Restrict type of parens and brackets to prevent negative use of NExprLoc

it is not generally appropriate to have higher-order parsers operate on
annotated locations as they are liable to perform changes to the parser which
are not captured in the annotation.

See haskell-nix#739 and
haskell-nix#744 for examples.

src/Nix/Parser.hs

@@ -197,7 +197,7 @@ nixNull :: Parser NExprLoc
 nixNull = annotateLocation1 (mkNullF <$ reserved "null" <?> "null")
 
 nixParens :: Parser NExprLoc
-nixParens = annotateLocation1 (stripAnn . unFix <$> (parens nixToplevelForm <?> "parens"))
+nixParens = annotateLocation1 (parens (stripAnn . unFix <$> nixToplevelForm) <?> "parens")
 
 nixList :: Parser NExprLoc
 nixList = annotateLocation1 (brackets (NList <$> many nixTerm) <?> "list")
@@ -408,7 +408,7 @@ nixBinders = (inherit <+> namedVar) `endBy` semi where
  <*> (equals *> nixToplevelForm)
  <*> pure p
  <?> "variable binding"
- scope = parens nixToplevelForm <?> "inherit scope"
+ scope = nixParens <?> "inherit scope"
 
 keyName :: Parser (NKeyName NExprLoc)
 keyName = dynamicKey <+> staticKey where
@@ -493,6 +493,13 @@ identifier = lexeme $ try $ do
  where
  identLetter x = isAlpha x || isDigit x || x == '_' || x == '\'' || x == '-'
 
+-- We restrict the type of 'parens' and 'brackets' here because if they were to
+-- take a @Parser NExprLoc@ argument they would parse additional text which
+-- wouldn't be captured in the source location annotation.
+--
+-- Braces and angles in hnix don't enclose a single expression so this type
+-- restriction would not be useful.
+parens, brackets :: Parser (NExprF f) -> Parser (NExprF f)
 parens = between (symbol "(") (symbol ")")
 braces = between (symbol "{") (symbol "}")
 -- angles = between (symbol "<") (symbol ">")