Make CFX_{Byte,Wide}String::Remove() no-touch if possible

core/fxcrt/fx_basic_wstring_unittest.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/fxcrt/include/fx_basic.h"
 #include "testing/fx_string_testhelpers.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 TEST(fxcrt, WideStringOperatorSubscript) {
   // CFX_WideString includes the NUL terminator for non-empty strings.
@@ skipping 276 matching lines @@
   freed.Remove(L'X');
   EXPECT_EQ(L"R", freed);
   freed.Remove(L'R');
   EXPECT_EQ(L"", freed);
 
   CFX_WideString empty;
   empty.Remove(L'X');
   EXPECT_EQ(L"", empty);
 }
 
+TEST(fxcrt, WideStringRemoveCopies) {
+  CFX_WideString freed(L"FREED");
+  const FX_WCHAR* old_buffer = freed.c_str();
+
+  // No change with single reference - no copy.
+  freed.Remove(L'Q');
+  EXPECT_EQ(L"FREED", freed);
+  EXPECT_EQ(old_buffer, freed.c_str());
+
+  // Change with single reference - no copy.
+  freed.Remove(L'E');
+  EXPECT_EQ(L"FRD", freed);
+  EXPECT_EQ(old_buffer, freed.c_str());
+
+  // No change with multiple references - no copy.
+  CFX_WideString shared(freed);
+  freed.Remove(L'Q');
+  EXPECT_EQ(L"FRD", freed);
+  EXPECT_EQ(old_buffer, freed.c_str());
+  EXPECT_EQ(old_buffer, shared.c_str());
+
+  // Change with multiple references -- must copy.
+  freed.Remove(L'D');
+  EXPECT_EQ(L"FR", freed);
+  EXPECT_NE(old_buffer, freed.c_str());
+  EXPECT_EQ(L"FRD", shared);
+  EXPECT_EQ(old_buffer, shared.c_str());
+}
+
 TEST(fxcrt, WideStringReplace) {
   CFX_WideString fred(L"FRED");
   fred.Replace(L"FR", L"BL");
   EXPECT_EQ(L"BLED", fred);
   fred.Replace(L"D", L"DDY");
   EXPECT_EQ(L"BLEDDY", fred);
   fred.Replace(L"LEDD", L"");
   EXPECT_EQ(L"BY", fred);
   fred.Replace(L"X", L"CLAMS");
   EXPECT_EQ(L"BY", fred);
@@ skipping 494 matching lines @@
   }
 }
 
 TEST(fxcrt, EmptyWideString) {
   CFX_WideString empty_str;
   EXPECT_TRUE(empty_str.IsEmpty());
   EXPECT_EQ(0, empty_str.GetLength());
   const FX_WCHAR* cstr = empty_str.c_str();
   EXPECT_EQ(0, FXSYS_wcslen(cstr));
 }