A more robust scheme for resetting denotations after Recheck (#18534)

The new scheme works also for arbitrary denotation changes in
PreRecheck.

Furthermore, recheck denot transformers are not run after Recheck has
ended. This
means that effectively only symbols touched by the Rechecker are
transformed and
reset again afterwards.

compiler/src/dotty/tools/dotc/Run.scala

@@ -168,10 +168,10 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
    */
   var pureFunsImportEncountered = false
 
-  /** Will be set to true if any of the compiled compilation units contains
-   *  a captureChecking language import.
+  /** Will be set to true if experimental.captureChecking is enabled
+   *  or any of the compiled compilation units contains a captureChecking language import.
    */
-  var ccImportEncountered = false
+  var ccEnabledSomewhere = Feature.enabledBySetting(Feature.captureChecking)(using ictx)
 
   private var myEnrichedErrorMessage = false
 

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

@@ -29,19 +29,22 @@ object CheckCaptures:
 
   class Pre extends PreRecheck, SymTransformer:
 
-    override def isEnabled(using Context) = true
+    override def isRunnable(using Context) = super.isRunnable && Feature.ccEnabledSomewhere
 
   	/**  - Reset `private` flags of parameter accessors so that we can refine them
      *     in Setup if they have non-empty capture sets.
      *   - Special handling of some symbols defined for case classes.
+     *  Enabled only until recheck is finished, and provided some compilation unit
+     *  is CC-enabled.
      */
     def transformSym(sym: SymDenotation)(using Context): SymDenotation =
-      if sym.isAllOf(PrivateParamAccessor) && !sym.hasAnnotation(defn.ConstructorOnlyAnnot) then
-        sym.copySymDenotation(initFlags = sym.flags &~ Private | Recheck.ResetPrivate)
-      else if Synthetics.needsTransform(sym) then
-        Synthetics.transform(sym, toCC = true)
-      else
-        sym
+      if !pastRecheck && Feature.ccEnabledSomewhere then
+        if sym.isAllOf(PrivateParamAccessor) && !sym.hasAnnotation(defn.ConstructorOnlyAnnot) then
+          sym.copySymDenotation(initFlags = sym.flags &~ Private | Recheck.ResetPrivate)
+        else if Synthetics.needsTransform(sym) then
+          Synthetics.transform(sym)
+        else sym
+      else sym
   end Pre
 
   enum EnvKind:
@@ -190,7 +193,8 @@ class CheckCaptures extends Recheck, SymTransformer:
   import CheckCaptures.*
 
   def phaseName: String = "cc"
-  override def isEnabled(using Context) = true
+
+  override def isRunnable(using Context) = super.isRunnable && Feature.ccEnabledSomewhere
 
   def newRechecker()(using Context) = CaptureChecker(ctx)
 
@@ -199,11 +203,7 @@ class CheckCaptures extends Recheck, SymTransformer:
   override def run(using Context): Unit =
     if Feature.ccEnabled then
       super.run
-
-  override def transformSym(sym: SymDenotation)(using Context): SymDenotation =
-    if Synthetics.needsTransform(sym) then Synthetics.transform(sym, toCC = false)
-    else super.transformSym(sym)
-
+
   override def printingContext(ctx: Context) = ctx.withProperty(ccStateKey, Some(new CCState))
 
   class CaptureChecker(ictx: Context) extends Rechecker(ictx):

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

@@ -306,7 +306,7 @@ extends tpd.TreeTraverser:
 
   /** Update info of `sym` for CheckCaptures phase only */
   private def updateInfo(sym: Symbol, info: Type)(using Context) =
-    sym.updateInfoBetween(preRecheckPhase, thisPhase, info, newOwnerFor(sym))
+    sym.updateInfo(preRecheckPhase, info, newOwnerFor(sym))
     sym.namedType match
       case ref: CaptureRef => ref.invalidateCaches()
       case _ =>

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

@@ -61,7 +61,7 @@ object Synthetics:
    *  @param  sym  The method to transform @pre needsTransform(sym) must hold.
    *  @param  toCC Whether to transform the type to capture checking or back.
    */
-  def transform(sym: SymDenotation, toCC: Boolean)(using Context): SymDenotation =
+  def transform(sym: SymDenotation)(using Context): SymDenotation =
 
