added warning for missing mli interface file

Changes

@@ -195,6 +195,9 @@ Working version
   (Nicolás Ojeda Bär, review by Alain Frisch, Gabriel Scherer and Damien
   Doligez)
 
+- #9407: added warning for missing mli interface file
+  (Anukriti Kumar, review by Octachron, vicuna)
+
 ### Internal/compiler-libs changes:
 
  - #463: a new Misc.Magic_number module for user-friendly parsing

typing/typemod.ml

@@ -2663,6 +2663,8 @@ let type_implementation sourcefile outputprefix modulename initial_env ast =
           gen_annot outputprefix sourcefile annots;
           (str, coercion)
         end else begin
+          Location.prerr_warning (Location.in_file sourcefile)
+            Warnings.Interface_not_found;
           let coercion =
             Includemod.compunit initial_env ~mark:Includemod.Mark_positive
               sourcefile sg "(inferred signature)" simple_sg

utils/warnings.ml

@@ -92,6 +92,7 @@ type t =
   | Redefining_unit of string               (* 65 *)
   | Unused_open_bang of string              (* 66 *)
   | Unused_functor_parameter of string      (* 67 *)
+  | Interface_not_found                     (* 68 *)
 ;;
 
 (* If you remove a warning, leave a hole in the numbering.  NEVER change
@@ -170,9 +171,10 @@ let number = function
   | Redefining_unit _ -> 65
   | Unused_open_bang _ -> 66
   | Unused_functor_parameter _ -> 67
+  | Interface_not_found -> 68
 ;;
 
-let last_warning_number = 67
+let last_warning_number = 68
 ;;
 
 (* Must be the max number returned by the [number] function. *)
@@ -631,6 +633,8 @@ let message = function
          which shadows the existing one.\n\
          Hint: Did you mean 'type %s = unit'?" name
   | Unused_functor_parameter s -> "unused functor parameter " ^ s ^ "."
+  | Interface_not_found ->
+    "Cannot find interface file"
 ;;
 
 let nerrors = ref 0;;

utils/warnings.mli

@@ -94,6 +94,7 @@ type t =
   | Redefining_unit of string               (* 65 *)
   | Unused_open_bang of string              (* 66 *)
   | Unused_functor_parameter of string      (* 67 *)
+  | Interface_not_found                     (* 68 *)
 ;;
 
 type alert = {kind:string; message:string; def:loc; use:loc}