Remove now-unnecessary hack in CoreFVs.ruleRhsFVS

compiler/coreSyn/CoreFVs.lhs

@@ -278,18 +278,16 @@ exprsOrphNames es = foldr (unionNameSets . exprOrphNames) emptyNameSet es
 -- | Those variables free in the right hand side of a rule
 ruleRhsFreeVars :: CoreRule -> VarSet
 ruleRhsFreeVars (BuiltinRule {}) = noFVs
-ruleRhsFreeVars (Rule { ru_fn = fn, ru_bndrs = bndrs, ru_rhs = rhs })
-  = delFromUFM fvs fn    -- Note [Rule free var hack]
-  where
-    fvs = addBndrs bndrs (expr_fvs rhs) isLocalVar emptyVarSet
+ruleRhsFreeVars (Rule { ru_fn = _, ru_bndrs = bndrs, ru_rhs = rhs })
+  = addBndrs bndrs (expr_fvs rhs) isLocalVar emptyVarSet
+      -- See Note [Rule free var hack]
 
 -- | Those variables free in the both the left right hand sides of a rule
 ruleFreeVars :: CoreRule -> VarSet
 ruleFreeVars (BuiltinRule {}) = noFVs
-ruleFreeVars (Rule { ru_fn = fn, ru_bndrs = bndrs, ru_rhs = rhs, ru_args = args })
-  = delFromUFM fvs fn   -- Note [Rule free var hack]
-  where
-    fvs = addBndrs bndrs (exprs_fvs (rhs:args)) isLocalVar emptyVarSet
+ruleFreeVars (Rule { ru_fn = _, ru_bndrs = bndrs, ru_rhs = rhs, ru_args = args })
+  = addBndrs bndrs (exprs_fvs (rhs:args)) isLocalVar emptyVarSet
+      -- See Note [Rule free var hack]
 
 idRuleRhsVars :: (Activation -> Bool) -> Id -> VarSet
 -- Just the variables free on the *rhs* of a rule
@@ -316,16 +314,16 @@ ruleLhsFreeIds (Rule { ru_bndrs = bndrs, ru_args = args })
   = addBndrs bndrs (exprs_fvs args) isLocalId emptyVarSet
 \end{code}
 
-
-Note [Rule free var hack]
+Note [Rule free var hack]  (Not a hack any more)
 ~~~~~~~~~~~~~~~~~~~~~~~~~
-Don't include the Id in its own rhs free-var set.
-Otherwise the occurrence analyser makes bindings recursive
-that shoudn't be.  E.g.
+We used not to include the Id in its own rhs free-var set.
+Otherwise the occurrence analyser makes bindings recursive:
+        f x y = x+y
         RULE:  f (f x y) z  ==>  f x (f y z)
-
-Also since rule_fn is a Name, not a Var, we have to use the grungy delUFM.
-
+However, the occurrence analyser distinguishes "non-rule loop breakers" 
+from "rule-only loop breakers" (see BasicTypes.OccInfo).  So it will
+put this 'f' in a Rec block, but will mark the binding as a non-rule loop
+breaker, which is perfectly inlinable.
 
 \begin{code}
 -- |Free variables of a vectorisation declaration