QDebug: make toString() SCARY

Extract the non-template-dependent code into an out-of-line function
and pass the actual streaming operation as a callback.

Saves 4% (174811→167936) in executable size for tst_qdebug and 2.7%
(93639→91122) for tst_tostring on Linux GCC 9 AMD64 release builds.

Change-Id: If232e5b26c66981ffcb614f1bdb7007c71e879bf
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 5cdd1f594d26e1d4f84b00741be1ab7231458512)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qdebug.cpp

@@ -1002,6 +1002,17 @@ QDebug &QDebug::resetFormat()
     \include qdebug-toString.qdocinc
 */
 
+/*! \internal */
+QString QDebug::toStringImpl(StreamTypeErased s, const void *obj)
+{
+    QString 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

@@ -223,13 +223,19 @@ public:
     QDebug &operator<<(T u128) { putUInt128(&u128); return maybeSpace(); }
 #endif // QT_SUPPORTS_INT128
 
+private:
+    template <typename T>
+    static void streamTypeErased(QDebug &d, const void *obj)
+    {
+        d << *static_cast<const T*>(obj);
+    }
+    using StreamTypeErased = void(*)(QDebug&, const void*);
+    QT7_ONLY(Q_CORE_EXPORT) static QString toStringImpl(StreamTypeErased s, const void *obj);
+public:
     template <typename T>
     static QString toString(const T &object)
     {
-        QString buffer;
+        return toStringImpl(&streamTypeErased<T>, &object);
-        QDebug stream(&buffer);
-        stream.nospace() << object;
-        return buffer;
     }
 };