diff --git a/src/compilerlib/pb_parsing_util.ml b/src/compilerlib/pb_parsing_util.ml
index a2c52ba5..19e96036 100644
--- a/src/compilerlib/pb_parsing_util.ml
+++ b/src/compilerlib/pb_parsing_util.ml
@@ -131,7 +131,7 @@ let option_list items = Pb_option.List_literal items
 
 let option_name_of_ident ident =
   let ident =
-    if String.starts_with ~prefix:"." ident then
+    if Pb_util.String.starts_with ~prefix:"." ident then
       String.sub ident 1 (String.length ident - 1)
     else
       ident
diff --git a/src/compilerlib/pb_util.ml b/src/compilerlib/pb_util.ml
index 6b273dff..dbe15c86 100644
--- a/src/compilerlib/pb_util.ml
+++ b/src/compilerlib/pb_util.ml
@@ -47,6 +47,22 @@ let string_fold_lefti f e0 s =
   in
   loop e0 0
 
+module String = struct
+  include String
+
+  let starts_with ~prefix s =
+    let len_s = length s and len_pre = length prefix in
+    let rec aux i =
+      if i = len_pre then
+        true
+      else if unsafe_get s i <> unsafe_get prefix i then
+        false
+      else
+        aux (i + 1)
+    in
+    len_s >= len_pre && aux 0
+end
+
 module Option = struct
   let default x = function
     | Some y -> y
diff --git a/src/compilerlib/pb_util.mli b/src/compilerlib/pb_util.mli
index 55426f89..5dc120ba 100644
--- a/src/compilerlib/pb_util.mli
+++ b/src/compilerlib/pb_util.mli
@@ -42,6 +42,14 @@ val string_fold_lefti : ('a -> int -> char -> 'a) -> 'a -> string -> 'a
 val indentation_prefix : int -> string
 (** [indentation_prefix level] returns a string of [2 * level] spaces *)
 
+module String : sig
+  val starts_with :
+    prefix:(* comment thwarts tools/sync_stdlib_docs *) string -> string -> bool
+  (** [starts_with ][~prefix s] is [true] if and only if [s] starts with
+    [prefix].
+    *)
+end
+
 module Option : sig
   val default : 'a -> 'a option -> 'a
   (** [option_default x o] returns [x] is [o] is [None] otherwise [y]
@@ -93,6 +101,7 @@ module List : sig
 
     Note: the [eq] function may be called even if the
     lists have different length. If you know your equality
+
     function is costly, you may want to check {!compare_lengths}
     first.
 *)