Improve the handling of default methods

See the long Note [INLINE and default methods].  

This patch changes a couple of data types, with a knock-on effect on
the format of interface files.  A lot of files get touched, but is a
relatively minor change.  The main tiresome bit is the extra plumbing
to communicate default methods between the type checker and the
desugarer.

4 files changed, 51 insertions, 26 deletions

diff --git a/compiler/iface/BinIface.hs b/compiler/iface/BinIface.hs
index beb39c00f1..2931ffa70a 100644
--- a/compiler/iface/BinIface.hs
+++ b/compiler/iface/BinIface.hs
@@ -600,16 +600,18 @@ instance Binary RuleMatchInfo where
                       else return FunLike
 
 instance Binary InlinePragma where
-    put_ bh (InlinePragma a b c) = do
+    put_ bh (InlinePragma a b c d) = do
             put_ bh a
             put_ bh b
             put_ bh c
+            put_ bh d
 
     get bh = do
            a <- get bh
            b <- get bh
            c <- get bh
-           return (InlinePragma a b c)
+           d <- get bh
+           return (InlinePragma a b c d)
 
 instance Binary StrictnessMark where
     put_ bh MarkedStrict    = putByte bh 0
@@ -1188,11 +1190,12 @@ instance Binary IfaceUnfolding where
     put_ bh (IfCoreUnfold e) = do
 	putByte bh 0
 	put_ bh e
-    put_ bh (IfInlineRule a b e) = do
+    put_ bh (IfInlineRule a b c d) = do
 	putByte bh 1
 	put_ bh a
 	put_ bh b
-	put_ bh e
+	put_ bh c
+	put_ bh d
     put_ bh (IfWrapper a n) = do
 	putByte bh 2
 	put_ bh a
@@ -1200,6 +1203,9 @@ instance Binary IfaceUnfolding where
     put_ bh (IfDFunUnfold as) = do
 	putByte bh 3
 	put_ bh as
+    put_ bh (IfCompulsory e) = do
+	putByte bh 4
+	put_ bh e
     get bh = do
 	h <- getByte bh
 	case h of
@@ -1207,13 +1213,16 @@ instance Binary IfaceUnfolding where
 		  return (IfCoreUnfold e)
 	  1 -> do a <- get bh
 		  b <- get bh
-		  e <- get bh
-		  return (IfInlineRule a b e)
+		  c <- get bh
+		  d <- get bh
+		  return (IfInlineRule a b c d)
 	  2 -> do a <- get bh
 		  n <- get bh
 		  return (IfWrapper a n)
-	  _ -> do as <- get bh
+	  3 -> do as <- get bh
 		  return (IfDFunUnfold as)
+	  _ -> do e <- get bh
+		  return (IfCompulsory e)
 
 instance Binary IfaceNote where
     put_ bh (IfaceSCC aa) = do
diff --git a/compiler/iface/IfaceSyn.lhs b/compiler/iface/IfaceSyn.lhs
index 9485dc9453..1db78220b7 100644
--- a/compiler/iface/IfaceSyn.lhs
+++ b/compiler/iface/IfaceSyn.lhs
@@ -211,11 +211,16 @@ data IfaceInfoItem
 
 data IfaceUnfolding 
   = IfCoreUnfold IfaceExpr
+  | IfCompulsory IfaceExpr	-- Only used for default methods, in fact
+
   | IfInlineRule Arity 
                  Bool		-- OK to inline even if *un*-saturated
+		 Bool		-- OK to inline even if context is boring
                  IfaceExpr 
+
   | IfWrapper    Arity Name	  -- NB: we need a Name (not just OccName) because the worker
 				  --     can simplify to a function in another module.
+
   | IfDFunUnfold [IfaceExpr]
 
 --------------------------------
@@ -676,10 +681,11 @@ instance Outputable IfaceInfoItem where
   ppr HsNoCafRefs	 = ptext (sLit "HasNoCafRefs")
 
 instance Outputable IfaceUnfolding where
+  ppr (IfCompulsory e)     = ptext (sLit "<compulsory>") <+> parens (ppr e)
   ppr (IfCoreUnfold e)     = parens (ppr e)
-  ppr (IfInlineRule a b e) = ptext (sLit "InlineRule:")
-                             <+> parens (ptext (sLit "arity") <+> int a <+> ppr b) 
-      		      	     <+> parens (ppr e)
+  ppr (IfInlineRule a uok bok e) = ptext (sLit "InlineRule")
+                                  <+> ppr (a,uok,bok) 
+      		      	          <+> parens (ppr e)
   ppr (IfWrapper a wkr)    = ptext (sLit "Worker:") <+> ppr wkr <+> parens (ptext (sLit "arity") <+> int a)
   ppr (IfDFunUnfold ns)    = ptext (sLit "DFun:") <+> brackets (pprWithCommas (pprIfaceExpr parens) ns)
 
