allow begin end on one line in safe cases

CHANGES.md

@@ -85,6 +85,10 @@ profile. This started with version 0.26.0.
   end
   ```
 
+- \* Allow `begin end` on one line in most cases. (#2745, @EmileTrotignon).
+  `f begin fun x -> y end` is not supported for now and is still always
+  multiline.
+
 ## 0.28.1
 
 ### Highlight

lib/Fmt_ast.ml

@@ -3051,10 +3051,9 @@ and fmt_lazy c ~ctx ?(pro = noop) ~fmt_atrs ~infix_ext_attrs ~parens e =
 and fmt_beginend c ~loc ?(box = true) ?(pro = noop) ~ctx ~ctx0 ~fmt_atrs
     ~infix_ext_attrs ~indent_wrap ?eol e =
   let cmts_before = Cmts.fmt_before c ?eol loc in
-  let begin_ = fmt_infix_ext_attrs c ~pro:(str "begin") infix_ext_attrs
-  and end_ =
-    (if not box then break 1000 (-2) else break 1000 0)
-    $ str "end" $ fmt_atrs
+  let begin_ = fmt_infix_ext_attrs c ~pro:(str "begin") infix_ext_attrs in
+  let end_ =
+    (if not box then break 1000 (-2) else break 1 0) $ str "end" $ fmt_atrs
   in
   let box_beginend_sb = Params.Exp.box_beginend_subexpr c.conf ~ctx ~ctx0 in
   let beginend_box =

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

@@ -576,11 +576,7 @@ let _ =
 
 let _ = ((module[@a] X) [@b])
 
-let _ =
-  begin[@a]
-    x
-  end
-  [@b]
+let _ = begin[@a] x end [@b]
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]

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

@@ -283,19 +283,10 @@ let _ = begin%ext end
 let _ = begin%ext end (* foo *)
 let _ = x y
 let _ = (* foo *) x y
-let _ =
-  begin%ext
-    x y
-  end
-let _ =
-  begin%ext
-    (* foo *) x y
-  end
+let _ = begin%ext x y end
+let _ = begin%ext (* foo *) x y end
 
-let _ =
-  begin[@landmark "parse_constant_dividends"]
-    market_data_items := ()
-  end
+let _ = begin[@landmark "parse_constant_dividends"] market_data_items := () end
 
 let () = if a then b (* asd *)
 
@@ -531,11 +522,7 @@ let main =
   | A -> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   | B -> bbbbbbbbbbbbbbbbbbbbbb
 
-let _ =
-  begin
-    a
-  end
-  [@a]
+let _ = begin a end [@a]
 
 let () =
   fooooo

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

@@ -316,27 +316,12 @@ let _ = begin end
 let _ = begin end (* foo *)
 let _ = begin%ext end
 let _ = begin%ext end (* foo *)
-let _ =
-  begin
-    x y
-  end
-let _ =
-  begin
-    (* foo *) x y
-  end
-let _ =
-  begin%ext
-    x y
-  end
-let _ =
-  begin%ext
-    (* foo *) x y
-  end
+let _ = begin x y end
+let _ = begin (* foo *) x y end
+let _ = begin%ext x y end
+let _ = begin%ext (* foo *) x y end
 
-let _ =
-  begin[@landmark "parse_constant_dividends"]
-    market_data_items := ()
-  end
+let _ = begin[@landmark "parse_constant_dividends"] market_data_items := () end
 
 let () =
   if a then begin
@@ -611,11 +596,7 @@ let main =
   | A -> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   | B -> bbbbbbbbbbbbbbbbbbbbbb
 
-let _ =
-  begin
-    a
-  end
-  [@a]
+let _ = begin a end [@a]
 
 let () =
   fooooo

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

@@ -1,34 +1,9 @@
-let _ =
-  begin%ext
-    y >>= z
-  end
-let _ =
-  [%ext
-    begin
-      y >>= z
-    end]
-let _ =
-  x
-  >>= begin%ext
-    y >>= z
-  end
-let _ =
-  x
-  >>= [%ext
-        begin
-          y >>= z
-        end]
-let _ =
-  f
-    begin%ext
-      y >>= z
-    end
-let _ =
-  f
-    [%ext
-      begin
-        y >>= z
-      end]
+let _ = begin%ext y >>= z end
+let _ = [%ext begin y >>= z end]
+let _ = x >>= begin%ext y >>= z end
+let _ = x >>= [%ext begin y >>= z end]
+let _ = f begin%ext y >>= z end
+let _ = f [%ext begin y >>= z end]
 
 let _ = (module%ext S)
 let _ = [%ext (module S)]

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

@@ -6,15 +6,9 @@ let _ = (* test *) "asd"
 
 let _ = "asd"
 
-let _ =
-  begin
-    (* te""st *) "asd"
-  end
-
-let _ =
-  begin
-    "asd"
-  end
+let _ = begin (* te""st *) "asd" end
+
+let _ = begin "asd" end
 
 let _ = 'a'
 

test/passing/refs.ahrefs/source.ml.err

@@ -1,4 +1,4 @@
-Warning: source.ml:3417 exceeds the margin
-Warning: source.ml:6551 exceeds the margin
-Warning: source.ml:6985 exceeds the margin
-Warning: source.ml:7817 exceeds the margin
