Add error message to @@directive (#6174)

* initial tests

* fix typo

* refactor

* add test

* update tests

* update CHANGELOG.md

Author: aspeddro

CHANGELOG.md

@@ -30,6 +30,7 @@
 #### :nail_care: Polish
 
 - Update list of reserved JS keywords. https://github.com/rescript-lang/rescript-compiler/pull/6167
+- Add error message to `@@directive`. https://github.com/rescript-lang/rescript-compiler/pull/6174
 
 # 11.0.0-alpha.3
 

jscomp/build_tests/super_errors/expected/directive_attr.res.expected

@@ -0,0 +1,8 @@
+
+  We've found a bug for you!
+  /.../fixtures/directive_attr.res:1:1-11
+
+  1 │ @@directive
+  2 │ 
+
+  expect string literal

jscomp/build_tests/super_errors/fixtures/directive_attr.res

@@ -0,0 +1 @@
+@@directive

jscomp/frontend/ast_config.ml

@@ -46,17 +46,10 @@ let process_directives str =
   str
   |> List.iter (fun (item : Parsetree.structure_item) ->
          match item.pstr_desc with
-         | Pstr_attribute
-             ( {txt = "directive"},
-               PStr
-                 [
-                   {
-                     pstr_desc =
-                       Pstr_eval
-                         ({pexp_desc = Pexp_constant (Pconst_string (d, _))}, _);
-                   };
-                 ] ) ->
-           Js_config.directives := !Js_config.directives @ [d]
+         | Pstr_attribute ({txt = "directive"}, payload) -> (
+           match Ast_payload.is_single_string payload with
+           | Some (d, _) -> Js_config.directives := !Js_config.directives @ [d]
+           | None -> Bs_syntaxerr.err item.pstr_loc Expect_string_literal)
          | Pstr_attribute ({txt = "uncurried"}, _) ->
            Config.uncurried := Uncurried
          | _ -> ())