fix(ppx): mark @@@mel.config used in interface files too

Changes.md

@@ -10,6 +10,8 @@ Unreleased
 - Make `'a Js.t` abstract (again), fixing a regression when bringing back
   OCaml-style objects BuckleScript
   ([#786](https://github.com/melange-re/melange/pull/786))
+- Don't issue "unused attribute" warning for well-formed `@@@mel.config` in
+  interface files ([#800](https://github.com/melange-re/melange/pull/800))
 
 2.0.0 2023-09-13
 ---------------

ppx/melange_ppx.ml

@@ -848,6 +848,14 @@ module Mapper = struct
                               :: pval_attributes;
                           };
                     })
+        | Psig_attribute
+            ({
+               (* TODO: remove support for bs.* *)
+               attr_name = { txt = "bs.config" | "mel.config" | "config"; _ };
+               _;
+             } as attr) ->
+            Bs_ast_invariant.mark_used_bs_attribute attr;
+            sigi
         | _ -> super#signature_item sigi
     end
 end

test/blackbox-tests/unused-attributes.t

@@ -23,10 +23,3 @@
   >   [@@@mel.config { flags = [|"-unboxed-types"|] }]
   > EOF
   $ melc -ppx melppx x.mli
-  File "x.mli", line 1, characters 6-16:
-  1 |   [@@@mel.config { flags = [|"-unboxed-types"|] }]
-            ^^^^^^^^^^
-  Alert unused: Unused attribute [@mel.config]
-  This means such annotation is not annotated properly.
-  For example, some annotations are only meaningful in externals
-