docs(CONTRIBUTING): add a guideline to not use dynamic casts/RTTI

And enforce it during compilation.

CONTRIBUTING.md

@@ -310,6 +310,20 @@ QObject notToMentionThatWeUseCamelCase;
 
 E.g. https://github.com/qTox/qTox/blob/master/src/misc/flowlayout.cpp
 
+
+## Dynamic casts / RTTI
+
+qTox is compiled without support for RTTI, as such PRs making use of
+`dynamic_cast()` will fail to compile and may be rejected on this basis. For
+manipulating Qt-based objects, use `qobject_cast()` instead.
+
+Compiling qTox without RTTI support gives up to 5-6% size reductions on
+compiled binary files. The usage of `dynamic_cast()` can also be completely
+mitigated when dealing with Qt objects through use of `qobject_cast()` which
+behaves very much like C++'s `dynamic_cast()` but without the RTTI overhead.
+
+Enforced with `-fno-rtti`.
+
 ## Documentaion
 
 If you added a new function, also add a doxygen comment before the

qtox.pro

@@ -40,7 +40,7 @@ FORMS    += \
 CONFIG   += c++11
 CONFIG   += link_pkgconfig
 
-QMAKE_CXXFLAGS += -fno-exceptions
+QMAKE_CXXFLAGS += -fno-exceptions -fno-rtti
 QMAKE_RESOURCE_FLAGS += -compress 9 -threshold 0
 
 # Rules for creating/updating {ts|qm}-files