used in `mightAccountFor`.
+     */
+    @sharable
+    private[CaptureSet] object ClosedUnrecorded extends Closed:
+      override def addHidden(hidden: HiddenSet, elem: CaptureRef): Boolean = true
 
-  @sharable
-  /** A special state that turns off recording of elements. Used only
-   *  in `addSub` to prevent cycles in recordings.
-   */
-  private object UnrecordedState extends VarState:
-    override def putElems(v: Var, refs: Refs) = true
-    override def putDeps(v: Var, deps: Deps) = true
-    override def rollBack(): Unit = ()
+  end VarState
 
+  @sharable
   /** The current VarState, as passed by the implicit context */
   def varState(using state: VarState): VarState = state