[5.4 compat] effects keyword can be an attribute id

test/passing/refs.ahrefs/attributes.ml.ref

@@ -584,3 +584,5 @@ let _ =
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]

test/passing/refs.default/attributes.ml.ref

@@ -463,3 +463,5 @@ let _ =
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]

test/passing/refs.janestreet/attributes.ml.ref

@@ -535,3 +535,5 @@ let _ =
 ;;
 
 let fold input ~init ~f = foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]

test/passing/refs.ocamlformat/attributes.ml.ref

@@ -513,3 +513,5 @@ let _ =
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]

test/passing/tests/attributes.ml

@@ -498,3 +498,5 @@ let _ = (begin[@a] x end [@b])
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]

vendor/parser-extended/parser.mly

@@ -4018,6 +4018,7 @@ single_attr_id:
   | DO { "do" }
   | DONE { "done" }
   | DOWNTO { "downto" }
+  | EFFECT { "effect" }
   | ELSE { "else" }
   | END { "end" }
   | EXCEPTION { "exception" }

vendor/parser-standard/parser.mly

@@ -4063,6 +4063,7 @@ single_attr_id:
   | DO { "do" }
   | DONE { "done" }
   | DOWNTO { "downto" }
+  | EFFECT { "effect" }
   | ELSE { "else" }
   | END { "end" }
   | EXCEPTION { "exception" }