Make RULE matching insensitive to eta-expansion

This patch fixes #19790 by making the rule matcher do on-the-fly
eta reduction.  See Note [Eta reduction the target] in GHC.Core.Rules

I found I also had to careful about casts when matching; see
Note [Casts in the target] and Note [Casts in the template]

Lots more comments and Notes in the rule matcher

3 files changed, 27 insertions, 1 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T19790.hs b/testsuite/tests/simplCore/should_compile/T19790.hs
new file mode 100644
index 0000000000..98f1d32836
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T19790.hs
@@ -0,0 +1,19 @@
+module T19790 where
+
+newtype Stream = MkStream { unStream :: Int -> Int }
+
+fromStream :: Stream -> [Int]
+{-# NOINLINE fromStream #-}
+fromStream (MkStream f) = map f [1,2]
+
+toStream :: [Int] -> Stream
+{-# NOINLINE toStream #-}
+toStream xs = MkStream (\x -> x + length xs)
+
+
+foo :: [Int] -> [Int]
+foo xs = fromStream (MkStream (\p -> unStream (toStream xs) p))
+
+-- The question is: does this rule fire?  It should!
+{-# RULES "This rule should fire!" forall xs. fromStream (toStream xs) = xs #-}
+
diff --git a/testsuite/tests/simplCore/should_compile/T19790.stderr b/testsuite/tests/simplCore/should_compile/T19790.stderr
new file mode 100644
index 0000000000..71632231f7
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T19790.stderr
@@ -0,0 +1,7 @@
+Rule fired: Class op + (BUILTIN)
+Rule fired: Class op length (BUILTIN)
+Rule fired: map (GHC.Base)
+Rule fired: fold/build (GHC.Base)
+Rule fired: This rule should fire! (T19790)
+Rule fired: length (GHC.List)
+Rule fired: lengthList (GHC.List)
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index 7285b91c45..9988457432 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -377,5 +377,5 @@ test('T20200a', normal, compile, ['-O2'])
 test('T20200b', normal, compile, ['-O2'])
 test('T20200KG', [extra_files(['T20200KGa.hs', 'T20200KG.hs-boot'])], multimod_compile, ['T20200KG', '-v0 -O2 -fspecialise-aggressively'])
 test('T20639', normal, compile, ['-O2'])
-
 test('T20894', normal, compile, ['-dcore-lint -O1 -ddebug-output'])
+test('T19790',  normal, compile, ['-O -ddump-rule-firings'])