Elide thunk allocation when using sleepInternal

build.sbt

@@ -643,7 +643,12 @@ lazy val core = crossProject(JSPlatform, JVMPlatform, NativePlatform)
         "cats.effect.IOFiberConstants.ExecuteRunnableR"),
       ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.effect.IOLocal.scope"),
       ProblemFilters.exclude[DirectMissingMethodProblem](
-        "cats.effect.IOFiberConstants.ContStateResult")
+        "cats.effect.IOFiberConstants.ContStateResult"),
+      // #3775, changes to internal timers APIs
+      ProblemFilters.exclude[IncompatibleResultTypeProblem](
+        "cats.effect.unsafe.TimerSkipList.insert"),
+      ProblemFilters.exclude[IncompatibleResultTypeProblem](
+        "cats.effect.unsafe.WorkerThread.sleep")
     ) ++ {
       if (tlIsScala3.value) {
         // Scala 3 specific exclusions

core/js-native/src/main/scala/cats/effect/unsafe/WorkStealingThreadPool.scala

@@ -30,7 +30,7 @@ private[effect] sealed abstract class WorkStealingThreadPool private ()
   private[effect] def reschedule(runnable: Runnable): Unit
   private[effect] def sleepInternal(
       delay: FiniteDuration,
-      callback: Right[Nothing, Unit] => Unit): Runnable
+      callback: Right[Nothing, Unit] => Unit): Function0[Unit] with Runnable
   private[effect] def sleep(
       delay: FiniteDuration,
       task: Runnable,

core/jvm/src/main/scala/cats/effect/unsafe/TimerSkipList.scala

@@ -68,18 +68,21 @@ private final class TimerSkipList() extends AtomicLong(MARKER + 1L) { sequenceNu
       cb: Callback,
       next: Node
   ) extends TimerSkipListNodeBase[Callback, Node](cb, next)
+      with Function0[Unit]
       with Runnable {
 
     /**
      * Cancels the timer
      */
-    final override def run(): Unit = {
+    final def apply(): Unit = {
       // TODO: We could null the callback here directly,
       // TODO: and the do the lookup after (for unlinking).
       TimerSkipList.this.doRemove(triggerTime, sequenceNum)
       ()
     }
 
+    final def run() = apply()
+
     private[TimerSkipList] final def isMarker: Boolean = {
       // note: a marker node also has `triggerTime == MARKER`,
       // but that's also a valid trigger time, so we need
@@ -158,7 +161,7 @@ private final class TimerSkipList() extends AtomicLong(MARKER + 1L) { sequenceNu
       delay: Long,
       callback: Right[Nothing, Unit] => Unit,
       tlr: ThreadLocalRandom
-  ): Runnable = {
+  ): Function0[Unit] with Runnable = {
     require(delay >= 0L)
     // we have to check for overflow:
     val triggerTime = computeTriggerTime(now = now, delay = delay)

core/jvm/src/main/scala/cats/effect/unsafe/WorkStealingThreadPool.scala

@@ -621,7 +621,9 @@ private[effect] final class WorkStealingThreadPool(
   /**
    * Tries to call the current worker's `sleep`, but falls back to `sleepExternal` if needed.
    */
-  def sleepInternal(delay: FiniteDuration, callback: Right[Nothing, Unit] => Unit): Runnable = {
+  def sleepInternal(
+      delay: FiniteDuration,
+      callback: Right[Nothing, Unit] => Unit): Function0[Unit] with Runnable = {
     val thread = Thread.currentThread()
     if (thread.isInstanceOf[WorkerThread]) {
       val worker = thread.asInstanceOf[WorkerThread]
@@ -642,7 +644,7 @@ private[effect] final class WorkStealingThreadPool(
    */
   private[this] final def sleepExternal(
       delay: FiniteDuration,
-      callback: Right[Nothing, Unit] => Unit): Runnable = {
+      callback: Right[Nothing, Unit] => Unit): Function0[Unit] with Runnable = {
     val random = ThreadLocalRandom.current()
     val idx = random.nextInt(threadCount)
     val tsl = sleepers(idx)

core/jvm/src/main/scala/cats/effect/unsafe/WorkerThread.scala

@@ -152,7 +152,9 @@ private final class WorkerThread(
     }
   }
 
-  def sleep(delay: FiniteDuration, callback: Right[Nothing, Unit] => Unit): Runnable = {
+  def sleep(
+      delay: FiniteDuration,
+      callback: Right[Nothing, Unit] => Unit): Function0[Unit] with Runnable = {
     // take the opportunity to update the current time, just in case other timers can benefit
     val _now = System.nanoTime()
     now = _now

core/shared/src/main/scala/cats/effect/IOFiber.scala

@@ -925,13 +925,17 @@ private final class IOFiber[A](
                 IO {
                   val scheduler = runtime.scheduler
 
-                  val cancel =
-                    if (scheduler.isInstanceOf[WorkStealingThreadPool])
-                      scheduler.asInstanceOf[WorkStealingThreadPool].sleepInternal(delay, cb)
-                    else
-                      scheduler.sleep(delay, () => cb(RightUnit))
+                  val cancelIO =
+                    if (scheduler.isInstanceOf[WorkStealingThreadPool]) {
+                      val cancel =
+                        scheduler.asInstanceOf[WorkStealingThreadPool].sleepInternal(delay, cb)
+                      IO.Delay(cancel, null)
+                    } else {
+                      val cancel = scheduler.sleep(delay, () => cb(RightUnit))
+                      IO(cancel.run())
+                    }
 
-                  Some(IO(cancel.run()))
+                  Some(cancelIO)
                 }
               }
             else IO.cede