QStringBuilder: use data_ptr().data() instead of constData()

As the comment says, this allows us to skip the check for non-detached
strings and for null ones. We know that the one we've just created isn't
either, so long as len != 0. All QConcatenable::appendTo() calls do
nothing if len == 0, including not calling memcpy() at all.

Moreover, because it no longer references the QByteArray::_empty
variable, the GCC -Warray-bounds warning should be gone too.

Task-number: QTBUG-116763
Pick-to: 6.5
Change-Id: Ica7a43f6147b49c187ccfffd179eba2897ce2aff
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
(cherry picked from commit 06c724881c4227acf308e54ebd3db8235bb6b740)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 35ffe8749e7af31b70e3e7b993dceab75aa95633)

src/corelib/text/qstringbuilder.h

@@ -76,9 +76,12 @@ private:
         const qsizetype len = QConcatenable< QStringBuilder<A, B> >::size(*this);
         T s(len, Qt::Uninitialized);
 
-        // we abuse const_cast / constData here because we know we've just
-        // allocated the data and we're the only reference count
-        typename T::iterator d = const_cast<typename T::iterator>(s.constData());
+        // Using data_ptr() here (private API) so we can bypass the
+        // isDetached() and the replacement of a null pointer with _empty in
+        // both QString and QByteArray's data() and constData(). The result is
+        // the same if len != 0.
+        auto d = reinterpret_cast<typename T::iterator>(s.data_ptr().data());
+
         typename T::const_iterator const start = d;
         QConcatenable< QStringBuilder<A, B> >::appendTo(*this, d);
 

src/gui/image/qimagereaderwriterhelpers.cpp

@@ -97,8 +97,6 @@ QList<QByteArray> supportedImageFormats(Capability cap)
     return formats;
 }
 
-QT_WARNING_PUSH
-QT_WARNING_DISABLE_GCC("-Warray-bounds") // QTBUG-116763
 QList<QByteArray> supportedMimeTypes(Capability cap)
 {
     QList<QByteArray> mimeTypes;
@@ -114,7 +112,6 @@ QList<QByteArray> supportedMimeTypes(Capability cap)
     mimeTypes.erase(std::unique(mimeTypes.begin(), mimeTypes.end()), mimeTypes.end());
     return mimeTypes;
 }
-QT_WARNING_POP
 
 QList<QByteArray> imageFormatsForMimeType(const QByteArray &mimeType, Capability cap)
 {