Don't include BufPos in interface files

Ticket #22162 pointed out that the build directory was leaking into the
ABI hash of a module because the BufPos depended on the location of the
build tree.

BufPos is only used in GHC.Parser.PostProcess.Haddock, and the
information doesn't need to be propagated outside the context of a
module.

Fixes #22162

compiler/GHC/Iface/Ext/Types.hs

@@ -781,5 +781,5 @@ toHieName name
   | isKnownKeyName name = KnownKeyName (nameUnique name)
   | isExternalName name = ExternalName (nameModule name)
                                        (nameOccName name)
-                                       (nameSrcSpan name)
-  | otherwise = LocalName (nameOccName name) (nameSrcSpan name)
+                                       (removeBufSpan $ nameSrcSpan name)
+  | otherwise = LocalName (nameOccName name) (removeBufSpan $ nameSrcSpan name)

compiler/GHC/Types/SrcLoc.hs

@@ -68,6 +68,7 @@ module GHC.Types.SrcLoc (
         getBufPos,
         BufSpan(..),
         getBufSpan,
+        removeBufSpan,
 
         -- * Located
         Located,
@@ -392,6 +393,10 @@ data UnhelpfulSpanReason
   | UnhelpfulOther !FastString
   deriving (Eq, Show)
 
+removeBufSpan :: SrcSpan -> SrcSpan
+removeBufSpan (RealSrcSpan s _) = RealSrcSpan s Strict.Nothing
+removeBufSpan s = s
+
 {- Note [Why Maybe BufPos]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 In SrcLoc we store (Maybe BufPos); in SrcSpan we store (Maybe BufSpan).

compiler/GHC/Utils/Binary.hs

@@ -1312,19 +1312,6 @@ instance Binary RealSrcSpan where
             return (mkRealSrcSpan (mkRealSrcLoc f sl sc)
                                   (mkRealSrcLoc f el ec))
 
-instance Binary BufPos where
-  put_ bh (BufPos i) = put_ bh i
-  get bh = BufPos <$> get bh
-
-instance Binary BufSpan where
-  put_ bh (BufSpan start end) = do
-    put_ bh start
-    put_ bh end
-  get bh = do
-    start <- get bh
-    end <- get bh
-    return (BufSpan start end)
-
 instance Binary UnhelpfulSpanReason where
   put_ bh r = case r of
     UnhelpfulNoLocationInfo -> putByte bh 0
@@ -1343,10 +1330,11 @@ instance Binary UnhelpfulSpanReason where
       _ -> UnhelpfulOther <$> get bh
 
 instance Binary SrcSpan where
-  put_ bh (RealSrcSpan ss sb) = do
+  put_ bh (RealSrcSpan ss _sb) = do
           putByte bh 0
+          -- BufSpan doesn't ever get serialised because the positions depend
+          -- on build location.
           put_ bh ss
-          put_ bh sb
 
   put_ bh (UnhelpfulSpan s) = do
           putByte bh 1
@@ -1356,8 +1344,7 @@ instance Binary SrcSpan where
           h <- getByte bh
           case h of
             0 -> do ss <- get bh
-                    sb <- get bh
-                    return (RealSrcSpan ss sb)
+                    return (RealSrcSpan ss Strict.Nothing)
             _ -> do s <- get bh
                     return (UnhelpfulSpan s)