+Warning: source.ml:3415 exceeds the margin
+Warning: source.ml:6549 exceeds the margin
+Warning: source.ml:6983 exceeds the margin
+Warning: source.ml:7815 exceeds the margin

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

@@ -96,9 +96,7 @@ let () =
   assert%foo[@foo] true;
   lazy%foo[@foo] x;
   object%foo[@foo] end;
-  begin%foo[@foo]
-    3
-  end;
+  begin%foo[@foo] 3 end;
   new%foo[@foo] x;
 
   match%foo[@foo] () with

test/passing/refs.default/attributes.ml.ref

@@ -454,12 +454,7 @@ let _ =
   [@b]
 
 let _ = ((module[@a] X) [@b])
-
-let _ =
-  begin[@a]
-    x
-  end
-  [@b]
+let _ = begin[@a] x end [@b]
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]

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

@@ -116,9 +116,7 @@ let a =
   | B -> bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
   end
 
-let a =
-  begin match f x i with A -> a | B -> b
-  end
+let a = begin match f x i with A -> a | B -> b end
 
 let a =
   begin[@a] match[@b]
@@ -177,9 +175,7 @@ let a =
   | B -> bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
   end
 
-let a =
-  begin try f x i with A -> a | B -> b
-  end
+let a = begin try f x i with A -> a | B -> b end
 
 let a =
   begin[@a] try[@b]

test/passing/refs.default/exp_grouping-parens.ml.ref

@@ -301,22 +301,9 @@ let _ = begin%ext end
 let _ = begin%ext end (* foo *)
 let _ = x y
 let _ = (* foo *) x y
-
-let _ =
-  begin%ext
-    x y
-  end
-
-let _ =
-  begin%ext
-    (* foo *) x y
-  end
-
-let _ =
-  begin[@landmark "parse_constant_dividends"]
-    market_data_items := ()
-  end
-
+let _ = begin%ext x y end
+let _ = begin%ext (* foo *) x y end
+let _ = begin[@landmark "parse_constant_dividends"] market_data_items := () end
 let () = if a then b (* asd *)
 
 let x =
@@ -548,11 +535,7 @@ let main =
   | A -> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   | B -> bbbbbbbbbbbbbbbbbbbbbb
 
-let _ =
-  begin
-    a
-  end
-  [@a]
+let _ = begin a end [@a]
 
 let () =
   fooooo

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

@@ -332,31 +332,11 @@ let _ = begin end
 let _ = begin end (* foo *)
 let _ = begin%ext end
 let _ = begin%ext end (* foo *)
-
-let _ =
-  begin
-    x y
-  end
-
-let _ =
-  begin
-    (* foo *) x y
-  end
-
-let _ =
-  begin%ext
-    x y
-  end
-
-let _ =
-  begin%ext
-    (* foo *) x y
-  end
-
-let _ =
-  begin[@landmark "parse_constant_dividends"]
-    market_data_items := ()
-  end
+let _ = begin x y end
+let _ = begin (* foo *) x y end
+let _ = begin%ext x y end
+let _ = begin%ext (* foo *) x y end
+let _ = begin[@landmark "parse_constant_dividends"] market_data_items := () end
 
 let () =
   if a then begin
@@ -629,11 +609,7 @@ let main =
   | A -> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   | B -> bbbbbbbbbbbbbbbbbbbbbb
 
-let _ =
-  begin
-    a
-  end
-  [@a]
+let _ = begin a end [@a]
 
 let () =
   fooooo

test/passing/refs.default/extensions_exp_grouping.ml.ref

@@ -1,40 +1,9 @@
-let _ =
-  begin%ext
-    y >>= z
-  end
-
-let _ =
-  [%ext
-    begin
-      y >>= z
-    end]
-
-let _ =
-  x
-  >>= begin%ext
-    y >>= z
-  end
-
-let _ =
-  x
-  >>= [%ext
-        begin
-          y >>= z
-        end]
-
-let _ =
-  f
-    begin%ext
-      y >>= z
-    end
-
-let _ =
-  f
-    [%ext
-      begin
-        y >>= z
-      end]
-
+let _ = begin%ext y >>= z end
+let _ = [%ext begin y >>= z end]
+let _ = x >>= begin%ext y >>= z end
+let _ = x >>= [%ext begin y >>= z end]
+let _ = f begin%ext y >>= z end
+let _ = f [%ext begin y >>= z end]
 let _ = (module%ext S)
 let _ = [%ext (module S)]
 let _ = f (module%ext S)

test/passing/refs.default/js_begin.ml.ref

@@ -9,6 +9,4 @@ let g = function
       foo;
       bar
 
-let () =
-  begin match foo with Bar -> snoo
-  end
+let () = begin match foo with Bar -> snoo end

test/passing/refs.default/js_pattern.ml.ref

@@ -2,10 +2,7 @@ let f = function _ -> 0
 let f x = match x with _ -> 0
 let f = function _ -> 0
 let f x = match x with _ -> 0
-
-let f x =
-  begin match x with _ -> 0
-  end
+let f x = begin match x with _ -> 0 end
 
 let check_price t = function
   | { Exec.trade_at_settlement = None | Some false } -> ()