Shim String.starts_with

As it's only available since 4.13.

src/cmdliner_base.ml

@@ -340,3 +340,20 @@ let env_bool_parse s = match String.lowercase_ascii s with
 | s ->
     let alts = alts_str ~quoted:true ["true"; "yes"; "false"; "no" ] in
     `Error (err_invalid_val s alts)
+
+let string_has_prefix ~prefix s =
+  let prefix_len = String.length prefix in
+  let s_len = String.length s in
+  if prefix_len > s_len then false else
+  let rec loop i =
+    if i = prefix_len then true
+    else if String.get prefix i = String.get s i then loop (i + 1)
+    else false
+  in
+  loop 0
+
+let string_drop_prefix ~prefix s =
+  if string_has_prefix ~prefix s then
+    let drop = String.length prefix in
+    Some (String.sub s drop (String.length s - drop))
+  else None

src/cmdliner_base.mli

@@ -58,4 +58,8 @@ val t4 :
   ('a * 'b * 'c * 'd) conv
 
 val env_bool_parse : bool parser
+
 val is_space : char -> bool
+
+val string_has_prefix : prefix:string -> string -> bool
+val string_drop_prefix : prefix:string -> string -> string option

src/cmdliner_cline.ml

@@ -90,11 +90,7 @@ let hint_matching_opt optidx s =
 let complete_prefix = "+cmdliner_complete:"
 
 let maybe_complete_token s =
-  if String.starts_with ~prefix:complete_prefix s
-  then
-    let drop = String.length complete_prefix in
-    Some (String.sub s drop (String.length s - drop))
-  else None
+  Cmdliner_base.string_drop_prefix ~prefix:complete_prefix s
 
 exception Completion_requested of 
   string * [ `Opt of Cmdliner_info.Arg.t | `Arg of Cmdliner_info.Arg.t | `Any ]

src/cmdliner_eval.ml

@@ -180,7 +180,7 @@ module Complete = struct
     Printf.ksprintf print_endline fmt
 
   let item ~prefix (name, doc) =
-    if String.starts_with ~prefix name then (
+    if Cmdliner_base.string_has_prefix ~prefix name then (
       print_endline "item";
       print_endline name;
       let doc = String.map (fun c -> if Cmdliner_base.is_space c then ' ' else c) doc in