[commit: ghc] master: Don't complain about unused Rule binders (57b4c55)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/57b4c5524fcbf02f61dfc8d9395906dc7f50f048/ghc

>---------------------------------------------------------------

commit 57b4c5524fcbf02f61dfc8d9395906dc7f50f048
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Tue Mar 1 17:07:26 2016 +0000

    Don't complain about unused Rule binders
    
    This fixes Trac #11643.  It's a corner case, now documented in
    Note [Linting rules] in CoreLint


>---------------------------------------------------------------

57b4c5524fcbf02f61dfc8d9395906dc7f50f048
 compiler/coreSyn/CoreLint.hs | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/compiler/coreSyn/CoreLint.hs b/compiler/coreSyn/CoreLint.hs
index 90e68e8..99625c9 100644
--- a/compiler/coreSyn/CoreLint.hs
+++ b/compiler/coreSyn/CoreLint.hs
@@ -1175,7 +1175,10 @@ lintCoreRule fun_ty (Rule { ru_name = name, ru_bndrs = bndrs
        ; ensureEqTys lhs_ty rhs_ty $
          (rule_doc <+> vcat [ text "lhs type:" <+> ppr lhs_ty
                             , text "rhs type:" <+> ppr rhs_ty ])
-       ; let bad_bndrs = filterOut (`elemVarSet` exprsFreeVars args) bndrs
+       ; let bad_bndrs = filterOut (`elemVarSet` exprsFreeVars args) $
+                         filter (`elemVarSet` exprFreeVars rhs) $
+                         bndrs
+
        ; checkL (null bad_bndrs)
                 (rule_doc <+> text "unbound" <+> ppr bad_bndrs)
             -- See Note [Linting rules]
@@ -1186,9 +1189,9 @@ lintCoreRule fun_ty (Rule { ru_name = name, ru_bndrs = bndrs
 {- Note [Linting rules]
 ~~~~~~~~~~~~~~~~~~~~~~~
 It's very bad if simplifying a rule means that one of the template
-variables (ru_bndrs) becomes not-mentioned in the template argumments
-(ru_args).  How can that happen?  Well, in Trac #10602, SpecConstr
-stupidly constructed a rule like
+variables (ru_bndrs) that /is/ mentioned on the RHS becomes
+not-mentioned in the LHS (ru_args).  How can that happen?  Well, in
+Trac #10602, SpecConstr stupidly constructed a rule like
 
   forall x,c1,c2.
      f (x |> c1 |> c2) = ....
@@ -1198,6 +1201,16 @@ Trac #10602, it collapsed to the identity and was removed altogether.)
 
 We don't have a great story for what to do here, but at least
 this check will nail it.
+
+NB (Trac #11643): it's possible that a variable listed in the
+binders becomes not-mentioned on both LHS and RHS.  Here's a silly
+example:
+   RULE forall x y. f (g x y) = g (x+1 (y-1)
+And suppose worker/wrapper decides that 'x' is Absent.  Then
+we'll end up with
+   RULE forall x y. f ($gw y) = $gw (x+1)
+This seems sufficiently obscure that there isn't enough payoff to
+try to trim the forall'd binder list.
 -}
 
 {-