Ignore extensions inside attributes for the unused extension check

Skepfyr · Skepfyr · commit 37c0ed97b8bf · 2025-12-10T12:19:55.000Z
This means ppxlib doesn't emit a warning for code like `[@attr [%ext]]`. Previously, it would warn that `[%ext]` wasn't translated, however this is essentially always not the desired behaviour; the ppx that interprets the attribute will be called before the one that interprets the extension (assuming context free ppxes) so the attribute handling will have to handle the extension itself. If the attribute substitues the extension into generated code the warning will still trigger if that substituted extension point isn't expanded. I'm slightly nervous this will cause weird errors if you put extensions inside the payload of builtin attributes, however I don't think there are any that can have an extension in their payload so it's probably fine. Signed-off-by: Jack Rickard <jrickard@janestreet.com>
diff --git a/src/extension.ml b/src/extension.ml
@@ -324,6 +324,10 @@ let collect_unhandled_extension_errors =
             "extension not expected here, Ppxlib.Extension needs updating!";
         ]
 
+    (* Skip extensions in attributes, they will be handled by the PPX that
+       handles each specific attribute. *)
+    method! attribute _x acc = acc
+
     method! core_type_desc x acc =
       match x with
       | Ptyp_extension ext -> acc @ unhandled_extension_error Core_type ext
diff --git a/test/driver/attributes/test.ml b/test/driver/attributes/test.ml
@@ -262,3 +262,28 @@ let e = [%flag_ghost "bye" [@flag]]
 [%%expect{|
 val e : string * string = ("bye", "bye")
 |}]
+
+(* Test extensions aren't flagged as unused inside attributes. *)
+
+let () =
+  let attr =
+    Attribute.declare
+      "ignore_me"
+      Attribute.Context.core_type
+      Ast_pattern.(__)
+      ignore
+  in
+  let ext =
+    Extension.V3.declare
+      "ignore_me"
+      Core_type
+      Ast_pattern.(ptyp __)
+      (fun ~ctxt:_ e -> let (_ : unit option) = Attribute.get attr e in e)
+  in
+  Driver.register_transformation "ignore_me" ~rules:[ Context_free.Rule.extension ext ]
+;;
+
+type t = [%ignore_me: int[@ignore_me [%doesn't_exist]]]
+[%%expect{|
+type t = int
+|}]
