Null-terminate Collection.toArray destination only in JVM

In other platforms the elements following the collection elements
should not be changed.

As a part of efforts to stabilize Native stdlib.

js/js.translator/testData/box/regression/stdlibTestSnippets/abstractCollectionToArray.kt

@@ -24,7 +24,7 @@ fun abstractCollectionToArray() {
     assertTrue("toArray1" in coll.invocations || "toArray2" in coll.invocations)
 
     val arr2: Array<String> = coll.toArray(Array(coll.size + 1) { "" })
-    assertEquals(data + listOf(null), arr2.asList())
+    assertEquals(data + listOf(""), arr2.asList())
 }
 
 fun box(): String {

kotlin-native/runtime/src/main/kotlin/kotlin/collections/ArraysNative.kt

@@ -30,8 +30,5 @@ internal actual fun <T> copyToArrayImpl(collection: Collection<*>, array: Array<
     while (iterator.hasNext()) {
         array[index++] = iterator.next() as T
     }
-    if (index < array.size) {
-        return array.copyOf(index) as Array<T>
-    }
-    return array
+    return terminateCollectionToArray(collection.size, array)
 }

libraries/stdlib/js/src/kotlin/collectionJs.kt

@@ -54,12 +54,11 @@ internal actual fun <T> copyToArrayImpl(collection: Collection<*>, array: Array<
     while (iterator.hasNext()) {
         array[index++] = iterator.next().unsafeCast<T>()
     }
-    if (index < array.size) {
-        array[index] = null.unsafeCast<T>()
-    }
-    return array
+    return terminateCollectionToArray(collection.size, array)
 }
 
+internal actual fun <T> terminateCollectionToArray(collectionSize: Int, array: Array<T>): Array<T> = array
+
 /**
  * Returns a new read-only list containing only the specified object [element].
  *

libraries/stdlib/js/src/kotlin/collections/ArrayList.kt

@@ -171,11 +171,7 @@ public actual open class ArrayList<E> internal constructor(private var array: Ar
 
         (this.array as Array<T>).copyInto(array)
 
-        if (array.size > size) {
-            array[size] = null as T // null-terminate
-        }
-
-        return array
+        return terminateCollectionToArray(size, array)
     }
 
     override fun toArray(): Array<Any?> {

libraries/stdlib/jvm/src/kotlin/collections/CollectionsJVM.kt

@@ -85,6 +85,14 @@ internal actual inline fun copyToArrayImpl(collection: Collection<*>): Array<Any
 internal actual inline fun <T> copyToArrayImpl(collection: Collection<*>, array: Array<T>): Array<T> =
     kotlin.jvm.internal.collectionToArray(collection, array as Array<Any?>) as Array<T>
 
+internal actual fun <T> terminateCollectionToArray(collectionSize: Int, array: Array<T>): Array<T> {
+    if (collectionSize < array.size) {
+        @Suppress("UNCHECKED_CAST")
+        array[collectionSize] = null as T // null-terminate
+    }
+    return array
+}
+
 // copies typed varargs array to array of objects
 internal actual fun <T> Array<out T>.copyToArrayOfAny(isVarargs: Boolean): Array<out Any?> =
     if (isVarargs && this.javaClass == Array<Any?>::class.java)

libraries/stdlib/jvm/src/kotlin/collections/builders/ListBuilder.kt

@@ -176,12 +176,7 @@ internal class ListBuilder<E> private constructor(
         @Suppress("UNCHECKED_CAST")
         (array as Array<T>).copyInto(destination, 0, startIndex = offset, endIndex = offset + length)
 
-        if (destination.size > length) {
-            @Suppress("UNCHECKED_CAST")
-            destination[length] = null as T // null-terminate
-        }
-
-        return destination
+        return terminateCollectionToArray(length, destination)
     }
 
     override fun toArray(): Array<Any?> {

libraries/stdlib/native-wasm/src/kotlin/collections/ArrayList.kt

@@ -221,11 +221,7 @@ actual class ArrayList<E> private constructor(
 
         (backingArray as Array<T>).copyInto(array, 0, startIndex = offset, endIndex = offset + length)
 
-        if (array.size > length) {
-            array[length] = null as T // null-terminate
-        }
-
-        return array
+        return terminateCollectionToArray(length, array)
     }
 
     override fun toArray(): Array<Any?> {

libraries/stdlib/native-wasm/src/kotlin/collections/Arrays.kt

@@ -93,6 +93,8 @@ internal actual fun copyToArrayImpl(collection: Collection<*>): Array<Any?> {
     return array
 }
 
