Keep the comments around a list pattern's only spread when formatting

changes/list-pattern-spread-comment.fix.md

@@ -0,0 +1 @@
+- Keep the comments around the spread of a list pattern without other items when formatting, as in `list{/* c */ ...r}`.

compiler/syntax/src/res_core.ml

@@ -1659,6 +1659,11 @@ and parse_list_pattern ~start_pos ~attrs p =
     else pattern
   in
   match list_patterns with
+  | [(true, pattern)] ->
+    (* [list{...p}] is [p] itself. Like [list{...e}] in expressions, it keeps
+       the location of [p], so that the comments around the spread stay
+       outside it and are printed with [p]. *)
+    {pattern with ppat_attributes = attrs}
   | (true, pattern) :: patterns ->
     let patterns = patterns |> List.map filter_spread |> List.rev in
     let pat = make_list_pattern loc patterns (Some pattern) in

tests/syntax_tests/data/printer/comments/expected/pattern.res.txt

@@ -47,6 +47,16 @@ let list{/* inside */} = list{}
 let /* before */ list{/* a1 */ a /* a2 */, /* b1 */ b /* b2 */} /* after */ = list{1, 2}
 let /* before */ list{/* a1 */ a /* a2 */, /* b1 */ ...b /* b2 */} /* after */ = list{1, 2}
 
+// A list pattern that is only a spread is the spread's operand
+let /* d */ r = list{1, 2}
+let r /* after */ = list{1, 2}
+let /* d1 */ /* d2 */ /* e */ r = list{1, 2}
+let f = /* d */ r => r
+let f = x =>
+  switch x {
+  /* d */ | r => r
+  }
+
 // Ppat_record
 let /* before */ {/* c0 */ name /* c1 */, /* c2 */ age /* c3 */} /* after */ = {
   name: "steve",

tests/syntax_tests/data/printer/comments/pattern.res

@@ -43,6 +43,16 @@ let list{/* inside */} = list{}
 let /* before */ list{/* a1 */ a /* a2 */, /* b1 */ b /* b2 */} /* after */ = list{1, 2}
 let /* before */ list{/* a1 */ a /* a2 */, /* b1 */ ...b /* b2 */} /* after */ = list{1, 2}
 
+// A list pattern that is only a spread is the spread's operand
+let list{/* d */ ...r} = list{1, 2}
+let list{...r /* after */} = list{1, 2}
+let list{/* d1 */ /* d2 */ ...list{/* e */ ...r}} = list{1, 2}
+let f = (list{/* d */ ...r}) => r
+let f = x =>
+  switch x {
+  | list{/* d */ ...r} => r
+  }
+
 // Ppat_record
 let /* before */ {/* c0 */ name /* c1 */, /* c2 */ age /* c3 */} /* after */ = {
   name: "steve",