add a warning when rest record would be empty

Author: tsnobip

compiler/ext/warnings.ml

@@ -88,6 +88,7 @@ type t =
   | Bs_toplevel_expression_unit of
       (string * top_level_unit_help) option (* 109 *)
   | Bs_todo of string option (* 110 *)
+  | Bs_record_rest_empty (* 111 *)
 
 (* If you remove a warning, leave a hole in the numbering.  NEVER change
    the numbers of existing warnings.
@@ -154,8 +155,9 @@ let number = function
   | Bs_uninterpreted_delimiters _ -> 108
   | Bs_toplevel_expression_unit _ -> 109
   | Bs_todo _ -> 110
+  | Bs_record_rest_empty -> 111
 
-let last_warning_number = 110
+let last_warning_number = 111
 
 let letter_all =
   let rec loop i = if i = 0 then [] else i :: loop (i - 1) in
@@ -532,6 +534,9 @@ let message = function
            `%s->ignore`"
           help_text help_text
       | _ -> "")
+  | Bs_record_rest_empty ->
+    "All fields of the rest type are already present in the explicit pattern. \
+     The rest record will always be empty."
   | Bs_todo maybe_text ->
     (match maybe_text with
     | None -> "Todo found."

compiler/ext/warnings.mli

@@ -81,6 +81,7 @@ type t =
   | Bs_toplevel_expression_unit of
       (string * top_level_unit_help) option (* 109 *)
   | Bs_todo of string option (* 110 *)
+  | Bs_record_rest_empty (* 111 *)
 
 val parse_options : bool -> string -> unit
 

compiler/ml/typecore.ml

@@ -1608,6 +1608,15 @@ and type_pat_aux ~constrs ~labels ~no_existentials ~mode ~explode ~env sp
               (fun (l : Types.label_declaration) -> Ident.name l.ld_id)
               rest_labels
           in
+          (* Warn if all rest fields are already explicit — the rest record will be empty *)
+          if
+            rest_field_names <> []
+            && List.for_all
+                 (fun f -> List.mem f explicit_fields)
+                 rest_field_names
+          then
+            Location.prerr_warning rest_pat.ppat_loc
+              Warnings.Bs_record_rest_empty;
           (* Validate: fields in both explicit and rest must be optional in the explicit pattern *)
           let not_optional =
             List.filter