How to correctly configure inlay hints

[Blue-Berry]

I am trying to configure inlay hints, I first came across the docs below:

  /**
  * Enable/Disable Inlay Hints
  * @default false
  * @since 1.18
  */
  inlayHints: { enable : boolean }

But after looking around a bit it seems to me that those docs are out of date. It looks like it takes "hintPatternVariables" and "hintLetBindings" as configuration options.

module InlayHints = struct
  type t =
    { hint_pattern_variables : bool [@key "hintPatternVariables"] [@default false]
    ; hint_let_bindings : bool [@key "hintLetBindings"] [@default false]
    }
    ...

After setting "hintLetBindings" to true I expect to get type annotations for the following let binding as seen in one of the inlay hints tests.

let f () = let y = 0 in y

(said test)

let%expect_test "let bindings" =
  let source = "let f () = let y = 0 in y" in
  apply_inlay_hints ~source ();
  [%expect {| let f () = let y = 0 in y |}];
  apply_inlay_hints ~hint_let_bindings:true ~source ();
  [%expect {| let f () = let y$: int$ = 0 in y |}]
;;

However i can't seem to get it working the only annotations I seem to have are for anonymous function parameters like the following:

let _ = (fun a -> a)

let _ = (fun a: a' -> a)

So i am obviously doing something incorrectly does anyone have any idea where I could have gone wrong?