Use /Fo instead of -o when invoking CL

The -o option in the Microsoft C Compiler is deprecated (and has been for
a very long time). The warning is tedious, so use /Fo instead. The only
problem with this is that "-o foo.obj" must become "/Fofoo.obj" with no
space, which requires a little support in Arg_spec.

Signed-off-by: David Allsopp <david.allsopp@metastack.com>

CHANGES.md

@@ -65,8 +65,12 @@ next
   `META.pkg.template`. This feature was unused and was making the code
   complicated (#370)
 
+
 - Remove read-only attribute on Windows before unlink (#247)
 
+- Use /Fo instead of -o when invoking the Microsoft C compiler to eliminate
+  deprecation warning when compiling C++ sources (#354)
+
 1.0+beta16 (05/11/2017)
 -----------------------
 

src/arg_spec.ml

@@ -6,6 +6,7 @@ type 'a t =
   | A        of string
   | As       of string list
   | S        of 'a t list
+  | Concat   of string * 'a t list
   | Dep      of Path.t
   | Deps     of Path.t list
   | Target   of Path.t
@@ -18,14 +19,16 @@ let rec add_deps ts set =
     match t with
     | Dep  fn  -> Pset.add fn set
     | Deps fns -> Pset.union set (Pset.of_list fns)
-    | S ts -> add_deps ts set
+    | S ts
+    | Concat (_, ts) -> add_deps ts set
     | _ -> set)
 
 let rec add_targets ts acc =
   List.fold_left ts ~init:acc ~f:(fun acc t ->
     match t with
     | Target fn  -> fn :: acc
-    | S ts -> add_targets ts acc
+    | S ts
+    | Concat (_, ts) -> add_targets ts acc
     | _ -> acc)
 
 let expand ~dir ts x =
@@ -45,6 +48,7 @@ let expand ~dir ts x =
     | Paths fns ->
       List.map fns ~f:(Path.reach ~from:dir)
     | S ts -> List.concat_map ts ~f:loop_dyn
+    | Concat (sep, ts) -> [String.concat ~sep (loop_dyn (S ts))]
     | Target _ -> die "Target not allowed under Dyn"
     | Dyn _ -> assert false
   in
@@ -54,6 +58,7 @@ let expand ~dir ts x =
     | (Dep fn | Path fn) -> [Path.reach fn ~from:dir]
     | (Deps fns | Paths fns) -> List.map fns ~f:(Path.reach ~from:dir)
     | S ts -> List.concat_map ts ~f:loop
+    | Concat (sep, ts) -> [String.concat ~sep (loop (S ts))]
     | Target fn -> [Path.reach fn ~from:dir]
     | Dyn f -> loop_dyn (f x)
   in

src/arg_spec.mli

@@ -4,6 +4,7 @@ type 'a t =
   | A        of string
   | As       of string list
   | S        of 'a t list
+  | Concat   of string * 'a t list (** Concatenation with a custom separator *)
   | Dep      of Path.t (** A path that is a dependency *)
   | Deps     of Path.t list
   | Target   of Path.t

src/gen_rules.ml

@@ -413,6 +413,13 @@ Add it to your jbuild file to remove this warning.
   let build_cxx_file (lib : Library.t) ~scope ~dir ~requires ~h_files c_name =
     let src = Path.relative dir (c_name ^ ".cpp") in
     let dst = Path.relative dir (c_name ^ ctx.ext_obj) in
+    let open Arg_spec in
+    let output_param =
+      if ctx.ccomp_type = "msvc" then
+        [Concat ("", [A "/Fo"; Target dst])]
+      else
+        [A "-o"; Target dst]
+    in
     SC.add_rule sctx
       (Build.paths h_files
        >>>
@@ -425,15 +432,15 @@ Add it to your jbuild file to remove this warning.
             the current directory *)
          ~dir
          (SC.resolve_program sctx ctx.c_compiler)
-         [ S [A "-I"; Path ctx.stdlib_dir]
-         ; As (SC.cxx_flags sctx)
-         ; Dyn (fun (cxx_flags, libs) ->
-             S [ Lib.c_include_flags libs
-               ; As cxx_flags
-               ])
-         ; A "-o"; Target dst
-         ; A "-c"; Dep src
-         ]);
+         ([ S [A "-I"; Path ctx.stdlib_dir]
+          ; As (SC.cxx_flags sctx)
+          ; Dyn (fun (cxx_flags, libs) ->
+              S [ Lib.c_include_flags libs
+                ; As cxx_flags
+                ])
+          ] @ output_param @
+          [ A "-c"; Dep src
+          ]));
     dst
 
   (* In 4.02, the compiler reads the cmi for module alias even with [-w -49