Use Lwt's pre-allocated promises

Author: MisterDA

src/core/lwt_list.ml

@@ -182,7 +182,7 @@ let rec find_s f l =
       (find_s [@ocaml.tailcall]) f l
 
 let _optionalize f x =
-  f x >>= fun b -> if b then Lwt.return (Some x) else Lwt.return None
+  f x >>= fun b -> if b then Lwt.return (Some x) else Lwt.return_none
 
 let filter_s f l =
   filter_map_s (_optionalize f) l

src/unix/lwt_io.ml

@@ -1504,7 +1504,7 @@ let rec delete_recursively directory =
   |> Lwt_stream.iter_s begin fun entry ->
     if entry = Filename.current_dir_name ||
        entry = Filename.parent_dir_name then
-      Lwt.return ()
+      Lwt.return_unit
     else
       let path = Filename.concat directory entry in
       Lwt_unix.lstat path >>= fun {Lwt_unix.st_kind; _} ->

src/unix/lwt_throttle.ml

@@ -83,7 +83,7 @@ module Make (H : Hashtbl.HashedType) : (S with type key = H.t) = struct
                 replacing. *)
              prerr_endline "internal error";
              Printexc.print_backtrace stderr;
-             Lwt.return ())
+             Lwt.return_unit)
       in
       Some t