Fix #262

Use the absolute path when evaluating a jbuild file in OCaml syntax.

src/jbuild_load.ml

@@ -124,7 +124,7 @@ end
             [ [ "-I"; "+compiler-libs" ]
             ; includes
             ; List.map cmas ~f:(Path.reach ~from:dir)
-            ; [ Path.reach ~from:dir wrapper ]
+            ; [ Path.to_absolute_filename wrapper ]
             ]
         in
         (* CR-someday jdimino: if we want to allow plugins to use findlib:

src/merlin.ml

@@ -12,15 +12,12 @@ type t =
   ; source_dirs: Path.Set.t
   }
 
-(* This must be forced after we change the cwd to the workspace root *)
-let root_absolute_name = lazy (Sys.getcwd ())
-
 let ppx_flags sctx ~dir ~src_dir:_ { preprocess; libname; _ } =
   match preprocess with
   | Pps { pps; flags } ->
     let exe = SC.PP.get_ppx_driver sctx pps ~dir ~dep_kind:Optional in
     let command =
-      List.map (Filename.concat (Lazy.force root_absolute_name) (Path.to_string exe)
+      List.map (Path.to_absolute_filename  exe
                 :: "--as-ppx"
                 :: SC.PP.cookie_library_name libname
                 @ flags)

src/path.ml

@@ -272,6 +272,14 @@ let absolute fn =
   else
     fn
 
+let to_absolute_filename t =
+  if is_local t then begin
+    let root = !Clflags.workspace_root in
+    assert (not (Filename.is_relative root));
+    Filename.concat root (to_string t)
+  end else
+    t
+
 let reach t ~from =
   match is_local t, is_local from with
   | false, _ -> t

src/path.mli

@@ -61,6 +61,10 @@ val relative : ?error_loc:Loc.t -> t -> string -> t
     relative to the initial directory jbuilder was launched in. *)
 val absolute : string -> t
 
+(** Convert a path to an absolute filename. Must be called after the
+    workspace root has been set. *)
+val to_absolute_filename : t -> string
+
 val reach : t -> from:t -> string
 val reach_for_running : t -> from:t -> string