Refactor the simplifier a bit to fix #22761

The core change in this commit, which fixes #22761, is that

* In a Core rule, ru_rhs is always occ-analysed.

This means adding a couple of calls to occurAnalyseExpr when
building a Rule, in
* GHC.Core.Rules.mkRule
* GHC.Core.Opt.Simplify.Iteration.simplRules

But diagosing the bug made me stare carefully at the code of the
Simplifier, and I ended up doing some only-loosely-related refactoring.

* I think that RULES could be lost because not every code path
  did addBndrRules

* The code around lambdas was very convoluted

It's mainly moving deck chairs around, but I like it more now.

3 files changed, 45 insertions, 0 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T22761.hs b/testsuite/tests/simplCore/should_compile/T22761.hs
new file mode 100644
index 0000000000..a396ecdc94
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T22761.hs
@@ -0,0 +1,40 @@
+module T22761 where
+
+import T22761a
+
+newtype Mod m = Mod m deriving Num
+
+gcdExt :: Integer -> (Integer, Integer)
+gcdExt x = go 0 x
+  where
+    go !_ 0 = (1, 1)
+    go r _ = go r r
+
+pow :: (Num m) => Mod m -> Mod m
+pow x = x*x*x
+{-# NOINLINE [1] pow #-}
+{-# RULES
+"powMod/3/Int" forall x. pow x = x*x*x
+#-}
+
+
+-- GHC puts `boo1` after `wom1` (since they don't appear connected)
+-- Then { wom1 = foo True }  rewrites to  { wom1 = boo False }
+-- so we need to do glomming.  And that triggers the bug
+-- in the RULE for `pow`!
+--
+-- wom2/boo2 are there to still elicit the bug if
+-- GHC reverses its default ordering
+
+{-# RULES
+"wombat1"  foo True = boo1 False
+#-}
+
+wom1 = foo True
+boo1 x = x
+
+{-# RULES
+"wombat2"  foo True = boo2 False
+#-}
+boo2 x = x
+wom2 = foo True
diff --git a/testsuite/tests/simplCore/should_compile/T22761a.hs b/testsuite/tests/simplCore/should_compile/T22761a.hs
new file mode 100644
index 0000000000..5d40815a2e
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T22761a.hs
@@ -0,0 +1,4 @@
+module T22761a where
+
+{-# NOINLINE [0] foo #-}
+foo x = x
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index 0ef452b743..12e2ecf042 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -471,5 +471,6 @@ test('T22611', [when(wordsize(32), skip), grep_errmsg(r'\$salterF') ], compile,
 test('T22715_2', normal, multimod_compile, ['T22715_2', '-v0 -O -fspecialise-aggressively'])
 test('T22802', normal, compile, ['-O'])
 test('T15205', normal, compile, ['-O -ddump-simpl -dno-typeable-binds -dsuppress-uniques'])
+test('T22761', normal, multimod_compile, ['T22761', '-O2 -v0'])
 
 test('RewriteHigherOrderPatterns', normal, compile, ['-O -ddump-rule-rewrites -dsuppress-all -dsuppress-uniques'])