Introduce first version of Dispatchers.IO for K/N

* Emulate expect declaration refinement via extension property as the only way to do it in a backwards-compatible manner: in the current model it is impossible to have
  common 'expect' Dispatchers declaration, then refined 'concurrent' Dispatchers declaration with 'expect val IO' and then JVM declaration with JVM-specific members. Current solutions seems to be the less intrusive one
* Elasticity is not supported as K/N Workers API lacks capability to gracefully shutdown workers, meaning that for any unlimited underlying pool, memory consumption is only going to grow in an unbound manner

Fixes #3205

kotlinx-coroutines-core/api/kotlinx-coroutines-core.api

@@ -302,6 +302,7 @@ public final class kotlinx/coroutines/Dispatchers {
 
 public final class kotlinx/coroutines/DispatchersKt {
  public static final field IO_PARALLELISM_PROPERTY_NAME Ljava/lang/String;
+ public static final synthetic fun getIO (Lkotlinx/coroutines/Dispatchers;)Lkotlinx/coroutines/CoroutineDispatcher;
 }
 
 public abstract interface class kotlinx/coroutines/DisposableHandle {

kotlinx-coroutines-core/common/src/Dispatchers.common.kt

@@ -12,7 +12,7 @@ import kotlin.coroutines.*
 public expect object Dispatchers {
  /**
  * The default [CoroutineDispatcher] that is used by all standard builders like
- * [launch][CoroutineScope.launch], [async][CoroutineScope.async], etc
+ * [launch][CoroutineScope.launch], [async][CoroutineScope.async], etc.
  * if neither a dispatcher nor any other [ContinuationInterceptor] is specified in their context.
  *
  * It is backed by a shared pool of threads on JVM. By default, the maximum number of threads used

kotlinx-coroutines-core/concurrent/src/Dispatchers.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2016-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines
+
+/**
+ * The [CoroutineDispatcher] that is designed for offloading blocking IO tasks to a shared pool of threads.
+ * Additional threads in this pool are created on demand.
+ *
+ * By default, the pool is backed by `64` standalone threads, for the additional details
+ * such as elasticity, dynamic sizing and interaction with [CoroutineDispatcher.limitedParallelism],
+ * please refer to the documentation of `Dispatchers.IO` on specific platform.
+ */
+@Suppress("EXTENSION_SHADOWED_BY_MEMBER")
+public expect val Dispatchers.IO: CoroutineDispatcher
+
+

kotlinx-coroutines-core/concurrent/test/DefaultDispatcherConcurrencyTest.kt → kotlinx-coroutines-core/concurrent/test/DefaultDispatchersConcurrencyTest.kt

@@ -6,3 +6,7 @@ package kotlinx.coroutines
 class DefaultDispatcherConcurrencyTest : AbstractDispatcherConcurrencyTest() {
  override val dispatcher: CoroutineDispatcher = Dispatchers.Default
 }
+
+class IoDispatcherConcurrencyTest : AbstractDispatcherConcurrencyTest() {
+ override val dispatcher: CoroutineDispatcher = Dispatchers.IO
+}

kotlinx-coroutines-core/jvm/src/Dispatchers.kt

@@ -1,9 +1,7 @@
 /*
- * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2016-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
-@file:Suppress("unused")
-
 package kotlinx.coroutines
 
 import kotlinx.coroutines.internal.*
@@ -161,3 +159,13 @@ public actual object Dispatchers {
  DefaultScheduler.shutdown()
  }
 }
+
+/**
+ * `actual` counterpart of the corresponding `expect` declaration.
+ * Should never be used directly from JVM sources, all accesses
+ * to `Dispatchers.IO` should be resolved to the corresponding member of [Dispatchers] object.
+ * @suppress
+ */
+@Suppress("EXTENSION_SHADOWED_BY_MEMBER")
+@Deprecated(message = "Should not be used directly", level = DeprecationLevel.HIDDEN)
+public actual val Dispatchers.IO: CoroutineDispatcher get() = Dispatchers.IO

kotlinx-coroutines-core/native/src/Dispatchers.kt

@@ -19,6 +19,19 @@ public actual object Dispatchers {
  internal fun injectMain(dispatcher: MainCoroutineDispatcher) {
  injectedMainDispatcher = dispatcher
  }
+
+ internal val IO: CoroutineDispatcher = newFixedThreadPoolContext(64, "Dispatchers.IO")
 }
 
+/**
+ * The [CoroutineDispatcher] that is designed for offloading blocking IO tasks to a shared pool of threads.
+ * Additional threads in this pool are created on demand.
+ *
+ * On Native platforms it is backed by a standalone [newFixedThreadPoolContext] with `64` worker threads in it.
+ * **NB**: this dispatcher **does not** share the same elasticity behaviour for [CoroutineDispatcher.limitedParallelism]
+ * as `Dispatchers.IO` on JVM.
+ */
+@Suppress("EXTENSION_SHADOWED_BY_MEMBER")
+public actual val Dispatchers.IO: CoroutineDispatcher get() = IO
+
 internal expect fun createMainDispatcher(default: CoroutineDispatcher): MainCoroutineDispatcher