Filter out BuiltinRules in occurrence analysis

Fixes Trac #11651.  Merge to 8.0.

1 files changed, 14 insertions, 0 deletions

diff --git a/compiler/simplCore/OccurAnal.hs b/compiler/simplCore/OccurAnal.hs
index d1c3ca809a..3eb20d0143 100644
--- a/compiler/simplCore/OccurAnal.hs
+++ b/compiler/simplCore/OccurAnal.hs
@@ -86,6 +86,7 @@ occurAnalysePgm this_mod active_rule imp_rules vects vectVars binds
     imp_rule_edges = foldr (plusVarEnv_C unionVarSet) emptyVarEnv
                             [ mapVarEnv (const maps_to) (exprFreeIds arg `delVarSetList` ru_bndrs imp_rule)
                             | imp_rule <- imp_rules
+                            , not (isBuiltinRule imp_rule)  -- See Note [Plugin rules]
                             , let maps_to = exprFreeIds (ru_rhs imp_rule)
                                              `delVarSetList` ru_bndrs imp_rule
                             , arg <- ru_args imp_rule ]
@@ -114,6 +115,19 @@ occurAnalyseExpr' enable_binder_swap expr
     -- To be conservative, we say that all inlines and rules are active
     all_active_rules = \_ -> True
 
+{- Note [Plugin rules]
+~~~~~~~~~~~~~~~~~~~~~~
+Conal Eliot (Trac #11651) built a GHC plugin that added some
+BuiltinRules (for imported Ids) to the mg_rules field of ModGuts, to
+do some domain-specific transformations that could not be expressed
+with an ordinary pattern-matching CoreRule.  But then we can't extract
+the dependencies (in imp_rule_edges) from ru_rhs etc, because a
+BuiltinRule doesn't have any of that stuff.
+
+So we simply assume that BuiltinRules have no dependencies, and filter
+them out from the imp_rule_edges comprehension.
+-}
+
 {-
 ************************************************************************
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