Warn if -- token is found. Don't warn if it's "--".

Cabal/Cabal.cabal

@@ -68,6 +68,7 @@ extra-source-files:
   tests/ParserTests/warnings/bom.cabal
   tests/ParserTests/warnings/bool.cabal
   tests/ParserTests/warnings/deprecatedfield.cabal
+  tests/ParserTests/warnings/doubledash.cabal
   tests/ParserTests/warnings/extratestmodule.cabal
   tests/ParserTests/warnings/gluedop.cabal
   tests/ParserTests/warnings/nbsp.cabal

Cabal/Distribution/Parsec/Class.hs

@@ -171,11 +171,21 @@ instance Parsec Bool where
 
 -- | @[^ ,]@
 parsecToken :: CabalParsing m => m String
-parsecToken = parsecHaskellString <|> (P.munch1 (\x -> not (isSpace x) && x /= ',')  P.<?> "identifier" )
+parsecToken = parsecHaskellString <|> ((P.munch1 (\x -> not (isSpace x) && x /= ',')  P.<?> "identifier" ) >>= checkNotDoubleDash)
 
 -- | @[^ ]@
 parsecToken' :: CabalParsing m => m String
-parsecToken' = parsecHaskellString <|> (P.munch1 (not . isSpace) P.<?> "token")
+parsecToken' = parsecHaskellString <|> ((P.munch1 (not . isSpace) P.<?> "token") >>= checkNotDoubleDash)
+
+checkNotDoubleDash ::  CabalParsing m => String -> m String
+checkNotDoubleDash s = do
+    when (s == "--") $ parsecWarning PWTDoubleDash $ unwords
+        [ "Double-dash token found."
+        , "Note: there are no end-of-line comments in .cabal files, only whole line comments."
+        , "Use \"--\" (quoted double dash) to silence this warning, if you actually want -- token"
+        ]
+
+    return s
 
 parsecFilePath :: CabalParsing m => m FilePath
 parsecFilePath = parsecToken

Cabal/Distribution/Parsec/Common.hs

@@ -43,6 +43,7 @@ data PWarnType
     | PWTLexNBSP
     | PWTLexBOM
     | PWTQuirkyCabalFile       -- ^ legacy cabal file that we know how to patch
+    | PWTDoubleDash            -- ^ Double dash token, most likely it's a mistake - it's not a comment
     deriving (Eq, Ord, Show, Enum, Bounded)
 
 -- | Parser warning.

Cabal/tests/ParserTests.hs

@@ -51,6 +51,7 @@ warningTests = testGroup "warnings triggered"
     , warningTest PWTUnknownField      "unknownfield.cabal"
     , warningTest PWTUnknownSection    "unknownsection.cabal"
     , warningTest PWTTrailingFields    "trailingfield.cabal"
+    , warningTest PWTDoubleDash        "doubledash.cabal"
     -- TODO: not implemented yet
     -- , warningTest PWTExtraTestModule   "extratestmodule.cabal"
     ]

Cabal/tests/ParserTests/regressions/encoding-0.8.cabal

@@ -1,6 +1,11 @@
 Name:   encoding
 Version:  0.8
 cabal-version: >=1.12
+-- double-dash files
+extra-source-files:
+  -- this is comment
+  README.md "--"
+  "--"
 
 custom-setup
   setup-depends:

Cabal/tests/ParserTests/regressions/encoding-0.8.format

@@ -1,6 +1,10 @@
 name: encoding
 version: 0.8
 cabal-version: >=1.12
+extra-source-files:
+    README.md
+    "--"
+    "--"
 
 custom-setup
     setup-depends: base <5,

Cabal/tests/ParserTests/warnings/doubledash.cabal

@@ -0,0 +1,9 @@
+name: bool
+version: 1
+cabal-version: >= 1.6
+extra-source-files:
+  README.md -- we include it
+
+library
+    build-depends: base >= 4.9 && <4.10
+    hs-source-dirs: .