fix: callback sequences formatted back from .ml (#2799)

* fix: callback sequences formatted back from .ml

fixes #2798

* chore: add changelog entry

CHANGES.md

@@ -32,6 +32,8 @@
   [#2794](https://github.com/reasonml/reason/pull/2794))
 - Parse universal type variables in signature items (@anmonteiro,
   [#2797](https://github.com/reasonml/reason/pull/2797))
+- Fix formatting of callbacks with sequence expressions (@anmonteiro,
+  [#2799](https://github.com/reasonml/reason/pull/2799))
 
 ## 3.12.0
 

src/reason-parser/reason_pprint_ast.ml

@@ -9913,9 +9913,13 @@ let createFormatter () =
                     "=> ", ")" ^ rightWrap
                   in
                   let wrap =
-                    if self#should_preserve_requested_braces retCb
-                    then leftWrap ^ "{", "}" ^ rightWrap
-                    else wrap
+                    match
+                      ( self#should_preserve_requested_braces retCb
+                      , self#isSeriesOfOpensFollowedByNonSequencyExpression
+                          { retCb with pexp_attributes = [] } )
+                    with
+                    | true, _ | _, false -> leftWrap ^ "{", "}" ^ rightWrap
+                    | _ -> wrap
                   in
                   let right =
                     source_map

test/mlFunctions.t/input.ml

@@ -0,0 +1,7 @@
+(* Copyright (c) 2015-present, Facebook, Inc. All rights reserved. *)
+
+let x =
+  ignore (fun y ->
+    let y = 4 in
+    y)
+

test/mlFunctions.t/run.t

@@ -0,0 +1,23 @@
+Format basic
+
+  $ refmt ./input.ml | tee formatted.re
+  /* Copyright (c) 2015-present, Facebook, Inc. All rights reserved. */
+
+  let x =
+    ignore(y => {
+      let y = 4;
+      y;
+    });
+
+Format the formatted file back
+  $ refmt ./formatted.re | tee formatted_back.re
+  /* Copyright (c) 2015-present, Facebook, Inc. All rights reserved. */
+
+  let x =
+    ignore(y => {
+      let y = 4;
+      y;
+    });
+
+Ensure idempotency: first format and second format are the same
+  $ diff formatted.re formatted_back.re