Add a comment, connecting the seq to the test (#4367) that shows its usefulness

diff --git a/compiler/basicTypes/IdInfo.lhs b/compiler/basicTypes/IdInfo.lhs
index c383f89..77f0325 100644 (file)
--- a/compiler/basicTypes/IdInfo.lhs
+++ b/compiler/basicTypes/IdInfo.lhs
@@ -243,7 +243,7 @@ setUnfoldingInfoLazily info uf      -- Lazy variant to avoid looking at the
 
 setUnfoldingInfo :: IdInfo -> Unfolding -> IdInfo
 setUnfoldingInfo info uf 
-  = seqUnfolding uf `seq`
+  = seqUnfolding uf `seq`      -- This seq makes a BIG difference to Trac #4367
     info { unfoldingInfo = uf }
 
 setArityInfo :: IdInfo -> ArityInfo -> IdInfo