This is a feature request.

If a function is annotated as 'pure', then it's checked that it doesn't operate against global state. ie, always returns inputs based only on arguments passed to it, and it doesn't do IO.

Also, functions annotated as 'pure' can only call other functions annotated as 'pure'

More or less, how the D lang describes them:

https://dlang.org/spec/function.html#pure-functions