Add a way to customize obj dir for executables

Use this function to simplify the obj dir for the utop runner

src/exe.ml

@@ -112,6 +112,7 @@ let link_exe
     SC.add_rules sctx (List.map rules ~f:(fun r -> libs_and_cm_and_flags >>> r))
 
 let build_and_link_many
+      ?obj_dir
       ~loc ~dir ~programs ~modules
       ~scope
       ~linkages
@@ -127,9 +128,13 @@ let build_and_link_many
       sctx
   =
   let item = (List.hd programs).Program.name in
-  (* Use "eobjs" rather than "objs" to avoid a potential conflict with a library of the
-     same name *)
-  let obj_dir = Path.relative dir ("." ^ item ^ ".eobjs") in
+  let obj_dir =
+    match obj_dir with
+    (* Use "eobjs" rather than "objs" to avoid a potential conflict with a
+       library of the same name *)
+    | None -> Path.relative dir ("." ^ item ^ ".eobjs")
+    | Some d -> d
+  in
   let dep_kind = Build.Required in
   let modules =
     String_map.map modules ~f:(Module.set_obj_name ~wrapper:None)
@@ -193,5 +198,5 @@ let build_and_link_many
 
   (obj_dir, real_requires)
 
-let build_and_link ~loc ~dir ~program =
-  build_and_link_many ~loc ~dir ~programs:[program]
+let build_and_link ?obj_dir ~loc ~dir ~program =
+  build_and_link_many ?obj_dir ~loc ~dir ~programs:[program]

src/exe.mli

@@ -38,7 +38,8 @@ end
     directory and the resolved list of library dependencies. *)
 
 val build_and_link
-  :  loc:Loc.t
+  :  ?obj_dir:Path.t
+  -> loc:Loc.t
   -> dir:Path.t
   -> program:Program.t
   -> modules:Module.t String_map.t
@@ -57,7 +58,8 @@ val build_and_link
   -> Path.t * (unit, Lib.t list) Build.t
 
 val build_and_link_many
-  :  loc:Loc.t
+  :  ?obj_dir:Path.t
+  -> loc:Loc.t
   -> dir:Path.t
   -> programs:Program.t list
   -> modules:Module.t String_map.t

src/utop.ml

@@ -66,6 +66,7 @@ let setup sctx ~dir ~(libs : Library.t list) ~scope =
     let _obj_dir, libs =
       Exe.build_and_link sctx
         ~loc
+        ~obj_dir:utop_exe_dir
         ~dir:utop_exe_dir
         ~program:{ name = exe_name ; main_module_name }
         ~modules

test/blackbox-tests/test-cases/utop/run.t

@@ -2,7 +2,7 @@
       ocamldep forutop/.utop/utop.ml.d
       ocamldep forutop/forutop.ml.d
         ocamlc forutop/.forutop.objs/forutop.{cmi,cmo,cmt}
-        ocamlc forutop/.utop/.utop.eobjs/utop.{cmi,cmo,cmt}
+        ocamlc forutop/.utop/utop.{cmi,cmo,cmt}
         ocamlc forutop/forutop.cma
         ocamlc forutop/.utop/utop.exe
   hello in utop