StringBuilder: Use intrinsic if input sequence is a string

runtime/src/main/cpp/KString.cpp

@@ -902,13 +902,14 @@ OBJ_GETTER(Kotlin_String_unsafeStringToUtf8OrThrow, KString thiz, KInt start, KI
   RETURN_RESULT_OF(unsafeUtf16ToUtf8Impl<utf16toUtf8OrThrow>, thiz, start, size);
 }
 
-KInt Kotlin_StringBuilder_insertString(KRef builder, KInt position, KString fromString) {
+KInt Kotlin_StringBuilder_insertString(KRef builder, KInt distIndex, KString fromString, KInt sourceIndex, KInt count) {
   auto toArray = builder->array();
-  RuntimeAssert(toArray->count_ >= fromString->count_ + position, "must be true");
-  memcpy(CharArrayAddressOfElementAt(toArray, position),
-         CharArrayAddressOfElementAt(fromString, 0),
-         fromString->count_ * sizeof(KChar));
-  return fromString->count_;
+  RuntimeAssert(sourceIndex >= 0 && sourceIndex + count <= fromString->count_, "must be true");
+  RuntimeAssert(distIndex >= 0 && distIndex + count <= toArray->count_, "must be true");
+  memcpy(CharArrayAddressOfElementAt(toArray, distIndex),
+         CharArrayAddressOfElementAt(fromString, sourceIndex),
+         count * sizeof(KChar));
+  return count;
 }
 
 KInt Kotlin_StringBuilder_insertInt(KRef builder, KInt position, KInt value) {

runtime/src/main/kotlin/kotlin/native/Text.kt

@@ -112,6 +112,9 @@ internal fun checkBoundsIndexes(startIndex: Int, endIndex: Int, size: Int) {
     }
 }
 
+internal fun insertString(array: CharArray, start: Int, value: String): Int =
+        insertString(array, start, value, 0, value.length)
+
 @SymbolName("Kotlin_ByteArray_unsafeStringFromUtf8")
 internal external fun ByteArray.unsafeStringFromUtf8(start: Int, size: Int) : String
 
@@ -128,7 +131,7 @@ internal external fun String.unsafeStringToUtf8OrThrow(start: Int, size: Int) :
 internal external fun unsafeStringFromCharArray(array: CharArray, start: Int, size: Int) : String
 
 @SymbolName("Kotlin_StringBuilder_insertString")
-internal external fun insertString(array: CharArray, start: Int, value: String): Int
+internal external fun insertString(array: CharArray, distIndex: Int, value: String, sourceIndex: Int, count: Int): Int
 
 @SymbolName("Kotlin_StringBuilder_insertInt")
 internal external fun insertInt(array: CharArray, start: Int, value: Int): Int

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

@@ -227,6 +227,10 @@ actual class StringBuilder private constructor (
         val toAppend = csq ?: "null"
         if (start < 0 || end < start || start > toAppend.length) throw IndexOutOfBoundsException()
         ensureExtraCapacity(end - start)
+        (toAppend as? String)?.let {
+            _length += insertString(array, _length, it, start, end - start)
+            return this
+        }
         var index = start
         while (index < end)
             array[_length++] = toAppend[index++]