Long live QDebug::toBytes()!

I've seen over and over again how test authors are plagued by the
impedance mismatch between their 8-bit actual data and the result of
QString::toString() being in UTF-16. They invariably ditch their 8-bit
inputs and go to QString, but the mere fact that they start out with
8-bit input and then run into problems means we have a gaping API hole
here. QTest::toString() also suffers from this.

So add the option to a) construct a QDebug object over a QByteArray
(already supported by underlying QTextStream) and b) to stream into
QByteArray like toString() streams into QString.

Finally, make QTest::toString() use the new toBytes() function instead
of the old toString() one.

This saves 1% (91122→90248) in tst_tostring exeutable size on
optimized Linux AMD64 GCC 9 builds.

[ChangeLog][QtCore][QDebug] Added ctor from QByteArray* and a static
toBytes() function.

Change-Id: I2b021513d6427a1a961f39e751eaf4faaf527ba8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qdebug.cpp

@@ -1000,6 +1000,8 @@ QDebug &QDebug::resetFormat()
     \since 6.0
 
     \include qdebug-toString.qdocinc
+
+    \sa toBytes()
 */
 
 /*! \internal */
@@ -1013,6 +1015,26 @@ QString QDebug::toStringImpl(StreamTypeErased s, const void *obj)
     return result;
 }
 
+/*!
+    \fn template <class T> QByteArray QDebug::toBytes(const T &object)
+    \since 6.9
+
+    This is equivalent to \c{QDebug::toString(object).toUtf8()}, but more efficient.
+
+    \sa toString()
+*/
+
+/*! \internal */
+QByteArray QDebug::toBytesImpl(StreamTypeErased s, const void *obj)
+{
+    QByteArray result;
+    {
+        QDebug d(&result);
+        s(d.nospace(), obj);
+    }
+    return result;
+}
+
 /*!
     \fn template <class T> QDebug operator<<(QDebug debug, const QList<T> &list)
     \relates QDebug

src/corelib/io/qdebug.h

@@ -51,6 +51,9 @@ class QT6_ONLY(Q_CORE_EXPORT) QDebug : public QIODeviceBase
         explicit Stream(QString *string)
             : ts(string, WriteOnly)
         {}
+        explicit Stream(QByteArray *ba)
+            : ts(ba, WriteOnly)
+        {}
         explicit Stream(QtMsgType t)
             : ts(&buffer, WriteOnly),
               type(t),
@@ -78,6 +81,7 @@ class QT6_ONLY(Q_CORE_EXPORT) QDebug : public QIODeviceBase
 public:
     explicit QDebug(QIODevice *device) : stream(new Stream(device)) {}
     explicit QDebug(QString *string) : stream(new Stream(string)) {}
+    explicit QDebug(QByteArray *bytes) : stream(new Stream(bytes)) {}
     explicit QDebug(QtMsgType t) : stream(new Stream(t)) {}
     QDebug(const QDebug &o) : stream(o.stream) { ++stream->ref; }
     QDebug(QDebug &&other) noexcept : stream{std::exchange(other.stream, nullptr)} {}
@@ -232,12 +236,19 @@ private:
     }
     using StreamTypeErased = void(*)(QDebug&, const void*);
     QT7_ONLY(Q_CORE_EXPORT) static QString toStringImpl(StreamTypeErased s, const void *obj);
+    QT7_ONLY(Q_CORE_EXPORT) static QByteArray toBytesImpl(StreamTypeErased s, const void *obj);
 public:
     template <typename T>
     static QString toString(const T &object)
     {
         return toStringImpl(&streamTypeErased<T>, &object);
     }
+
+    template <typename T>
+    static QByteArray toBytes(const T &object)
+    {
+        return toBytesImpl(&streamTypeErased<T>, &object);
+    }
 };
 
 Q_DECLARE_SHARED(QDebug)

src/testlib/qtesttostring.h

@@ -55,7 +55,7 @@ inline typename std::enable_if<!QtPrivate::IsQEnumHelper<T>::Value && !std::is_e
     char *result = nullptr;
 #ifndef QT_NO_DEBUG_STREAM
     if constexpr (QTypeTraits::has_ostream_operator_v<QDebug, T>) {
-        result = qstrdup(QDebug::toString(t).toUtf8().constData());
+        result = qstrdup(QDebug::toBytes(t).constData());
     } else {
         static_assert(!QMetaTypeId2<T>::IsBuiltIn,
                 "Built-in type must implement debug streaming operator "