QtGui: use QStringRef to optimize memory allocation

Replace substring functions that return QString with
corresponding functions that return QStringRef where
it's possible.

Create QString from QStringRef only where necessary.

Change-Id: Id83c60cc7568b20ef008b51c582cfdf466a47e35
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/gui/image/qpnghandler.cpp

@@ -761,7 +761,7 @@ static void set_text(const QImage &image, png_structp png_ptr, png_infop info_pt
     QMap<QString, QString>::ConstIterator it = text.constBegin();
     int i = 0;
     while (it != text.constEnd()) {
-        text_ptr[i].key = qstrdup(it.key().left(79).toLatin1().constData());
+        text_ptr[i].key = qstrdup(it.key().leftRef(79).toLatin1().constData());
         bool noCompress = (it.value().length() < 40);
 
 #ifdef PNG_iTXt_SUPPORTED

src/gui/kernel/qkeysequence.cpp

@@ -1154,21 +1154,22 @@ int QKeySequencePrivate::decodeString(const QString &str, QKeySequence::Sequence
     }
 
     int p = accel.lastIndexOf(QLatin1Char('+'), str.length() - 2); // -2 so that Ctrl++ works
+    QStringRef accelRef(&accel);
     if(p > 0)
-        accel = accel.mid(p + 1);
+        accelRef = accelRef.mid(p + 1);
 
     int fnum = 0;
-    if (accel.length() == 1) {
+    if (accelRef.length() == 1) {
 #if defined(Q_OS_MACX)
-        int qtKey = qtkeyForMacSymbol(accel.at(0));
+        int qtKey = qtkeyForMacSymbol(accelRef.at(0));
         if (qtKey != -1) {
             ret |= qtKey;
         } else
 #endif
         {
-            ret |= accel.at(0).toUpper().unicode();
+            ret |= accelRef.at(0).toUpper().unicode();
         }
-    } else if (accel.at(0) == QLatin1Char('f') && (fnum = accel.mid(1).toInt()) >= 1 && fnum <= 35) {
+    } else if (accelRef.at(0) == QLatin1Char('f') && (fnum = accelRef.mid(1).toInt()) >= 1 && fnum <= 35) {
         ret |= Qt::Key_F1 + fnum - 1;
     } else {
         // For NativeText, check the traslation table first,
@@ -1182,7 +1183,7 @@ int QKeySequencePrivate::decodeString(const QString &str, QKeySequence::Sequence
                 QString keyName(tran == 0
                                 ? QCoreApplication::translate("QShortcut", keyname[i].name)
                                 : QString::fromLatin1(keyname[i].name));
-                if (accel == keyName.toLower()) {
+                if (accelRef == keyName.toLower()) {
                     ret |= keyname[i].key;
                     found = true;
                     break;

src/gui/text/qcssparser.cpp

@@ -1510,7 +1510,7 @@ QRect Declaration::rectValue() const
     const QStringList func = v.variant.toStringList();
     if (func.count() != 2 || func.at(0).compare(QLatin1String("rect")) != 0)
         return QRect();
-    QStringList args = func[1].split(QLatin1Char(' '), QString::SkipEmptyParts);
+    const auto args = func[1].splitRef(QLatin1Char(' '), QString::SkipEmptyParts);
     if (args.count() != 4)
         return QRect();
     QRect rect(args[0].toInt(), args[1].toInt(), args[2].toInt(), args[3].toInt());
@@ -1915,9 +1915,8 @@ bool StyleSelector::basicSelectorMatches(const BasicSelector &sel, NodePtr node)
                 return false;
 
             if (a.valueMatchCriterium == QCss::AttributeSelector::MatchContains) {
-
-                QStringList lst = attrValue.split(QLatin1Char(' '));
-                if (!lst.contains(a.value))
+                const auto lst = attrValue.splitRef(QLatin1Char(' '));
+                if (!lst.contains(QStringRef(&a.value)))
                     return false;
             } else if (
                 (a.valueMatchCriterium == QCss::AttributeSelector::MatchEqual

src/gui/text/qfont.cpp

@@ -2033,7 +2033,7 @@ uint qHash(const QFont &font, uint seed) Q_DECL_NOTHROW
  */
 bool QFont::fromString(const QString &descrip)
 {
-    QStringList l(descrip.split(QLatin1Char(',')));
+    const auto l = descrip.splitRef(QLatin1Char(','));
 
     int count = l.count();
     if (!count || (count > 2 && count < 9) || count > 11) {
@@ -2042,7 +2042,7 @@ bool QFont::fromString(const QString &descrip)
         return false;
     }
 
-    setFamily(l[0]);
+    setFamily(l[0].toString());
     if (count > 1 && l[1].toDouble() > 0.0)
         setPointSizeF(l[1].toDouble());
     if (count == 9) {

src/gui/text/qfontdatabase.cpp

@@ -697,15 +697,15 @@ static QStringList familyList(const QFontDef &req)
     if (req.family.isEmpty())
         return family_list;
 
-    QStringList list = req.family.split(QLatin1Char(','));
+    const auto list = req.family.splitRef(QLatin1Char(','));
     const int numFamilies = list.size();
     family_list.reserve(numFamilies);
     for (int i = 0; i < numFamilies; ++i) {
-        QString str = list.at(i).trimmed();
+        QStringRef str = list.at(i).trimmed();
         if ((str.startsWith(QLatin1Char('"')) && str.endsWith(QLatin1Char('"')))
             || (str.startsWith(QLatin1Char('\'')) && str.endsWith(QLatin1Char('\''))))
             str = str.mid(1, str.length() - 2);
-        family_list << str;
+        family_list << str.toString();
     }
 
     // append the substitute list for each family in family_list