Add warning for probable identities (fromIntegral and friends)

[ghc-hetmet.git] / compiler / deSugar / Desugar.lhs

diff --git a/compiler/deSugar/Desugar.lhs b/compiler/deSugar/Desugar.lhs
index e5d763c..073e873 100644 (file)
--- a/compiler/deSugar/Desugar.lhs
+++ b/compiler/deSugar/Desugar.lhs
@@ -337,8 +337,9 @@ dsRule (L loc (HsRule name act vars lhs _tv_lhs rhs _fv_rhs))
   = putSrcSpanDs loc $ 
     do { let bndrs' = [var | RuleBndr (L _ var) <- vars]
 
-       ; lhs' <- unsetOptM Opt_EnableRewriteRules $
-                 dsLExpr lhs   -- Note [Desugaring RULE left hand sides]
+        ; lhs' <- unsetOptM Opt_EnableRewriteRules $
+                  unsetOptM Opt_WarnIdentities $
+                  dsLExpr lhs   -- Note [Desugaring RULE left hand sides]
 
        ; rhs' <- dsLExpr rhs
 
@@ -359,6 +360,7 @@ dsRule (L loc (HsRule name act vars lhs _tv_lhs rhs _fv_rhs))
        ; return (Just rule)
        } } }
 \end{code}
+
 Note [Desugaring RULE left hand sides]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 For the LHS of a RULE we do *not* want to desugar
@@ -369,4 +371,6 @@ switching off EnableRewriteRules.  See DsExpr.dsExplicitList.
 
 That keeps the desugaring of list comprehensions simple too.
 
-
+Nor do we want to warn of conversion identities on the LHS;
+the rule is precisly to optimise them:
+  {-# RULES "fromRational/id" fromRational = id :: Rational -> Rational #-}