Comment by safercplusplus

Yeah, but C++ now supports "user-defined" annotations which effectively allow you to add the equivalent of any keyword you need, right? (Even if it's not the prettiest syntax.) For example, the scpptool static analyzer supports (and enforces) lifetime annotations with similar meaning to Rust's lifetime annotations.

I believe gcc actually does support `__attribute__ ((pure))` to indicate function purity. (I assume it doesn't actually enforce it, but presumably it theoretically could at some point.)