I just discovered this novel #emacs #emacslisp #elisp feature:

As part of their definitions, functions can be declared as being pure and side-effect free. This shows up in *Help* buffers and is also used by the compiler for optimisation. These types of declarations are associated with the symbol as opposed to the function value(https://www.gnu.org/software/emacs/manual/html_node/elisp/Symbol-Properties.html), just like #clojure metadata(https://clojure.org/reference/metadata).

I know #dlang has a similar feature where functions can be marked as being pure (https://tour.dlang.org/tour/en/gems/functional-programming), but I haven't seen it anywhere else. I suppose #rust immutable references are kind of similar too, although that's stretching it.

Are there any there any other cool or novel features in elisp?