Fix comment before in forcing unnecessary line break in let bindings

`let x = value (* comment *) in` was being broken across multiple lines
regardless of configuration because comments near the `in` keyword were
formatted with `force_break` as their prefix, which unconditionally
forced the containing hvbox to vertical mode.

Fix: change `~pro:force_break` to `~pro:(break 1 0)` in two places in
fmt_value_binding — the comment is now a regular break that becomes a
space when the hvbox fits horizontally. When the content exceeds the
margin, the box still goes vertical naturally.

This also fixes a non-idempotency: previously, `let x = v (* c *) in`
and its pre-broken equivalent produced different outputs. The bug
affects all profiles (default, ocamlformat, janestreet, ahrefs).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Boris Yakobowski

lib/Fmt_ast.ml

@@ -4886,7 +4886,7 @@ and fmt_value_binding c ~ctx0 ~rec_flag ?in_ ?epi
         , fmt_item_attributes c ~pre:(Break (1, 2)) at_at_attrs $ in_ indent
         , fmt_opt epi
         , Cmts.fmt_before c lb_loc
-        , Cmts.fmt_after c lb_loc ~pro:force_break )
+        , Cmts.fmt_after c lb_loc ~pro:(break 1 0) )
     | None ->
         let epi =
           fmt_item_attributes c ~pre:(Break (1, 0)) at_at_attrs $ fmt_opt epi
@@ -4965,7 +4965,7 @@ and fmt_value_binding c ~ctx0 ~rec_flag ?in_ ?epi
               ( hvbox_if toplevel indent decl_and_body
               $ cmts_after
               $ opt loc_in
-                  (Cmts.fmt_before c ~pro:force_break ~epi:noop ~eol:noop) )
+                  (Cmts.fmt_before c ~pro:(break 1 0) ~epi:noop ~eol:noop) )
           $ in_ )
       $ opt loc_in (Cmts.fmt_after ~pro:force_break c)
       $ epi )

test/passing/refs.ahrefs/let_binding-deindent-fun.ml.ref

@@ -303,18 +303,12 @@ let rewrite_export =
 
 (* Comment before in should stay on same line when it fits *)
 let () =
-  let threshold =
-    3
-    (* max depth *)
-  in
+  let threshold = 3 (* max depth *) in
   use threshold
 ;;
 
 let () =
-  let left _ =
-    true
-    (* value vs nothing *)
-  in
+  let left _ = true (* value vs nothing *) in
   use left
 ;;
 

test/passing/refs.ahrefs/let_binding-in_indent.ml.ref

@@ -303,18 +303,12 @@ let rewrite_export =
 
 (* Comment before in should stay on same line when it fits *)
 let () =
-  let threshold =
-    3
-    (* max depth *)
-  in
+  let threshold = 3 (* max depth *) in
       use threshold
 ;;
 
 let () =
-  let left _ =
-    true
-    (* value vs nothing *)
-  in
+  let left _ = true (* value vs nothing *) in
       use left
 ;;
 

test/passing/refs.ahrefs/let_binding-indent.ml.ref

@@ -303,18 +303,12 @@ let rewrite_export =
 
 (* Comment before in should stay on same line when it fits *)
 let () =
-  let threshold =
-    3
-    (* max depth *)
-in
+  let threshold = 3 (* max depth *) in
   use threshold
 ;;
 
 let () =
-  let left _ =
-    true
-    (* value vs nothing *)
-in
+  let left _ = true (* value vs nothing *) in
   use left
 ;;
 

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

@@ -303,18 +303,12 @@ let rewrite_export =
 
 (* Comment before in should stay on same line when it fits *)
 let () =
-  let threshold =
-    3
-    (* max depth *)
-  in
+  let threshold = 3 (* max depth *) in
   use threshold
 ;;
 
 let () =
-  let left _ =
-    true
-    (* value vs nothing *)
-  in
+  let left _ = true (* value vs nothing *) in
   use left
 ;;
 

test/passing/refs.ahrefs/let_punning-denied.ml.ref

@@ -22,16 +22,10 @@ let q =
 
 let r =
   let* (* 1 *) x (* 2 *) = (* 3 *) x (* 4 *)
-  and* (* 5 *) y =
-    y
-    (* 6 *)
-  in
+  and* (* 5 *) y = y (* 6 *) in
   x, y
 
 let s =
   let%foo (* 1 *) x (* 2 *) = (* 3 *) x (* 4 *)
-  and (* 5 *) y =
-    y
-    (* 6 *)
-  in
+  and (* 5 *) y = y (* 6 *) in
   x, y

test/passing/refs.ahrefs/let_punning-preferred.ml.ref

@@ -26,9 +26,7 @@ let r =
     (* 3 *) x
   (* 2 *)
   (* 4 *)
-  and* (* 5 *) y
-    (* 6 *)
-  in
+  and* (* 5 *) y (* 6 *) in
   x, y
 
 let s =
@@ -37,7 +35,5 @@ let s =
     (* 3 *) x
   (* 2 *)
   (* 4 *)
-  and (* 5 *) y
-    (* 6 *)
-  in
+  and (* 5 *) y (* 6 *) in
   x, y

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

@@ -22,14 +22,10 @@ let q =
 
 let r =
   let* (* 1 *) x (* 2 *) = (* 3 *) x (* 4 *)
-  and* (* 5 *) y
-    (* 6 *)
-  in
+  and* (* 5 *) y (* 6 *) in
   x, y
 
 let s =
   let%foo (* 1 *) x (* 2 *) = (* 3 *) x (* 4 *)
-  and (* 5 *) y
-    (* 6 *)
-  in
+  and (* 5 *) y (* 6 *) in
   x, y

test/passing/refs.default/let_binding-deindent-fun.ml.ref

@@ -276,18 +276,12 @@ let rewrite_export =
 
 (* Comment before in should stay on same line when it fits *)
 let () =
-  let threshold =
-    3
-    (* max depth *)
-  in
+  let threshold = 3 (* max depth *) in
   use threshold
 ;;
 
 let () =
-  let left _ =
-    true
-    (* value vs nothing *)
-  in
+  let left _ = true (* value vs nothing *) in
   use left
 ;;
 

test/passing/refs.default/let_binding-in_indent.ml.ref

@@ -276,18 +276,12 @@ let rewrite_export =
 
 (* Comment before in should stay on same line when it fits *)
 let () =
-  let threshold =
-    3
-    (* max depth *)
-  in
+  let threshold = 3 (* max depth *) in
       use threshold
 ;;
 
 let () =
-  let left _ =
-    true
-    (* value vs nothing *)
-  in
+  let left _ = true (* value vs nothing *) in
       use left
 ;;