+internal actual fun <T> terminateCollectionToArray(collectionSize: Int, array: Array<T>): Array<T> = array
+
 /**
  * Returns a new array which is a copy of the original array with new elements filled with null values.
  */

libraries/stdlib/src/kotlin/collections/ArrayDeque.kt

@@ -541,12 +541,9 @@ public class ArrayDeque<E> : AbstractMutableList<E> {
             elementData.copyInto(dest, destinationOffset = 0, startIndex = head, endIndex = elementData.size)
             elementData.copyInto(dest, destinationOffset = elementData.size - head, startIndex = 0, endIndex = tail)
         }
-        if (dest.size > size) {
-            dest[size] = null // null-terminate
-        }
 
         @Suppress("UNCHECKED_CAST")
-        return dest as Array<T>
+        return terminateCollectionToArray(size, dest) as Array<T>
     }
 
     @Suppress("NOTHING_TO_OVERRIDE")

libraries/stdlib/src/kotlin/collections/Collections.kt

@@ -479,3 +479,9 @@ internal fun throwIndexOverflow() { throw ArithmeticException("Index overflow ha
 @SinceKotlin("1.3")
 internal fun throwCountOverflow() { throw ArithmeticException("Count overflow has happened.") }
 
+/**
+ * In JVM if the size of [array] is bigger than [collectionSize], sets `array[collectionSize] = null`.
+ * In other platforms does nothing.
+ * Returns the given [array].
+ */
+internal expect fun <T> terminateCollectionToArray(collectionSize: Int, array: Array<T>): Array<T>

libraries/stdlib/src/kotlin/collections/SlidingWindow.kt

@@ -146,9 +146,8 @@ private class RingBuffer<T>(private val buffer: Array<Any?>, filledSize: Int) :
             widx++
             idx++
         }
-        if (result.size > this.size) result[this.size] = null
 
-        return result as Array<T>
+        return terminateCollectionToArray(size, result) as Array<T>
     }
 
     override fun toArray(): Array<Any?> {

libraries/stdlib/test/collections/ArrayDequeTest.kt

@@ -5,6 +5,7 @@
 
 package test.collections
 
+import test.*
 import test.collections.behaviors.iteratorBehavior
 import test.collections.behaviors.listIteratorBehavior
 import test.collections.behaviors.listIteratorProperties
@@ -622,12 +623,20 @@ class ArrayDequeTest {
 
             val dest = Array(expected.size + 2) { it + 100 }
 
-            @Suppress("UNCHECKED_CAST")
-            val nullTerminatedExpected = (expected as Array<Any?>) + null + (expected.size + 101)
+            val expectedDest = buildList {
+                addAll(expected)
+                if (TestPlatform.current == TestPlatform.Jvm) {
+                    add(null)
+                } else {
+                    add(expected.size + 100)
+                }
+                add(expected.size + 101)
+            }.toTypedArray()
+
             val actual = deque.testToArray(dest)
             assertTrue(
-                nullTerminatedExpected contentEquals actual,
-                message = "Expected: ${nullTerminatedExpected.contentToString()}, Actual: ${actual.contentToString()}"
+                expectedDest contentEquals actual,
+                message = "Expected: ${expectedDest.contentToString()}, Actual: ${actual.contentToString()}"
             )
         }
 

libraries/stdlib/test/collections/CollectionTest.kt

@@ -1311,7 +1311,12 @@ class CollectionTest {
         assertTrue("toArray1" in coll.invocations || "toArray2" in coll.invocations)
 
         val arr2: Array<String> = coll.toArray(Array(coll.size + 1) { "" })
-        assertEquals(data + listOf(null), arr2.asList())
+        testOnlyOn(TestPlatform.Jvm) {
+            assertEquals(data + listOf(null), arr2.asList())
+        }
+        testExceptOn(TestPlatform.Jvm) {
+            assertEquals(data + listOf(""), arr2.asList())
+        }
     }
 
     @Test

libraries/stdlib/wasm/src/kotlin/collections/ArraysWasm.kt

@@ -34,8 +34,5 @@ internal actual fun <T> copyToArrayImpl(collection: Collection<*>, array: Array<
     while (iterator.hasNext()) {
         array[index++] = iterator.next() as T
     }
-    if (index < array.size) {
-        (array as Array<T?>).fill(null, index)
-    }
-    return array
+    return terminateCollectionToArray(collection.size, array)
 }