     /** Add capture dependencies to the type of the `apply` or `copy` method of a case class.
      *  An apply method in a case class like this:
@@ -92,31 +92,18 @@ object Synthetics:
       case _ =>
         info
 
-    /** Drop capture dependencies from the type of `apply` or `copy` method of a case class */
-    def dropCaptureDeps(tp: Type): Type = tp match
-      case tp: MethodOrPoly =>
-        tp.derivedLambdaType(resType = dropCaptureDeps(tp.resType))
-      case CapturingType(parent, _) =>
-        dropCaptureDeps(parent)
-      case RefinedType(parent, _, _) =>
-        dropCaptureDeps(parent)
-      case _ =>
-        tp
-
     /** Add capture information to the type of the default getter of a case class copy method
-     *  if toCC = true, or remove the added info again if toCC = false.
      */
     def transformDefaultGetterCaptures(info: Type, owner: Symbol, idx: Int)(using Context): Type = info match
       case info: MethodOrPoly =>
         info.derivedLambdaType(resType = transformDefaultGetterCaptures(info.resType, owner, idx))
       case info: ExprType =>
         info.derivedExprType(transformDefaultGetterCaptures(info.resType, owner, idx))
       case EventuallyCapturingType(parent, _) =>
-        if toCC then transformDefaultGetterCaptures(parent, owner, idx)
-        else parent
+        transformDefaultGetterCaptures(parent, owner, idx)
       case info @ AnnotatedType(parent, annot) =>
         info.derivedAnnotatedType(transformDefaultGetterCaptures(parent, owner, idx), annot)
-      case _ if toCC && idx < owner.asClass.paramGetters.length =>
+      case _ if idx < owner.asClass.paramGetters.length =>
         val param = owner.asClass.paramGetters(idx)
         val pinfo = param.info
         atPhase(ctx.phase.next) {
@@ -126,49 +113,29 @@ object Synthetics:
       case _ =>
         info
 
-    /** Augment an unapply of type `(x: C): D` to `(x: C^{cap}): D^{x}` if toCC is true,
-     *  or remove the added capture sets again if toCC = false.
-     */
+    /** Augment an unapply of type `(x: C): D` to `(x: C^{cap}): D^{x}` */
     def transformUnapplyCaptures(info: Type)(using Context): Type = info match
       case info: MethodType =>
-        if toCC then
-          val paramInfo :: Nil = info.paramInfos: @unchecked
-          val newParamInfo = CapturingType(paramInfo, CaptureSet.universal)
-          val trackedParam = info.paramRefs.head
-          def newResult(tp: Type): Type = tp match
-            case tp: MethodOrPoly =>
-              tp.derivedLambdaType(resType = newResult(tp.resType))
-            case _ =>
-              CapturingType(tp, CaptureSet(trackedParam))
-          info.derivedLambdaType(paramInfos = newParamInfo :: Nil, resType = newResult(info.resType))
-            .showing(i"augment unapply type $info to $result", capt)
-        else info.paramInfos match
-          case CapturingType(oldParamInfo, _) :: Nil =>
-            def oldResult(tp: Type): Type = tp match
-              case tp: MethodOrPoly =>
-                tp.derivedLambdaType(resType = oldResult(tp.resType))
-              case CapturingType(tp, _) =>
-                tp
-            info.derivedLambdaType(paramInfos = oldParamInfo :: Nil, resType = oldResult(info.resType))
+        val paramInfo :: Nil = info.paramInfos: @unchecked
+        val newParamInfo = CapturingType(paramInfo, CaptureSet.universal)
+        val trackedParam = info.paramRefs.head
+        def newResult(tp: Type): Type = tp match
+          case tp: MethodOrPoly =>
+            tp.derivedLambdaType(resType = newResult(tp.resType))
           case _ =>
-            info
+            CapturingType(tp, CaptureSet(trackedParam))
+        info.derivedLambdaType(paramInfos = newParamInfo :: Nil, resType = newResult(info.resType))
+          .showing(i"augment unapply type $info to $result", capt)
       case info: PolyType =>
         info.derivedLambdaType(resType = transformUnapplyCaptures(info.resType))
 
