QFont: fix a QT_ASCII_CAST_WARN

Says GCC:

  qfont.cpp:2202:81:   required from here
   2202 |         fontDescription += comma + tag.toString() + u'=' + QString::number(value);
        |                                                                                 ^
  qstringbuilder.h:403:37: warning: ‘static void QConcatenable<QByteArray>::appendTo(const QByteArray&, QChar*&)’ is deprecated: Use fromUtf8, QStringLiteral, or QLatin1StringView [-Wdeprecated-declarations]
    403 |         QConcatenableEx<B>::appendTo(p.b, out);
        |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
  qstringbuilder.h:372:43: note: declared here
    372 |     QT_ASCII_CAST_WARN static inline void appendTo(const QByteArray &ba, QChar *&out)
        |                                           ^~~~~~~~

This is because Tag::toString() returns QByteArray, not QString.

To fix, wrap the toString() result in QLatin1StringView. This is
safe, because the temporary QByteArray will be kept alive until the
end of the full-expression, so until after op+= has executed.

It does change the serialization vis-a-vis the old code, but a) only
for non-US-ASCII tags, which are not really valid, and b) we make it
more robust now, because each random 4-octet sequence is a valid L1
string, but many are not valid UTF-8, so their interpretation as U8
by the old code (QByteArray → QString assumes U8) depends on
behavior of Qt outside its spec. GIGO applies, of course, but we
don't tend to specify the GO for any given GI, so it's better to use
an algorithm for which no input is "garbage".

Amends 8fe6ad3eed40a8c851bf5d49e322f9794b9f7dcf.

Task-number: QTBUG-141412
Change-Id: Ic45f9147ca54201a33d953da57b768d2a5d115e4
Reviewed-by: Ahmad Samir <[email protected]>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/gui/text/qfont.cpp b/src/gui/text/qfont.cpp
index b60cad34a8d..2b2f2a27fcd 100644
--- a/src/gui/text/qfont.cpp
+++ b/src/gui/text/qfont.cpp
@@ -2201,7 +2201,7 @@ QString QFont::toString() const
 
     fontDescription += comma + QString::number(sortedFeatures.size());
     for (const auto &[tag, value] : std::as_const(sortedFeatures).asKeyValueRange())
-        fontDescription += comma + tag.toString() + u'=' + QString::number(value);
+        fontDescription += comma + QLatin1StringView{tag.toString()} + u'=' + QString::number(value);
 
     return fontDescription;
 }