[menhir] Call menhir from root scope directory as to improve error re…

…porting.

This is a common problem in a few Dune tools: if a menhir file is
inside a sub-directory, the tool will be invoked from the subdir,
however the Dune call was likely from the top-level directory and the
error/warning messages won't be correct.

I am not sure this is the right way to address this problem, in
particular what will happen if Dune is called from some parallel
sub-directory. Suggestions welcome.

Signed-off-by: Emilio Jesus Gallego Arias <e+git@x80.org>

src/menhir.ml

@@ -40,6 +40,11 @@ module type PARAMS = sig
      is of the form [_build/<context>/src], e.g., [_build/default/src]. *)
   val dir : Path.t
 
+  (* [root_dir] is the base directory of the context, usually where
+     the build is invoked from; we run menhir from this directoy to we
+     get correct error paths. *)
+  val root_dir : Path.t
+
   (* [stanza] is the [(menhir ...)] stanza, as found in the [jbuild] file. *)
 
   val stanza: stanza
@@ -122,7 +127,7 @@ module Run (P : PARAMS) : sig end = struct
   (* [menhir args] generates a Menhir command line (a build action). *)
 
   let menhir (args : 'a args) : (string list, Action.t) Build.t =
-    Build.run ~dir menhir_binary args
+    Build.run ~dir:root_dir menhir_binary args
 
   let rule ?(mode=stanza.mode) : (unit, Action.t) Build.t -> unit =
     SC.add_rule sctx ~dir ~mode ~loc:stanza.loc
@@ -322,6 +327,7 @@ let gen_rules ~dir cctx stanza =
     Run (struct
       let cctx = cctx
       let dir = dir
+      let root_dir = Scope.root (Compilation_context.scope cctx)
       let stanza = stanza
     end) in
   ()