     def transformComposeCaptures(symd: SymDenotation) =
       val (pt: PolyType) = symd.info: @unchecked
       val (mt: MethodType) = pt.resType: @unchecked
       val (enclThis: ThisType) = symd.owner.thisType: @unchecked
-      val mt1 =
-        if toCC then
-          MethodType(mt.paramNames)(
-            mt1 => mt.paramInfos.map(_.capturing(CaptureSet.universal)),
-            mt1 => CapturingType(mt.resType, CaptureSet(enclThis, mt1.paramRefs.head)))
-        else
-          MethodType(mt.paramNames)(
-            mt1 => mt.paramInfos.map(_.stripCapturing),
-            mt1 => mt.resType.stripCapturing)
-      pt.derivedLambdaType(resType = mt1)
+      pt.derivedLambdaType(resType = MethodType(mt.paramNames)(
+        mt1 => mt.paramInfos.map(_.capturing(CaptureSet.universal)),
+        mt1 => CapturingType(mt.resType, CaptureSet(enclThis, mt1.paramRefs.head))))
 
     def transformCurriedTupledCaptures(symd: SymDenotation) =
       val (et: ExprType) = symd.info: @unchecked
@@ -179,26 +146,18 @@ object Synthetics:
           defn.FunctionOf(args, mapFinalResult(res, f), isContextual)
         else
           f(tp)
-      val resType1 =
-        if toCC then
-          mapFinalResult(et.resType, CapturingType(_, CaptureSet(enclThis)))
-        else
-          et.resType.stripCapturing
-      ExprType(resType1)
+      ExprType(mapFinalResult(et.resType, CapturingType(_, CaptureSet(enclThis))))
 
     def transformCompareCaptures =
-      if toCC then
-        MethodType(defn.ObjectType.capturing(CaptureSet.universal) :: Nil, defn.BooleanType)
-      else
-        defn.methOfAnyRef(defn.BooleanType)
+      MethodType(defn.ObjectType.capturing(CaptureSet.universal) :: Nil, defn.BooleanType)
 
     sym.copySymDenotation(info = sym.name match
       case DefaultGetterName(nme.copy, n) =>
         transformDefaultGetterCaptures(sym.info, sym.owner, n)
       case nme.unapply =>
         transformUnapplyCaptures(sym.info)
       case nme.apply | nme.copy =>
-        if toCC then addCaptureDeps(sym.info) else dropCaptureDeps(sym.info)
+        addCaptureDeps(sym.info)
       case nme.andThen | nme.compose =>
         transformComposeCaptures(sym)
       case nme.curried | nme.tupled =>

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -103,8 +103,8 @@ object Feature:
 
   /** Is captureChecking enabled for any of the currently compiled compilation units? */
   def ccEnabledSomewhere(using Context) =
-    enabledBySetting(captureChecking)
-    || ctx.run != null && ctx.run.nn.ccImportEncountered
+    if ctx.run != null then ctx.run.nn.ccEnabledSomewhere
+    else enabledBySetting(captureChecking)
 
   def sourceVersionSetting(using Context): SourceVersion =
     SourceVersion.valueOf(ctx.settings.source.value)
@@ -174,7 +174,7 @@ object Feature:
       true
     else if fullFeatureName == captureChecking then
       ctx.compilationUnit.needsCaptureChecking = true
-      if ctx.run != null then ctx.run.nn.ccImportEncountered = true
+      if ctx.run != null then ctx.run.nn.ccEnabledSomewhere = true
       true
     else
       false

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

@@ -299,13 +299,24 @@ object Phases {
      */
     def phaseName: String
 
+    /** This property is queried when phases are first assembled.
+     *  If it is false, the phase will be dropped from the set of phases to traverse.
+     */
+    def isEnabled(using Context): Boolean = true
+
+    /** This property is queried before a phase is run.
+     *  If it is false, the phase is skipped.
+     */
     def isRunnable(using Context): Boolean =
       !ctx.reporter.hasErrors
         // TODO: This might test an unintended condition.
         // To find out whether any errors have been reported during this
         // run one calls `errorsReported`, not `hasErrors`.
         // But maybe changing this would prevent useful phases from running?
 
