WIP: disambiguate optional labels

When disambiguating record types, there's a check that all the labels are supplied when constructing a record.
While not supplying all the labels is supported in case of optional labels, the order of disambiguation is affected by the presence of optional labels.

Example:

```res
type t1 = {x:int, y:int}
type t2 = {x:int, y:int, z?:int}

let v = {x:3, y:4}
```

Currently `v` has type `t1`, while it's perfectly fine for it to have type `t2`.
In particular, the normal shadowing behaviour that applies without optional labels, does not happen. (If you remove `z` from the second type definition, then the normal shadowing happens, and `v` gets type `t2`.

This wip changes the disambiguation so that supplying at least all the mandatory labels is enough in disambiguation.

The change also addresses the issue #6752 of spurious warning of unused open.

Author: cristianoc

jscomp/ml/env.ml

@@ -2048,6 +2048,8 @@ let open_signature
       );
     let shadowed = ref [] in
     let slot s b =
+      let () = Printf.eprintf "XXX slot %s\n" s in
+      (* let _ = assert false in *)
       begin match check_shadowing env b with
       | Some kind when not (List.mem (kind, s) !shadowed) ->
           shadowed := (kind, s) :: !shadowed;

jscomp/ml/typecore.ml

@@ -840,14 +840,23 @@ module Label = NameChoice (struct
 end)
 
 let disambiguate_label_by_ids keep closed ids labels =
+  Printf.eprintf "XXX disambiguate_label_by_ids labels:%d\n" (List.length labels);
   let check_ids (lbl, _) =
     let lbls = Hashtbl.create 8 in
     Array.iter (fun lbl -> Hashtbl.add lbls lbl.lbl_name ()) lbl.lbl_all;
     List.for_all (Hashtbl.mem lbls) ids
   and check_closed (lbl, _) =
-    (not closed || List.length ids = Array.length lbl.lbl_all)
+    let all_lbls = ref (Array.length lbl.lbl_all) in
+    all_lbls := 0;
+    for i = 0 to Array.length lbl.lbl_all - 1 do
+      let lbl = lbl.lbl_all.(i) in
+      if not (label_is_optional lbl) then incr all_lbls
+    done;
+    Printf.eprintf "XXX check_closed %s %d %d\n" lbl.lbl_name (List.length ids) !all_lbls;
+    (not closed || List.length ids >= !all_lbls)
   in
   let labels' = Ext_list.filter labels check_ids in
+  Printf.eprintf "XXX labels':%d\n" (List.length labels);
   if keep && labels' = [] then (false, labels) else
   let labels'' = Ext_list.filter labels' check_closed in
   if keep && labels'' = [] then (false, labels') else (true, labels'')
@@ -864,6 +873,7 @@ let disambiguate_lid_a_list loc closed env opath lid_a_list =
     | _ -> Location.prerr_warning loc msg
   in
   let process_label lid =
+    let () = Printf.eprintf "XXX process_label %s\n" (Longident.last lid.txt) in
     (* Strategy for each field:
        * collect all the labels in scope for that name
        * if the type is known and principal, just eventually warn
@@ -882,6 +892,7 @@ let disambiguate_lid_a_list loc closed env opath lid_a_list =
         Some (_, _) -> (true, scope) (* disambiguate only checks scope *)
       | _  -> disambiguate_label_by_ids (opath=None) closed ids scope
     in
+    Printf.eprintf "ok: %b\n" ok;
     if ok then Label.disambiguate lid env opath labels ~warn ~scope
           else fst (List.hd labels) (* will fail later *)
   in