QRunnable: Extract Method warnNullCallable()

The warning doesn't depend on the template argument, so factor it into
an out-of-line Q_DECL_COLD_FUNCTION. This will automatically mark the
is_null check as [[unlikely]], too.

As a drive-by, use printf-style qWarning() (expands to less code and
doesn't require #include <qdebug.h>).

Change-Id: I9ac2d912eb1d4f3d1d9b41d77700bed6d6850b0a
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/corelib/thread/qrunnable.cpp

@@ -9,6 +9,16 @@ QRunnable::~QRunnable()
 {
 }
 
+/*!
+    \internal
+    Prints a warning and returns nullptr.
+*/
+QRunnable *QRunnable::warnNullCallable()
+{
+    qWarning("Trying to create null QRunnable. This may stop working.");
+    return nullptr;
+}
+
 /*!
     \class QRunnable
     \inmodule QtCore

src/corelib/thread/qrunnable.h

@@ -5,7 +5,6 @@
 #define QRUNNABLE_H
 
 #include <QtCore/qcompilerdetection.h>
-#include <QtCore/qdebug.h>
 
 #include <functional>
 #include <type_traits>
@@ -64,6 +63,9 @@ protected:
         {
         }
     };
+
+private:
+    static Q_DECL_COLD_FUNCTION QRunnable *warnNullCallable();
 };
 
 class QGenericRunnable : public QRunnable
@@ -105,10 +107,8 @@ QRunnable *QRunnable::create(Callable &&functionToRun)
         const void *functionPtr = reinterpret_cast<void *>(functionToRun);
         is_null = !functionPtr;
     }
-    if (is_null) {
+    if (is_null)
-        qWarning() << "Trying to create null QRunnable. This may stop working.";
+        return warnNullCallable();
-        return nullptr;
-    }
 
     return new QGenericRunnable(
             new QGenericRunnableHelper<std::decay_t<Callable>>(