Use BitSet for boolean TASTy attributes (#19092)

Author: bishabosha

compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala

@@ -2,14 +2,15 @@ package dotty.tools.dotc
 package core.tasty
 
 import scala.language.unsafeNulls
+import scala.collection.immutable.BitSet
 
 import dotty.tools.tasty.{TastyFormat, TastyReader, TastyBuffer}
 
 class AttributeUnpickler(reader: TastyReader):
   import reader._
 
   lazy val attributes: Attributes = {
-    val booleanTags = List.newBuilder[Int]
+    val booleanTags = BitSet.newBuilder
 
     while !isAtEnd do
       booleanTags += readByte()

compiler/src/dotty/tools/dotc/core/tasty/Attributes.scala

@@ -1,23 +1,26 @@
 package dotty.tools.dotc.core.tasty
 
-import dotty.tools.tasty.TastyFormat
+import dotty.tools.tasty.TastyFormat.*
 
-class Attributes(
-  val booleanTags: List[Int],
+import scala.collection.immutable.BitSet
+
+class Attributes private[tasty](
+  private[tasty] val booleanTags: BitSet,
 ) {
-  def scala2StandardLibrary: Boolean =
-    booleanTags.contains(TastyFormat.SCALA2STANDARDLIBRARYattr)
-  def explicitNulls: Boolean =
-    booleanTags.contains(TastyFormat.EXPLICITNULLSattr)
+  def scala2StandardLibrary: Boolean = booleanTags(SCALA2STANDARDLIBRARYattr)
+  def explicitNulls: Boolean = booleanTags(EXPLICITNULLSattr)
 }
 
 object Attributes:
   def apply(
     scala2StandardLibrary: Boolean,
     explicitNulls: Boolean,
   ): Attributes =
-    val booleanTags = List.newBuilder[Int]
-    if scala2StandardLibrary then booleanTags += TastyFormat.SCALA2STANDARDLIBRARYattr
-    if explicitNulls then booleanTags += TastyFormat.EXPLICITNULLSattr
+    val booleanTags = BitSet.newBuilder
+    if scala2StandardLibrary then booleanTags += SCALA2STANDARDLIBRARYattr
+    if explicitNulls then booleanTags += EXPLICITNULLSattr
     new Attributes(booleanTags.result())
   end apply
+
+  val empty: Attributes =
+    new Attributes(BitSet.empty)

compiler/src/dotty/tools/dotc/core/tasty/DottyUnpickler.scala

@@ -57,7 +57,7 @@ class DottyUnpickler(bytes: Array[Byte], mode: UnpickleMode = UnpickleMode.TopLe
   private val treeUnpickler = unpickler.unpickle(treeSectionUnpickler(posUnpicklerOpt, commentUnpicklerOpt, attributeUnpicklerOpt)).get
 
   def tastyAttributes: Attributes =
-    attributeUnpicklerOpt.map(_.attributes).getOrElse(new Attributes(booleanTags = Nil))
+    attributeUnpicklerOpt.map(_.attributes).getOrElse(Attributes.empty)
 
   /** Enter all toplevel classes and objects into their scopes
    *  @param roots          a set of SymDenotations that should be overwritten by unpickling