Make a trailing colon for stanzas a parse failure

Here's a mistake I make semi-regularly:

    source-repository-package:
        type: git
        location: https://github.com/parsonsmatt/foundation
        tag: 688c32ccd9a951bc96dd09423a6e6684f091d510
        subdir: basement
        subdir: foundation

Cabal treats this as a warning, so it prints:

    Warning: cabal.project: Unrecognized field
    'source-repository-package' on line 52

This is fine (if you already know the mistake you've made, at least!),
but it's very easy to miss amidst lots of output.

I often re-run `cabal` when I see a ton of output to attempt to get a
smaller error message. (Usually it works and I get an error message
that's got less "compiling module such and such" noise in it.) However,
re-running `cabal` will discard this warning entirely!

Let's make it a hard error instead. This is a backwards-compatibility
break.

cabal-install/src/Distribution/Client/ParseUtils.hs

@@ -1,6 +1,8 @@
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
 
 -----------------------------------------------------------------------------
 
@@ -51,6 +53,7 @@ import Distribution.Deprecated.ParseUtils
   ( Field (..)
   , FieldDescr (..)
   , LineNo
+  , PError (..)
   , ParseResult (..)
   , liftField
   , lineNo
@@ -260,13 +263,16 @@ parseFieldsAndSections fieldDescrs sectionDescrs fgSectionDescrs =
     setField a (F line name value) =
       case Map.lookup name fieldMap of
         Just (FieldDescr _ _ set) -> set line value a
-        Nothing -> do
-          warning $
-            "Unrecognized field '"
-              ++ name
-              ++ "' on line "
-              ++ show line
-          return a
+        Nothing ->
+          case Left <$> Map.lookup name sectionMap <|> Right <$> Map.lookup name fgSectionMap of
+            Just _ -> ParseFailed $ FieldShouldBeStanza name line
+            Nothing -> do
+              warning $
+                "Unrecognized field '"
+                  ++ name
+                  ++ "' on line "
+                  ++ show line
+              return a
     setField a (Section line name param fields) =
       case Left <$> Map.lookup name sectionMap <|> Right <$> Map.lookup name fgSectionMap of
         Just (Left (SectionDescr _ fieldDescrs' sectionDescrs' _ set sectionEmpty)) -> do

cabal-install/src/Distribution/Deprecated/ParseUtils.hs

@@ -91,6 +91,7 @@ data PError
   = AmbiguousParse String LineNo
   | NoParse String LineNo
   | TabsError LineNo
+  | FieldShouldBeStanza String LineNo
   | FromString String (Maybe LineNo)
   deriving (Eq, Show)
 
@@ -186,6 +187,10 @@ locatedErrorMsg (NoParse f n) =
   , "Parse of field '" ++ f ++ "' failed."
   )
 locatedErrorMsg (TabsError n) = (Just n, "Tab used as indentation.")
+locatedErrorMsg (FieldShouldBeStanza name lineNumber) =
+  ( Just lineNumber
+  , "'" ++ name ++ "' is a stanza, not a field. Remove the trailing ':' to parse a stanza."
+  )
 locatedErrorMsg (FromString s n) = (n, s)
 
 syntaxError :: LineNo -> String -> ParseResult a