use special case beginend syntax in match cases when possible

Author: EmileTrotignon

lib/Params.ml

@@ -535,12 +535,18 @@ let get_cases (c : Conf.t) ~fmt_infix_ext_attrs ~ctx ~first ~last
     else (parenze_exp xast && not body_has_parens, Some false)
   in
   let indent = if align_nested_match then 0 else indent in
+  let nested_exp_has_special_beginend exp =
+    match exp with
+    | Pexp_match _ | Pexp_try _ | Pexp_function _ | Pexp_ifthenelse _ -> true
+    | _ -> false
+  in
   let open_paren_branch, close_paren_branch, branch_expr =
     match ast with
     | { pexp_desc= Pexp_beginend (nested_exp, infix_ext_attrs)
       ; pexp_attributes= []
       ; _ }
-      when not cmts_before ->
+      when (not cmts_before)
+           && not (nested_exp_has_special_beginend nested_exp.pexp_desc) ->
         let close_paren =
           let offset =
             match c.fmt_opts.break_cases.v with `Nested -> 0 | _ -> -2

test/passing/refs.ahrefs/cases_exp_grouping.ml.ref

@@ -1,26 +1,30 @@
 let _ =
   match x with
-  | A -> begin match B with A -> fooooooooooooo end
-  | A -> begin match B with A -> fooooooooooooo | B -> fooooooooooooo end
-  | A -> begin
-    match B with
+  | A ->
+    begin match B with A -> fooooooooooooo
+    end
+  | A ->
+    begin match B with A -> fooooooooooooo | B -> fooooooooooooo
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
     | C -> fooooooooooooo
     | D -> fooooooooooooo
-  end
+    end
 [@@ocamlformat "break-cases=fit"]
 
 let _ =
   match x with
-  | A -> begin
-    match B with A -> fooooooooooooo
-  end
-  | A -> begin
-    match B with A -> fooooooooooooo | B -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+  | A ->
+    begin match B with A -> fooooooooooooo
+    end
+  | A ->
+    begin match B with A -> fooooooooooooo | B -> fooooooooooooo
+    end
+  | A ->
+    begin match B with
     | A ->
       fooooooooooooo
     | B ->
@@ -34,62 +38,62 @@ let _ =
 
 let _ =
   match x with
-  | A -> begin
-    match B with
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
     | C -> fooooooooooooo
     | D -> fooooooooooooo
-  end
+    end
 [@@ocamlformat "break-cases=toplevel"]
 
 let _ =
   match x with
-  | A -> begin
-    match B with
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
     | C -> fooooooooooooo
     | D -> fooooooooooooo
-  end
+    end
 [@@ocamlformat "break-cases=fit-or-vertical"]
 
 let _ =
   match x with
-  | A -> begin
-    match B with
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
     | C -> fooooooooooooo
     | D -> fooooooooooooo
-  end
+    end
 [@@ocamlformat "break-cases=all"]
 
 let a =

test/passing/refs.ahrefs/effects.ml.ref

@@ -33,16 +33,16 @@ let run (main : unit -> unit) : unit =
     | effect Fork f, k ->
       enqueue k ();
       spawn f
-    | effect Xchg n, k -> begin
-      match !exchanger with
+    | effect Xchg n, k ->
+      begin match !exchanger with
       | Some (n', k') ->
         exchanger := None;
         enqueue k' n;
         continue k n'
       | None ->
         exchanger := Some (n, k);
         dequeue ()
-    end
+      end
   in
   spawn main
 

test/passing/refs.ahrefs/exp_grouping.ml.ref

@@ -281,22 +281,22 @@ let _ =
 
 let _ =
   match x with
-  | A -> begin
-    match B with
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
-  end
-  | A -> begin
-    match B with
+    end
+  | A ->
+    begin match B with
     | A -> fooooooooooooo
     | B -> fooooooooooooo
     | C -> fooooooooooooo
     | D -> fooooooooooooo
-  end
+    end
 
 let () =
   begin

test/passing/refs.ahrefs/ifand.ml.ref

@@ -1,4 +1,6 @@
 let _ = if cond1 && cond2 then _
 
 let _ = function
-  | _ when x = 2 && y = 3 -> begin if a = b || (b = c && c = d) then _ end
+  | _ when x = 2 && y = 3 ->
+    begin if a = b || (b = c && c = d) then _
+    end