Introduce associateWith and associateWithTo functions

#KT-13814

libraries/stdlib/test/collections/CollectionTest.kt

@@ -358,6 +358,18 @@ class CollectionTest {
         assertEquals(namesByTeam, mutableNamesByTeam)
     }
 
+    @Test fun associateWith() {
+        val items = listOf("Alice", "Bob", "Carol")
+        val itemsWithTheirLength = items.associateWith { it.length }
+
+        assertEquals(mapOf("Alice" to 5, "Bob" to 3, "Carol" to 5), itemsWithTheirLength)
+
+        val updatedLength =
+            items.drop(1).associateWithTo(itemsWithTheirLength.toMutableMap()) { name -> name.toLowerCase().count { it in "aeuio" }}
+
+        assertEquals(mapOf("Alice" to 5, "Bob" to 1, "Carol" to 2), updatedLength)
+    }
+
     @Test fun plusRanges() {
         val range1 = 1..3
         val range2 = 4..7

libraries/stdlib/test/collections/SequenceTest.kt

@@ -548,6 +548,18 @@ public class SequenceTest {
         assertEquals(listOf("act", "wast", "test"), sequenceOf("act", "test", "wast").sortedWith(comparator).toList())
     }
 
+    @Test fun associateWith() {
+        val items = sequenceOf("Alice", "Bob", "Carol")
+        val itemsWithTheirLength = items.associateWith { it.length }
+
+        assertEquals(mapOf("Alice" to 5, "Bob" to 3, "Carol" to 5), itemsWithTheirLength)
+
+        val updatedLength =
+            items.drop(1).associateWithTo(itemsWithTheirLength.toMutableMap()) { name -> name.toLowerCase().count { it in "aeuio" }}
+
+        assertEquals(mapOf("Alice" to 5, "Bob" to 1, "Carol" to 2), updatedLength)
+    }
+
     @Test fun orEmpty() {
         val s1: Sequence<Int>? = null
         assertEquals(emptySequence(), s1.orEmpty())

libraries/stdlib/test/text/StringTest.kt

@@ -1159,6 +1159,14 @@ class StringTest {
         assertEquals(listOf('A', 'A', 'B', 'D'), result[true])
     }
 
+    @Test fun associateWith() = withOneCharSequenceArg("abc") { data ->
+        val result = data.associateWith { it + 1 }
+        assertEquals(mapOf('a' to 'b', 'b' to 'c', 'c' to 'd'), result)
+
+        val mutableResult = data.drop(1).associateWithTo(result.toMutableMap()) { it - 1 }
+        assertEquals(mapOf('a' to 'b', 'b' to 'a', 'c' to 'b'), mutableResult)
+    }
+
     @Test fun joinToString() {
         val data = "abcd".toList()
         val result = data.joinToString("_", "(", ")")