(wip) claude please make this pass the capture checker

Author: lrytz

library/src/scala/collection/immutable/LazyListIterableBase.scala

@@ -26,28 +26,33 @@ import java.util.concurrent.atomic.AtomicReferenceFieldUpdater
  * class has access to the corresponding field. So it needs to be called in the class where the field is
  * declared (fields are always private in Scala).
  */
-abstract class LazyListIterableBase[+A] private[immutable] (initialTail: AnyRef | Null) extends Iterable[A] with Serializable {
+abstract class LazyListIterableBase[+A] private[immutable] (initialTail: (AnyRef | Null)^) extends Iterable[A] with Serializable {
   /** See [[LazyList._head]] for the possible states of this field. */
-  @volatile private var _tail: AnyRef^{this} | Null /* () => LazyList[A] | Thread | InRace | LazyList[A] | Null */ = initialTail
+  @volatile private var _tail: AnyRef^{this} | Null /* () => LazyList[A] | Thread | InRace | LazyList[A] | Null */ =
+    caps.unsafe.unsafeAssumePure(initialTail)
 
   private[immutable] def rawTail: AnyRef^{this} | Null = _tail
 
-  private[immutable] def setRawTail(value: AnyRef): Unit = _tail = value
+  private[immutable] def setRawTail(value: AnyRef^): Unit = _tail = caps.unsafe.unsafeAssumePure(value)
 
   @noinline private[immutable] def makeTailUpdater: LazyListIterableBase.TailUpdater =
     new LazyListIterableBase.TailUpdater(AtomicReferenceFieldUpdater.newUpdater(classOf[LazyListIterableBase[?]], classOf[AnyRef], "_tail"))
 }
 
 private[immutable] object LazyListIterableBase {
+  import caps.unsafe.unsafeAssumePure
+
   final class TailUpdater(u: AtomicReferenceFieldUpdater[LazyListIterableBase[?], AnyRef]) {
-    def compareAndSet(ll: LazyListIterableBase[?], expected: AnyRef, value: AnyRef): Boolean = u.compareAndSet(ll, expected, value)
-    def getAndSet(ll: LazyListIterableBase[?], value: AnyRef | Null): AnyRef | Null = u.getAndSet(ll, value)
+    def compareAndSet(ll: LazyListIterableBase[?]^, expected: AnyRef^, value: AnyRef^): Boolean =
+      u.compareAndSet(unsafeAssumePure(ll), unsafeAssumePure(expected), unsafeAssumePure(value))
+    def getAndSet(ll: LazyListIterableBase[?]^, value: (AnyRef | Null)^): (AnyRef | Null)^ =
+      u.getAndSet(unsafeAssumePure(ll), value.asInstanceOf[AnyRef | Null])
   }
 
   // this utility is constant `true` on Scala.js -> enables DCE in LazyList
-  def isCurrentThread(t: Thread): Boolean = t eq Thread.currentThread
+  def isCurrentThread(t: Thread^): Boolean = t eq Thread.currentThread
   // also for Scala.js
-  def InRace(t: Thread): InRace = new InRace(t)
+  def InRace(t: Thread^): InRace = new InRace(unsafeAssumePure(t))
 
   final class InRace private[LazyListIterableBase] (val owner: Thread) {
     private val done: CountDownLatch = new CountDownLatch(1)