Add Sys.override_argv, needed for toplevel_expect_tests.

src/core_sys.ml

@@ -153,3 +153,18 @@ external opaque_identity : 'a -> 'a = "%opaque"
 module Private = struct
   let unix_quote = unix_quote
 end
+
+[%%if ocaml_version < (4, 09, 0)]
+let override_argv args =
+  let len = Array.length args in
+  assert (len <= Array.length Sys.argv);
+  Array.blit ~src:args ~src_pos:0 ~dst:Sys.argv ~dst_pos:0 ~len;
+  (Caml.Obj.truncate [@ocaml.alert "-deprecated"]) (Obj.repr Sys.argv) len;
+  Arg.current := 0;
+;;
+[%%else]
+external caml_sys_modify_argv : string array -> unit = "caml_sys_modify_argv";;
+let override_argv new_argv =
+  caml_sys_modify_argv new_argv;
+  Arg.current := 0
+[%%endif]

src/core_sys.mli

@@ -182,6 +182,18 @@ val home_directory : unit -> string
 *)
 external opaque_identity : 'a -> 'a = "%opaque"
 
+(** [override_argv new_argv] makes subsequent calls to {!get_argv} return [new_argv].
+
+    Prior to OCaml version 4.09, this function has two noteworthy behaviors:
+
+    - it may raise if the length of [new_argv] is greater than the length of [argv] before
+      the call;
+    - it re-uses and mutates the previous [argv] value instead of using the new one; and
+    - it even mutates its length, which can be observed by inspecting the array returned
+      by an earlier call to {!get_argv}.
+*)
+val override_argv : string array -> unit
+
 
 (**/**)