Don't move trailing comments in let anymore

While this was initially introduced for good reasons, as can also be
seen in the diff, that code was written with documentation comments in
mind and not code comments. As per the bug report, the downsides were
deemed to high to keep it in

Fixes NixOS#259

src/Nixfmt/Pretty.hs

@@ -646,31 +646,13 @@ instance Pretty Expression where
       convertTrailing Nothing = []
       convertTrailing (Just (TrailingComment t)) = [LineComment (" " <> t)]
 
-      -- Extract detached comments at the bottom.
-      -- This uses a custom variant of span/spanJust/spanMaybe.
-      -- Note that this is a foldr which walks from the bottom, but the lists
-      -- are constructed in a way that they end up correct again.
-      (binderComments, bindersWithoutComments) =
-        foldr
-          ( \item (start, rest) -> case item of
-              (Comments inner)
-                | null rest ->
-                    -- Only move all non-empty-line trivia below the `in`
-                    let (comments, el) = break (== EmptyLine) (reverse inner)
-                    in (reverse comments : start, Comments (reverse el) : rest)
-              _ -> (start, item : rest)
-          )
-          ([], [])
-          (unItems binders)
-
       letPart = group $ pretty let_ <> hardline <> letBody
-      letBody = nest $ prettyItems (Items bindersWithoutComments)
+      letBody = nest $ prettyItems binders
       inPart =
         group $
           pretty in_
             <> hardline
-            -- Take our trailing and inject it between `in` and body
-            <> pretty (concat binderComments ++ preTrivia ++ convertTrailing trailComment)
+            <> pretty (preTrivia ++ convertTrailing trailComment)
             <> pretty expr
   pretty (Assert assert cond semicolon expr) =
     group $

test/diff/comment/out-pure.nix

@@ -146,8 +146,8 @@
       #6
 
       d = 1;
+      #7
     in
-    #7
     d
   )
 

test/diff/comment/out.nix

@@ -146,8 +146,8 @@
       #6
 
       d = 1;
+      #7
     in
-    #7
     d
   )
 

test/diff/idioms_lib_3/out-pure.nix

@@ -148,8 +148,8 @@ rec {
           [${mkSectionName sectName}]
         ''
         + toKeyValue { inherit mkKeyValue listsAsDuplicateKeys; } sectValues;
+      # map input to ini sections
     in
-    # map input to ini sections
     mapAttrsToStringsSep "\n" mkSection attrsOfAttrs;
 
   # Generate an INI-style config file from an attrset

test/diff/idioms_lib_3/out.nix

@@ -151,8 +151,8 @@ rec {
           [${mkSectionName sectName}]
         ''
         + toKeyValue { inherit mkKeyValue listsAsDuplicateKeys; } sectValues;
+      # map input to ini sections
     in
-    # map input to ini sections
     mapAttrsToStringsSep "\n" mkSection attrsOfAttrs;
 
   # Generate an INI-style config file from an attrset

test/diff/let_in/out-pure.nix

@@ -65,9 +65,9 @@ let
     let
       b = 0;
 
+      # foo
+      # bar
     in
-    # foo
-    # bar
     # baz
     # qux
     null;

test/diff/let_in/out.nix

@@ -65,9 +65,9 @@ let
     let
       b = 0;
 
+      # foo
+      # bar
     in
-    # foo
-    # bar
     # baz
     # qux
     null;