+    /** True for all phases except NoPhase */
+    def exists: Boolean = true
+
     /** If set, allow missing or superfluous arguments in applications
      *  and type applications.
      */
@@ -360,10 +371,6 @@ object Phases {
     /** Can this transform change the base types of a type? */
     def changesBaseTypes: Boolean = changesParents
 
-    def isEnabled(using Context): Boolean = true
-
-    def exists: Boolean = true
-
     def initContext(ctx: FreshContext): Unit = ()
 
     /** A hook that allows to transform the usual context passed to the function

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

@@ -22,7 +22,10 @@ abstract class TypeError(using creationContext: Context) extends Exception(""):
    *  This is expensive and only useful for debugging purposes.
    */
   def computeStackTrace: Boolean =
-    ctx.debug || (cyclicErrors != noPrinter && this.isInstanceOf[CyclicReference] && !(ctx.mode is Mode.CheckCyclic))
+    ctx.debug
+    || (cyclicErrors != noPrinter && this.isInstanceOf[CyclicReference] && !(ctx.mode is Mode.CheckCyclic))
+    || ctx.settings.YdebugTypeError.value
+    || ctx.settings.YdebugError.value
 
   override def fillInStackTrace(): Throwable =
     if computeStackTrace then super.fillInStackTrace().nn

compiler/src/dotty/tools/dotc/transform/PreRecheck.scala

@@ -14,6 +14,8 @@ abstract class PreRecheck extends Phase, DenotTransformer:
 
   override def changesBaseTypes: Boolean = true
 
+  var pastRecheck = false
+
   def run(using Context): Unit = ()
 
   override def isCheckable = false

compiler/src/dotty/tools/dotc/transform/Recheck.scala

@@ -48,26 +48,19 @@ object Recheck:
 
   extension (sym: Symbol)
 
-    /** Update symbol's info to newInfo from prevPhase.next to lastPhase.
+    /** Update symbol's info to newInfo after `prevPhase`.
      *  Also update owner to newOwnerOrNull if it is not null.
-     *  Reset to previous info and owner for phases after lastPhase.
+     *  The update is valid until after Recheck. After that the symbol's denotation
+     *  is reset to what it was before PreRecheck.
      */
-    def updateInfoBetween(prevPhase: DenotTransformer, lastPhase: DenotTransformer, newInfo: Type, newOwnerOrNull: Symbol | Null = null)(using Context): Unit =
+    def updateInfo(prevPhase: DenotTransformer, newInfo: Type, newOwnerOrNull: Symbol | Null = null)(using Context): Unit =
       val newOwner = if newOwnerOrNull == null then sym.owner else newOwnerOrNull
       if (sym.info ne newInfo) || (sym.owner ne newOwner)  then
         val flags = sym.flags
-        sym.copySymDenotation(
-            initFlags =
-              if flags.isAllOf(ResetPrivateParamAccessor)
-              then flags &~ ResetPrivate | Private
-              else flags
-          ).installAfter(lastPhase) // reset
         sym.copySymDenotation(
             owner = newOwner,
             info = newInfo,
-            initFlags =
-              if newInfo.isInstanceOf[LazyType] then flags &~ Touched
-              else flags
+            initFlags = if newInfo.isInstanceOf[LazyType] then flags &~ Touched else flags
           ).installAfter(prevPhase)
 
     /** Does symbol have a new denotation valid from phase.next that is different
@@ -158,16 +151,21 @@ abstract class Recheck extends Phase, SymTransformer:
     // One failing test is pos/i583a.scala
 
   /** Change any `ResetPrivate` flags back to `Private` */
-  def transformSym(sym: SymDenotation)(using Context): SymDenotation =
-    if sym.isAllOf(Recheck.ResetPrivateParamAccessor) then
-      sym.copySymDenotation(initFlags = sym.flags &~ Recheck.ResetPrivate | Private)
-    else sym
+  def transformSym(symd: SymDenotation)(using Context): SymDenotation =
+    val sym = symd.symbol
+    if sym.isUpdatedAfter(preRecheckPhase)
+    then atPhase(preRecheckPhase)(sym.denot.copySymDenotation())
+    else symd
 
   def run(using Context): Unit =
     val rechecker = newRechecker()
     rechecker.checkUnit(ctx.compilationUnit)
     rechecker.reset()
 
+  override def runOn(units: List[CompilationUnit])(using runCtx: Context): List[CompilationUnit] =
+    try super.runOn(units)
+    finally preRecheckPhase.pastRecheck = true
+
   def newRechecker()(using Context): Rechecker
 
   /** The typechecker pass */