@@ -799,10 +805,11 @@ freeNamesItem (HsUnfold _ u) = freeNamesIfUnfold u
 freeNamesItem _              = emptyNameSet
 
 freeNamesIfUnfold :: IfaceUnfolding -> NameSet
-freeNamesIfUnfold (IfCoreUnfold e)     = freeNamesIfExpr e
-freeNamesIfUnfold (IfInlineRule _ _ e) = freeNamesIfExpr e
-freeNamesIfUnfold (IfWrapper _ v)      = unitNameSet v
-freeNamesIfUnfold (IfDFunUnfold vs)    = fnList freeNamesIfExpr vs
+freeNamesIfUnfold (IfCoreUnfold e)       = freeNamesIfExpr e
+freeNamesIfUnfold (IfCompulsory e)       = freeNamesIfExpr e
+freeNamesIfUnfold (IfInlineRule _ _ _ e) = freeNamesIfExpr e
+freeNamesIfUnfold (IfWrapper _ v)        = unitNameSet v
+freeNamesIfUnfold (IfDFunUnfold vs)      = fnList freeNamesIfExpr vs
 
 freeNamesIfExpr :: IfaceExpr -> NameSet
 freeNamesIfExpr (IfaceExt v)	  = unitNameSet v
diff --git a/compiler/iface/MkIface.lhs b/compiler/iface/MkIface.lhs
index 1c34edca3c..702a744d34 100644
--- a/compiler/iface/MkIface.lhs
+++ b/compiler/iface/MkIface.lhs
@@ -1503,20 +1503,21 @@ toIfaceIdInfo id_info
 
 --------------------------
 toIfUnfolding :: Bool -> Unfolding -> Maybe IfaceInfoItem
-toIfUnfolding lb (CoreUnfolding { uf_tmpl = rhs, uf_arity = arity
-                                , uf_src = src, uf_guidance = guidance })
-  = case src of
-	InlineWrapper w -> Just (HsUnfold lb (IfWrapper arity (idName w)))
-	InlineRule {}   -> Just (HsUnfold lb (IfInlineRule arity sat (toIfaceExpr rhs)))
-        _other          -> Just (HsUnfold lb (IfCoreUnfold (toIfaceExpr rhs)))
+toIfUnfolding lb unf@(CoreUnfolding { uf_tmpl = rhs, uf_arity = arity
+                                    , uf_src = src, uf_guidance = guidance })
+  = Just $ HsUnfold lb $
+    case src of
+	InlineRule {}
+          -> case guidance of
+               UnfWhen unsat_ok boring_ok -> IfInlineRule arity unsat_ok boring_ok (toIfaceExpr rhs)
+               _other                     -> pprPanic "toIfUnfolding" (ppr unf)
+	InlineWrapper w  -> IfWrapper arity (idName w)
+        InlineCompulsory -> IfCompulsory (toIfaceExpr rhs)
+        InlineRhs        -> IfCoreUnfold (toIfaceExpr rhs)
 	-- Yes, even if guidance is UnfNever, expose the unfolding
 	-- If we didn't want to expose the unfolding, TidyPgm would
 	-- have stuck in NoUnfolding.  For supercompilation we want 
 	-- to see that unfolding!
-  where
-    sat = case guidance of
-            UnfWhen unsat_ok _ -> unsat_ok
-            _other             -> needSaturated
 
 toIfUnfolding lb (DFunUnfolding _con ops)
   = Just (HsUnfold lb (IfDFunUnfold (map toIfaceExpr ops)))
diff --git a/compiler/iface/TcIface.lhs b/compiler/iface/TcIface.lhs
index c9c33dbde6..7d0d02ee12 100644
--- a/compiler/iface/TcIface.lhs
+++ b/compiler/iface/TcIface.lhs
@@ -1015,11 +1015,19 @@ tcUnfolding name _ info (IfCoreUnfold if_expr)
     		     Just sig -> isBottomingSig sig
  		     Nothing  -> False
 
-tcUnfolding name _ _ (IfInlineRule arity unsat_ok if_expr)
+tcUnfolding name _ _ (IfCompulsory if_expr)
   = do 	{ mb_expr <- tcPragExpr name if_expr
 	; return (case mb_expr of
 		    Nothing   -> NoUnfolding
-		    Just expr -> mkInlineRule unsat_ok expr arity) }
+		    Just expr -> mkCompulsoryUnfolding expr) }
+
+tcUnfolding name _ _ (IfInlineRule arity unsat_ok boring_ok if_expr)
+  = do 	{ mb_expr <- tcPragExpr name if_expr
+	; return (case mb_expr of
+		    Nothing   -> NoUnfolding
+		    Just expr -> mkCoreUnfolding True InlineRule expr arity 
+                                                 (UnfWhen unsat_ok boring_ok))
+    }
 
 tcUnfolding name ty info (IfWrapper arity wkr)
   = do 	{ mb_wkr_id <- forkM_maybe doc (tcIfaceExtId wkr)