Fix creation of SmallPersistentList from mutable root buffer

Author: Abduqodiri Qurbonzoda

core/commonMain/src/implementations/immutableList/PersistentVector.kt

@@ -180,7 +180,8 @@ internal class PersistentVector<E>(private val root: Array<Any?>,
      */
     private fun pullLastBufferFromRoot(root: Array<Any?>, rootSize: Int, shift: Int): PersistentList<E> {
         if (shift == 0) {
-            return SmallPersistentVector(root)
+            val buffer = if (root.size == MUTABLE_BUFFER_SIZE) root.copyOf(MAX_BUFFER_SIZE) else root
+            return SmallPersistentVector(buffer)
         }
         val tailCarry = ObjectRef(null)
         val newRoot = pullLastBuffer(root, shift, rootSize - 1, tailCarry)!!

core/commonTest/src/contract/list/ImmutableListTest.kt

@@ -96,6 +96,14 @@ class ImmutableListTest {
         assertEquals(emptyList<Char>(), list.clear())
     }
 
+    @Test
+    fun smallPersistentListFromMutableBuffer() {
+        val list = List(33) { it }
+        var vector = persistentListOf<Int>().mutate { it.addAll(list) }
+        vector = vector.removeAt(vector.lastIndex)
+        assertEquals(list.dropLast(1), vector)
+    }
+
     @Test fun subList() {
         val list = "abcxaxyz12".toImmutableList()
         val subList = list.subList(2, 5) // 2, 3, 4