Cache result of toCaptureSet in RetainingAnnotation

Author: odersky

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

@@ -10,34 +10,38 @@ import config.Feature
 /** A class for annotations @retains, @retainsByName and @retainsCap */
 class RetainingAnnotation(tpe: Type) extends CompactAnnotation(tpe):
 
-    /** Sanitize @retains arguments to approximate illegal types that could cause a compilation
-     *  time blowup before they are dropped ot detected. This means mapping all all skolems
-     *  (?n: T) to (?n: Any), and mapping all recursive captures that are not on CapSet to `^`.
-     *  Skolems and capturing types on types other than CapSet are not allowed in a
-     *  @retains annotation anyway, so the underlying type does not matter as long as it is also
-     *  illegal. See i24556.scala and i24556a.scala.
-     */
-    override protected def sanitize(tp: Type)(using Context): Type = tp match
-      case SkolemType(_) =>
-        SkolemType(defn.AnyType)
-      case tp @ AnnotatedType(parent, ann)
-      if ann.symbol.isRetainsLike && parent.typeSymbol != defn.Caps_CapSet =>
-        tp.derivedAnnotatedType(parent, ann.derivedClassAnnotation(defn.RetainsCapAnnot))
-      case tp @ OrType(tp1, tp2) =>
-        tp.derivedOrType(sanitize(tp1), sanitize(tp2))
-      case _ =>
-        tp
-
-    override def mapWith(tm: TypeMap)(using Context): Annotation =
-      if Feature.ccEnabledSomewhere then mapWithCtd(tm) else EmptyAnnotation
-
-    def isStrict(using Context): Boolean = symbol.isRetains
-
-    def retainedType(using Context): Type =
-      if symbol == defn.RetainsCapAnnot then defn.captureRoot.termRef
-      else argumentType(0)
-
-    def toCaptureSet(using Context): CaptureSet =
-      CaptureSet(retainedType.retainedElements*)
+  /** Sanitize @retains arguments to approximate illegal types that could cause a compilation
+   *  time blowup before they are dropped ot detected. This means mapping all all skolems
+   *  (?n: T) to (?n: Any), and mapping all recursive captures that are not on CapSet to `^`.
+   *  Skolems and capturing types on types other than CapSet are not allowed in a
+   *  @retains annotation anyway, so the underlying type does not matter as long as it is also
+   *  illegal. See i24556.scala and i24556a.scala.
+   */
+  override protected def sanitize(tp: Type)(using Context): Type = tp match
+    case SkolemType(_) =>
+      SkolemType(defn.AnyType)
+    case tp @ AnnotatedType(parent, ann)
+    if ann.symbol.isRetainsLike && parent.typeSymbol != defn.Caps_CapSet =>
+      tp.derivedAnnotatedType(parent, ann.derivedClassAnnotation(defn.RetainsCapAnnot))
+    case tp @ OrType(tp1, tp2) =>
+      tp.derivedOrType(sanitize(tp1), sanitize(tp2))
+    case _ =>
+      tp
+
+  override def mapWith(tm: TypeMap)(using Context): Annotation =
+    if Feature.ccEnabledSomewhere then mapWithCtd(tm) else EmptyAnnotation
+
+  def isStrict(using Context): Boolean = symbol.isRetains
+
+  def retainedType(using Context): Type =
+    if symbol == defn.RetainsCapAnnot then defn.captureRoot.termRef
+    else argumentType(0)
+
+  private var myCaptureSet: CaptureSet | Null = null
+
+  def toCaptureSet(using Context): CaptureSet =
+    if myCaptureSet == null then
+      myCaptureSet = CaptureSet(retainedType.retainedElements*)
+    myCaptureSet.nn
 
 end RetainingAnnotation