stdlib: StringBuilder: insert "null" if the argument is null

This patch changes the behaviour of StringBuilder.append and
StringBuilder.insert methods if the argument is null in accordance
with Kotlin JVM.

backend.native/tests/runtime/text/string_builder0.kt

@@ -21,7 +21,7 @@ fun assertEquals(value1: Int, value2: Int) {
 
 fun assertEquals(builder: StringBuilder, content: String) = assertEquals(builder.toString(), content)
 
-// IndexOutOfBoundsException
+// IndexOutOfBoundsException.
 fun assertException(body: () -> Unit) {
     try {
         body()
@@ -101,7 +101,7 @@ fun testInsertSingle(value: Char) {
 }
 
 fun testInsert() {
-    // String/CharSequence/CharArray
+    // String/CharSequence/CharArray.
     testInsertString("abcd", 0, "12", "12abcd")
     testInsertString("abcd", 4, "12", "abcd12")
     testInsertString("abcd", 2, "12", "ab12cd")
@@ -109,31 +109,39 @@ fun testInsert() {
     testInsertStringException("a", -1, "1")
     testInsertStringException("a", 2, "1")
 
-    // Subsequence of CharSequence
+    // Null inserting.
-    // Insert in the beginning
+    assertEquals(StringBuilder("abcd").insert(0, null as CharSequence?), "nullabcd")
-    assertEquals(StringBuilder("abcd").insert(0, "1234", 0, 0), "abcd")     // 0 symbols
+    assertEquals(StringBuilder("abcd").insert(4, null as CharSequence?), "abcdnull")
-    assertEquals(StringBuilder("abcd").insert(0, "1234", 0, 1), "1abcd")    // 1 symbol
+    assertEquals(StringBuilder("abcd").insert(2, null as CharSequence?), "abnullcd")
-    assertEquals(StringBuilder("abcd").insert(0, "1234", 1, 3), "23abcd")   // 2 symbols
+    assertEquals(StringBuilder("").insert(0, null as CharSequence?), "null")
 
-    // Insert in the end
+    // Subsequence of CharSequence.
+    // Insert in the beginning.
+    assertEquals(StringBuilder("abcd").insert(0, "1234", 0, 0), "abcd")                    // 0 symbols
+    assertEquals(StringBuilder("abcd").insert(0, "1234", 0, 1), "1abcd")                   // 1 symbol
+    assertEquals(StringBuilder("abcd").insert(0, "1234", 1, 3), "23abcd")                  // 2 symbols
+    assertEquals(StringBuilder("abcd").insert(0, null as CharSequence?, 1, 3), "ulabcd")   // 2 symbols of null
+
+    // Insert in the end.
     assertEquals(StringBuilder("abcd").insert(4, "1234", 0, 0), "abcd")
     assertEquals(StringBuilder("abcd").insert(4, "1234", 0, 1), "abcd1")
     assertEquals(StringBuilder("abcd").insert(4, "1234", 1, 3), "abcd23")
+    assertEquals(StringBuilder("abcd").insert(4, null as CharSequence?, 1, 3), "abcdul")
 
-    // Insert in the middle
+    // Insert in the middle.
     assertEquals(StringBuilder("abcd").insert(2, "1234", 0, 0), "abcd")
     assertEquals(StringBuilder("abcd").insert(2, "1234", 0, 1), "ab1cd")
     assertEquals(StringBuilder("abcd").insert(2, "1234", 1, 3), "ab23cd")
+    assertEquals(StringBuilder("abcd").insert(2, null as CharSequence?, 1, 3), "abulcd")
 
-    // Incorrect indices
+    // Incorrect indices.
     assertException { StringBuilder("a").insert(-1, "1", 0, 0) }
     assertException { StringBuilder("a").insert(2, "1", 0, 0) }
     assertException { StringBuilder("a").insert(1, "1", -1, 0) }
     assertException { StringBuilder("a").insert(1, "1", 0, 2) }
     assertException { StringBuilder("a").insert(1, "123", 2, 0) }
 
-
+    // Other types.
-    // Other types
     testInsertSingle(true)
     testInsertSingle(42.toByte())
     testInsertSingle(42.toShort())
@@ -147,7 +155,6 @@ fun testInsert() {
         }
     })
     testInsertSingle('a')
-
 }
 
 // Reverse ==================================================================================================
@@ -248,6 +255,9 @@ fun testBasic() {
     sb.append(12345678L)
     assertEquals(15, sb.length)
     assertEquals("1, true12345678", sb.toString())
+    sb.append(null as CharSequence?)
+    assertEquals(19, sb.length)
+    assertEquals("1, true12345678null", sb.toString())
 
     sb.length = 0
     assertEquals(0, sb.length)

runtime/src/main/kotlin/kotlin/text/StringBuilder.kt

@@ -146,15 +146,15 @@ class StringBuilder private constructor (
     }
 
     fun insert(index: Int, csq: CharSequence?): StringBuilder {
-        // TODO: how to treat nulls properly?
+        // Kotlin JVM inserts the "null" string if the argument is null.
-        if (csq == null) return this
+        val toInsert = csq ?: "null"
-        return insert(index, csq, 0, csq.length)
+        return insert(index, toInsert, 0, toInsert.length)
     }
 
     fun insert(index: Int, csq: CharSequence?, start: Int, end: Int): StringBuilder {
-        // TODO: how to treat nulls properly?
+        // Kotlin JVM processes null as if the argument was "null" char sequence.
-        if (csq == null) return this
+        val toInsert = csq ?: "null"
-        if (start < 0 || end < start || start > csq.length) throw IndexOutOfBoundsException()
+        if (start < 0 || end < start || start > toInsert.length) throw IndexOutOfBoundsException()
         checkInsertIndex(index)
 
         val extraLength = end - start
@@ -168,7 +168,7 @@ class StringBuilder private constructor (
         from = start
         to = index
         while (from < end) {
-            array[to++] = csq[from++]
+            array[to++] = toInsert[from++]
         }
 
         length += extraLength
@@ -214,19 +214,19 @@ class StringBuilder private constructor (
     }
 
     override fun append(csq: CharSequence?): StringBuilder {
-        // TODO: how to treat nulls properly?
+        // Kotlin JVM processes null as if the argument was "null" char sequence.
-        if (csq == null) return this
+        val toAppend = csq ?: "null"
-        return append(csq, 0, csq.length)
+        return append(toAppend, 0, toAppend.length)
     }
 
     override fun append(csq: CharSequence?, start: Int, end: Int): StringBuilder {
-        // TODO: how to treat nulls properly?
+        // Kotlin JVM processes null as if the argument was "null" char sequence.
-        if (csq == null) return this
+        val toAppend = csq ?: "null"
-        if (start < 0 || end < start || start > csq.length) throw IndexOutOfBoundsException()
+        if (start < 0 || end < start || start > toAppend.length) throw IndexOutOfBoundsException()
         ensureExtraCapacity(end - start)
         var index = start
         while (index < end)
-            array[length++] = csq[index++]
+            array[length++] = toAppend[index++]
         return this
     }