Fix staged pps on Windows (ocaml#10869)

* Use backslashes when passing staged pps driver on Windows

Signed-off-by: Nicolás Ojeda Bär <n.oje.bar@gmail.com>

doc/changes/10869.md

@@ -0,0 +1,2 @@
+- Fix staged pps preprocessors on Windows (which were not working at all
+  previously) (#10869, @nojb)

otherlibs/stdune/src/string.ml

@@ -278,7 +278,22 @@ let need_quoting s =
 ;;
 
 let quote_for_shell s = if need_quoting s then Stdlib.Filename.quote s else s
-let quote_list_for_shell l = List.map l ~f:quote_for_shell |> concat ~sep:" "
+
+let quote_list_for_shell = function
+  | [] -> ""
+  | prog :: args ->
+    let prog =
+      if Sys.win32 && contains prog '/'
+      then
+        map
+          ~f:(function
+            | '/' -> '\\'
+            | c -> c)
+          prog
+      else prog
+    in
+    quote_for_shell prog :: List.map ~f:quote_for_shell args |> concat ~sep:" "
+;;
 
 let of_list chars =
   let s = Bytes.make (List.length chars) '0' in

otherlibs/stdune/src/string.mli

@@ -116,8 +116,8 @@ val need_quoting : string -> bool
     [true] *)
 val quote_for_shell : string -> string
 
-(** [quote_list_for_shell l] is
-    [List.map l ~f:quote_for_shell |> concat ~sep:" "] *)
+(** [quote_list_for_shell l] quotes a command-line so that it can be passed to
+    the system shell (eg by using [Sys.command]). *)
 val quote_list_for_shell : string list -> string
 
 val filter_map : string -> f:(char -> char option) -> string

test/blackbox-tests/test-cases/dune

@@ -29,7 +29,7 @@
   (alias runtest-js)))
 
 (cram
- (applies_to windows-diff)
+ (applies_to windows-diff github6644)
  (alias runtest-windows))
 
 ; DISABLED TESTS