diff --git a/src/analysis/signature_help.ml b/src/analysis/signature_help.ml
index 9a199d71d2..898cdd6dda 100644
--- a/src/analysis/signature_help.ml
+++ b/src/analysis/signature_help.ml
@@ -155,6 +155,40 @@ let application_signature ~prefix = function
     Some { result with active_param }
   | _ -> None
 
+module String = struct
+    include String
+    let rfindi =
+      let rec loop s ~f i =
+        if i < 0 then None
+        else if f (String.unsafe_get s i) then Some i
+        else loop s ~f (i - 1)
+      in
+      fun ?from s ~f ->
+        let from =
+          let len = String.length s in
+          match from with
+          | None -> len - 1
+          | Some i ->
+            if i > len - 1 then failwith "rfindi: invalid from"
+            else i
+        in
+        loop s ~f from
+
+    let rec check_prefix s ~prefix len i =
+      i = len || (s.[i] = prefix.[i] && check_prefix s ~prefix len (i + 1))
+
+    let lsplit2 s ~on =
+      match String.index_opt s on with
+      | None -> None
+      | Some i ->
+        Some (sub s ~pos:0 ~len:i, sub s ~pos:(i + 1) ~len:(length s - i - 1))
+
+    let is_prefix s ~prefix =
+      let len = length s in
+      let prefix_len = length prefix in
+      len >= prefix_len && check_prefix s ~prefix prefix_len 0
+  end
+
 (** @see <https://ocaml.org/manual/lex.html> reference *)
 let prefix_of_position ~short_path source position =
   match Msource.text source with
@@ -216,8 +250,8 @@ let prefix_of_position ~short_path source position =
     (* if we reconstructed [~f:ignore] or [?f:ignore], we should take only
        [ignore], so: *)
     if
-      String.is_prefixed reconstructed_prefix ~by:"~"
-      || String.is_prefixed reconstructed_prefix ~by:"?"
+      String.is_prefix reconstructed_prefix ~prefix:"~"
+      || String.is_prefix reconstructed_prefix ~prefix:"?"
     then
       match String.lsplit2 reconstructed_prefix ~on:':' with
       | Some (_, s) -> s
diff --git a/src/utils/std.ml b/src/utils/std.ml
index 2e3b4009aa..0031c04b04 100644
--- a/src/utils/std.ml
+++ b/src/utils/std.ml
@@ -393,32 +393,6 @@ module String = struct
   (* Drop characters from beginning of string *)
   let drop n s = sub s ~pos:n ~len:(length s - n)
 
-  let rfindi =
-    let rec loop s ~f i =
-      if i < 0 then None
-      else if f (String.unsafe_get s i) then Some i
-      else loop s ~f (i - 1)
-    in
-    fun ?from s ~f ->
-      let from =
-        let len = String.length s in
-        match from with
-        | None -> len - 1
-        | Some i ->
-          if i > len - 1 then failwith "rfindi: invalid from"
-          else i
-      in
-      loop s ~f from
-
-  let rec check_prefix s ~prefix len i =
-    i = len || (s.[i] = prefix.[i] && check_prefix s ~prefix len (i + 1))
-
-  let lsplit2 s ~on =
-    match String.index_opt s on with
-    | None -> None
-    | Some i ->
-      Some (sub s ~pos:0 ~len:i, sub s ~pos:(i + 1) ~len:(length s - i - 1))
-
   module Set = struct
     include MoreLabels.Set.Make (struct type t = string let compare = compare end)
     let of_list l = List.fold_left ~f:(fun s elt -> add elt s) l ~init:empty