Ignore extensions inside attributes for the unused extension check

CHANGES.md

@@ -3,6 +3,8 @@ unreleased
 
 - Fix infinite loop when duplicate attributes are present, raising
   an error instead (#613, @ceastlund, @patricoferris)
+- Ignore extensions inside attributes for the unused extension check
+  (#616, @Skepfyr)
 
 0.37.0
 ------

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

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
+|}]