Add support for a trailing comment without a newline (#2309)

… as standardized in dhall-lang/dhall-lang#1226

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: Gabriella439

dhall/src/Dhall/Parser/Expression.hs

@@ -241,7 +241,11 @@ parsers :: forall a. Parser a -> Parsers a
 parsers embedded = Parsers{..}
   where
     completeExpression_ =
-        many shebang *> whitespace *> expression <* whitespace
+            many shebang
+        *>  whitespace
+        *>  expression
+        <*  whitespace
+        <*  optional lineCommentPrefix
 
     shebang = do
         _ <- text "#!"

dhall/src/Dhall/Parser/Token.hs

@@ -8,6 +8,7 @@ module Dhall.Parser.Token (
     validCodepoint,
     whitespace,
     lineComment,
+    lineCommentPrefix,
     blockComment,
     nonemptyWhitespace,
     bashEnvironmentVariable,
@@ -464,20 +465,23 @@ hexNumber = choice [ hexDigit, hexUpper, hexLower ]
       where
         predicate c = 'a' <= c && c <= 'f'
 
--- | Parse a Dhall's single-line comment, starting from `--` and until the
---   last character of the line /before/ the end-of-line character
-lineComment :: Parser Text
-lineComment = do
+-- | Same as `lineComment` except that this doesn't parse the end-of-line
+--   character
+lineCommentPrefix :: Parser Text
+lineCommentPrefix = do
     _ <- text "--"
 
     let predicate c = ('\x20' <= c && c <= '\x10FFFF') || c == '\t'
 
     commentText <- Dhall.Parser.Combinators.takeWhile predicate
 
-    _ <- endOfLine
-
     return ("--" <> commentText)
 
+-- | Parse a Dhall's single-line comment, starting from `--` and until the
+--   last character of the line /before/ the end-of-line character
+lineComment :: Parser Text
+lineComment = try (lineCommentPrefix <* endOfLine)
+
 -- | Parsed text doesn't include opening braces
 blockComment :: Parser Text
 blockComment = do