Don't let comments escape from empty export lists

CHANGELOG.md

@@ -8,6 +8,9 @@
 * Support the (deprecated) `DatatypeContexts` extension to avoid surprises.
   [Issue 1012](https://github.com/tweag/ormolu/issues/1012).
 
+* Don't let comments escape from empty export lists. [Issue
+  906](https://github.com/tweag/ormolu/issues/906).
+
 ## Ormolu 0.6.0.1
 
 * Fix false positives in AST diffing related to `UnicodeSyntax`. [PR

data/examples/module-header/multiline-empty-comment-out.hs

@@ -0,0 +1,4 @@
+module Foo
+  ( -- test
+  )
+where

data/examples/module-header/multiline-empty-comment.hs

@@ -0,0 +1,2 @@
+module Foo ( -- test
+           ) where

src/Ormolu/Printer/Combinators.hs

@@ -23,6 +23,7 @@ module Ormolu.Printer.Combinators
     inciIf,
     askSourceType,
     askFixityOverrides,
+    encloseLocated,
     askFixityMap,
     located,
     located',
@@ -77,7 +78,7 @@ import GHC.Data.Strict qualified as Strict
 import GHC.Types.SrcLoc
 import Ormolu.Printer.Comments
 import Ormolu.Printer.Internal
-import Ormolu.Utils (HasSrcSpan (..))
+import Ormolu.Utils (HasSrcSpan (..), getLoc')
 
 ----------------------------------------------------------------------------
 -- Basic
@@ -111,6 +112,23 @@ located (L l' a) f = case loc' l' of
       switchLayout [RealSrcSpan l Strict.Nothing] (f a)
     spitFollowingComments l
 
+-- | Similar to 'located', but when the "payload" is an empty list, print
+-- virtual elements at the start and end of the source span to prevent comments
+-- from "floating out".
+encloseLocated ::
+  (HasSrcSpan l) =>
+  GenLocated l [a] ->
+  ([a] -> R ()) ->
+  R ()
+encloseLocated la f = located la $ \a -> do
+  when (null a) $ located (L startSpan ()) pure
+  f a
+  when (null a) $ located (L endSpan ()) pure
+  where
+    l = getLoc' la
+    (startLoc, endLoc) = (srcSpanStart l, srcSpanEnd l)
+    (startSpan, endSpan) = (mkSrcSpan startLoc startLoc, mkSrcSpan endLoc endLoc)
+
 -- | A version of 'located' with arguments flipped.
 located' ::
   (HasSrcSpan l) =>

src/Ormolu/Printer/Meat/Module.hs

@@ -54,7 +54,7 @@ p_hsModule mstackHeader pragmas HsModule {..} = do
         case hsmodExports of
           Nothing -> return ()
           Just l -> do
-            located l $ \exports -> do
+            encloseLocated l $ \exports -> do
               inci (p_hsmodExports exports)
             breakpoint
         txt "where"