Make renamer to be more flexible with parens in the LHS of the rules

We used to reject LHS like `(f a) b` in RULES and requires it to be written as
`f a b`. It will be handy to allow both as the expression may be more
readable with extra parens in some cases when infix operator is involved.
Espceially when TemplateHaskell is used, extra parens may be added out of
user's control and result in "valid" rules being rejected and there
are not always ways to workaround it.

Fixes #24621

compiler/GHC/Rename/Module.hs

@@ -1226,6 +1226,15 @@ with LHSs with a complicated desugaring (and hence unlikely to match);
 But there are legitimate non-trivial args ei, like sections and
 lambdas.  So it seems simpler not to check at all, and that is why
 check_e is commented out.
+
+Note [Parens on the LHS of a RULE]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+You may think that no one would write
+
+   {-# RULES "foo" (f True) = blah #-}
+
+with the LHS wrapped in parens. But Template Haskell does (#24621)!
+So we should accommodate them.
 -}
 
 checkValidRule :: FastString -> [Name] -> LHsExpr GhcRn -> NameSet -> RnM ()
@@ -1253,6 +1262,8 @@ validRuleLhs foralls lhs
     check (HsAppType _ e _)               = checkl e
     check (HsVar _ lv)
       | (unLoc lv) `notElem` foralls      = Nothing
+    -- See Note [Parens on the LHS of a RULE]
+    check (HsPar _ e)                     = checkl e
     check other                           = Just other  -- Failure
 
         -- Check an argument

testsuite/tests/rename/should_compile/T24621_normal.hs

+{-# OPTIONS_GHC -Wno-inline-rule-shadowing #-}
+module T24621_normal where
+
+import Data.Function
+
+foo :: a -> a
+foo x = x
+
+{-# RULES "" forall a b c. a * c + b * c = (a + b) * c :: Int #-}
+{-# RULES "." forall f g. (f . g) foo = f (g foo) #-}
+{-# RULES "foo" forall a b. (foo a) b = a b #-}
+{-# RULES "on" forall a b. (flip compare `on` foo) a b = compare b a #-}

testsuite/tests/rename/should_compile/T24621_th.hs

+{-# LANGUAGE TemplateHaskell #-}
+{-# OPTIONS_GHC -Wno-inline-rule-shadowing #-}
+module T24621_th where
+
+import Data.Function
+
+foo :: a -> a
+foo x = x
+
+$( [d| {-# RULES "" forall a b c. a * c + b * c = (a + b) * c :: Int #-} |] )
+$( [d| {-# RULES "." forall a b. (.) a b foo = a (b foo) #-} |] )
+$( [d| {-# RULES "foo" forall a b. foo a b = a b #-} |] )

testsuite/tests/rename/should_compile/all.T

@@ -223,4 +223,6 @@ test('T22478a', req_th, compile, [''])
 test('RecordWildCardDeprecation', normal, multimod_compile, ['RecordWildCardDeprecation', '-Wno-duplicate-exports'])
 test('T14032b', normal, compile_and_run, [''])
 test('T14032d', normal, compile, [''])
+test('T24621_normal', normal, compile, [''])
+test('T24621_th', req_th, compile, [''])
 test('T24732', normal, compile_and_run, ['-package "base(Prelude, Text.Printf as P\')"'])