User defined rules are disabled under Safe Haskell - warning is scary
Does this warning means that just defining new rules is forbidden, or does it mean that no rules fire ever in {-# LANGUAGE Safe #-}
modules.
If it's former, then the
Defining user rules is disabled under Safe Haskell
wording would be more precise.
The manual says
RULES — Rewrite rules defined in a module M compiled with Safe are dropped. Rules defined in Trustworthy modules that M imports are still valid and will fire as usual.
So it's the former, I'd suggest changing the warning's wording.