Check for invalid UTF8 when parsing .cabal files

This assumes a permissive UTF8 decoder has inserted '\xfffd' as a replacement
character. We should check strictly for errors when we decode instead, though
it's nice to do that where we have some kind of error reporting infrastructure
which is why I've added it to the parser for the moment. The current error
message is not too bad, as it reports the line number.

Distribution/ParseUtils.hs

@@ -90,6 +90,7 @@ type LineNo = Int
 data PError = AmbigousParse String LineNo
             | NoParse String LineNo
             | TabsError LineNo
+            | UTFError LineNo
             | FromString String (Maybe LineNo)
         deriving Show
 
@@ -129,6 +130,7 @@ locatedErrorMsg :: PError -> (Maybe LineNo, String)
 locatedErrorMsg (AmbigousParse f n) = (Just n, "Ambiguous parse in field '"++f++"'")
 locatedErrorMsg (NoParse f n)       = (Just n, "Parse of field '"++f++"' failed: ")
 locatedErrorMsg (TabsError n)       = (Just n, "Tab used as indentation.")
+locatedErrorMsg (UTFError  n)       = (Just n, "Invalid UTF-8 text.")
 locatedErrorMsg (FromString s n)    = (n, s)
 
 syntaxError :: LineNo -> String -> ParseResult a
@@ -137,6 +139,9 @@ syntaxError n s = ParseFailed $ FromString s (Just n)
 tabsError :: LineNo -> ParseResult a
 tabsError ln = ParseFailed $ TabsError ln
 
+utf8Error :: LineNo -> ParseResult a
+utf8Error ln = ParseFailed $ UTFError ln
+
 warning :: String -> ParseResult ()
 warning s = ParseOk [s] ()
 
@@ -263,15 +268,18 @@ fName (Section _ n _ _) = n
 fName _ = error "fname: not a field or section"
 
 readFields :: String -> ParseResult [Field]
-readFields input = 
-      ifelse
-  =<< mapM (mkField 0)
-  =<< mkTree (tokenise input)
-
-  where tokenise = concatMap tokeniseLine
-                 . trimLines
-                 . lines
-                 . normaliseLineEndings
+readFields input =
+  case [ n | (n,l) <- zip [1..] ls
+           , '\xfffd' `elem` l ] of
+
+    (n:_) -> utf8Error n
+
+    []    -> ifelse
+         =<< mapM (mkField 0)
+         =<< mkTree tokens
+
+  where ls = (lines . normaliseLineEndings) input
+        tokens = (concatMap tokeniseLine . trimLines) ls
 
 -- attach line number and determine indentation
 trimLines :: [String] -> [(LineNo, Indent, HasTabs, String)]