Add warning for .cxx pre 1.8

Signed-off-by: Rudi Grinberg <[email protected]>

src/c.ml

@@ -10,16 +10,21 @@ module Kind = struct
     | C -> Format.pp_print_string fmt "c"
     | Cxx -> Format.pp_print_string fmt "cpp"
 
+  type split =
+    | Unrecognized
+    | Not_allowed_until of Syntax.Version.t
+    | Recognized of string * t
+
   let split_extension fn ~dune_version =
     match String.lsplit2 fn ~on:'.' with
-    | Some (obj, "c") -> Some (obj, C)
-    | Some (obj, "cpp") -> Some (obj, Cxx)
+    | Some (obj, "c") -> Recognized (obj, C)
+    | Some (obj, "cpp") -> Recognized (obj, Cxx)
     | Some (obj, "cxx") ->
       if dune_version >= (1, 8) then
-        Some (obj, Cxx)
+        Recognized (obj, Cxx)
       else
-        None
-    | _ -> None
+        Not_allowed_until (1, 8)
+    | _ -> Unrecognized
 
   let possible_fns t fn ~dune_version =
     match t with

src/c.mli

@@ -7,10 +7,15 @@ module Kind : sig
 
   val pp : t Fmt.t
 
+  type split =
+    | Unrecognized
+    | Not_allowed_until of Syntax.Version.t
+    | Recognized of string * t
+
   val split_extension
     :  string
     -> dune_version:Syntax.Version.t
-    -> (string * t) option
+    -> split
 
   (** [possible_fns t s] returns the possible filenames given the extension-less
       basenames [s] *)

src/c_sources.ml

@@ -49,8 +49,14 @@ let load_sources ~dune_version ~dir ~files =
   let init = C.Kind.Dict.make String.Map.empty in
   String.Set.fold files ~init ~f:(fun fn acc ->
     match C.Kind.split_extension fn ~dune_version with
-    | None -> acc
-    | Some (obj, kind) ->
+    | Unrecognized -> acc
+    | Not_allowed_until version ->
+      let loc = Loc.in_dir dir in
+      Errors.warn loc
+        "Source file %s with extension %s is not allowed before version %a"
+        fn (Filename.extension fn) Syntax.Version.pp version;
+      acc
+    | Recognized (obj, kind) ->
       let path = Path.relative dir fn in
       C.Kind.Dict.update acc kind ~f:(fun v ->
         String.Map.add v obj (C.Source.make ~kind ~path)