Fix the occurrence analyser

Ticket #18603 demonstrated that the occurrence analyser's
handling of

  local RULES for imported Ids

(which I now call IMP-RULES) was inadequate.  It led the simplifier
into an infnite loop by failing to label a binder as a loop breaker.

The main change in this commit is to treat IMP-RULES in a simple and
uniform way: as extra rules for the local binder.  See
  Note [IMP-RULES: local rules for imported functions]

This led to quite a bit of refactoring.  The result is still tricky,
but it's much better than before, and better documented I think.

Oh, and it fixes the bug.

2 files changed, 30 insertions, 0 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T18603.hs b/testsuite/tests/simplCore/should_compile/T18603.hs
new file mode 100644
index 0000000000..d85f77c66a
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T18603.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE NoImplicitPrelude #-}
+
+module Test where
+
+import GHC.Base (build, foldr, id, Maybe(..))
+
+catMaybes :: [Maybe a] -> [a]
+catMaybes = mapMaybe id
+
+mapMaybe          :: (a -> Maybe b) -> [a] -> [b]
+mapMaybe _ []     = []
+mapMaybe f (x:xs) =
+ let rs = mapMaybe f xs in
+ case f x of
+  Nothing -> rs
+  Just r  -> r:rs
+{-# NOINLINE [1] mapMaybe #-}
+
+{-# RULES
+"mapMaybe"     [~1] forall f xs. mapMaybe f xs
+                    = build (\c n -> foldr (mapMaybeFB c f) n xs)
+"mapMaybeList" [1]  forall f. foldr (mapMaybeFB (:) f) [] = mapMaybe f
+  #-}
+
+{-# INLINE [0] mapMaybeFB #-} -- See Note [Inline FB functions] in GHC.List
+mapMaybeFB :: (b -> r -> r) -> (a -> Maybe b) -> a -> r -> r
+mapMaybeFB cons f x next = case f x of
+  Nothing -> next
+  Just r -> cons r next
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index d377cfd06b..1535e32253 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -333,6 +333,7 @@ test('T18347', normal, compile, ['-dcore-lint -O'])
 test('T18355', [ grep_errmsg(r'OneShot') ], compile, ['-O -ddump-simpl -dsuppress-uniques'])
 test('T18399', normal, compile, ['-dcore-lint -O'])
 test('T18589', normal, compile, ['-dcore-lint -O'])
+test('T18603', normal, compile, ['-dcore-lint -O'])
 
 # T18649 should /not/ generate a specialisation rule
 test('T18649', normal, compile, ['-O -ddump-rules -Wno-simplifiable-class-constraints'])