Fix comments formatted away in function with no args

CHANGELOG.md

@@ -34,6 +34,7 @@
 - Fix parsing issue with nested variant pattern type spreads. https://github.com/rescript-lang/rescript-compiler/pull/7080
 - Fix JSX settings inheritance: only 'version' propagates to dependencies, preserving their 'mode' and 'module'. https://github.com/rescript-lang/rescript-compiler/pull/7094
 - Fix variant cast to int. https://github.com/rescript-lang/rescript-compiler/pull/7058
+- Fix comments formatted away in function without arguments. https://github.com/rescript-lang/rescript-compiler/pull/7095
 
 #### :nail_care: Polish
 

compiler/syntax/src/res_printer.ml

@@ -4842,9 +4842,22 @@ and print_arguments_with_callback_in_last_position ~state args cmt_tbl =
 and print_arguments ~state ?(partial = false)
     (args : (Asttypes.arg_label * Parsetree.expression) list) cmt_tbl =
   match args with
-  | [(Nolabel, {pexp_desc = Pexp_construct ({txt = Longident.Lident "()"}, _)})]
-    ->
-    Doc.text "()"
+  | [
+   ( Nolabel,
+     {
+       pexp_desc = Pexp_construct ({txt = Longident.Lident "()"}, _);
+       pexp_loc = loc;
+     } );
+  ] ->
+    if has_leading_line_comment cmt_tbl loc then
+      let cmt = print_comments Doc.nil cmt_tbl loc in
+      Doc.concat
+        [
+          Doc.lparen;
+          Doc.indent (Doc.group (Doc.concat [Doc.soft_line; cmt]));
+          Doc.rparen;
+        ]
+    else Doc.text "()"
   | [(Nolabel, arg)] when ParsetreeViewer.is_huggable_expression arg ->
     let arg_doc =
       let doc = print_expression_with_comments ~state arg cmt_tbl in

tests/syntax_tests/printer/comments/expected/expr.res.txt

@@ -234,6 +234,32 @@ let multiply = (
   /* c2 */ m2 /* c3 */,
 ) => ()
 
+f(
+  // a
+  // b
+  // c
+  )
+
+f(
+  // a
+  // b
+  // c
+  )
+
+let x = {
+  f(
+    // a
+    // b
+    // c
+    )
+
+  f(
+    // a
+    // b
+    // c
+    )
+}
+
 f(() => 1)
 // c1
 f(_ => 1)

tests/syntax_tests/printer/comments/expr.res

@@ -223,6 +223,35 @@ let f = (/* c0 */ ~greeting /* c1 */, /* c2 */ ~from /* c3 */ as /* c4 */ hometo
 let multiply = (/* c-2 */ type t /* c-1 */,/* c0 */ m1 /* c1 */, /* c2 */ m2 /* c3 */) => () 
 let multiply = (/* c-4 */ type t /* c-3 */,/* c0 */ m1 /* c1 */, /* c-2 */ type s /* c-1 */, /* c2 */ m2 /* c3 */) => () 
 
+f(
+  // a
+  // b
+// c
+)
+
+f(
+  // a 
+  // b
+    // c
+  ()
+)
+
+let x = {
+  f(
+    // a 
+    // b
+      // c
+  )
+
+
+  f(
+    // a 
+    // b
+  // c
+    ()
+  )
+}
+
 f(()
   // c1
   => 1);