Print "Entering directory '...'" when necessary

When the root of the workspace is not the current directory, print:

  Entering directory '<absolute path to root>'

This way editors such as emacs or vim knows how to interpret filenames
reported by the compiler.

Fixes #138

doc/usage.rst

@@ -17,6 +17,15 @@ jbuild-workspace
 The root of the current workspace is determined by looking up a
 ``jbuild-workspace`` file in the current directory and parent directories.
 
+``jbuilder`` prints out the root when starting if it is not the
+current directory:
+
+.. code:: bash
+
+    $ jbuilder runtest
+    Entering directory '/home/jdimino/code/jbuilder'
+    ...
+
 More precisely, it will choose the outermost ancestor directory containing a
 ``jbuild-workspace`` file as root. For instance if you are in
 ``/home/me/code/myproject/src``, then jbuilder will look for all these files in

src/future.ml

@@ -661,5 +661,7 @@ module Scheduler = struct
     Lazy.force Ansi_color.setup_env_for_colors;
     Log.info log ("Workspace root: " ^ !Clflags.workspace_root);
     let cwd = Sys.getcwd () in
+    if cwd <> initial_cwd then
+      Printf.eprintf "Entering directory '%s'\n%!" cwd;
     go_rec cwd log t
 end

src/import.ml

@@ -12,6 +12,8 @@ let () = Printexc.record_backtrace true
 let sprintf = Printf.sprintf
 let ksprintf = Printf.ksprintf
 
+let initial_cwd = Sys.getcwd ()
+
 (* An error in the code of jbuild, that should be reported upstream *)
 exception Code_error of string
 let code_errorf fmt = ksprintf (fun msg -> raise (Code_error msg)) fmt

src/path.ml

@@ -266,13 +266,11 @@ let of_string ?error_loc s =
 let t sexp = of_string (Sexp.Of_sexp.string sexp) ~error_loc:(Sexp.Ast.loc sexp)
 let sexp_of_t t = Sexp.Atom (to_string t)
 
-let absolute =
-  let initial_dir = Sys.getcwd () in
-  fun fn ->
-    if is_local fn then
-      Filename.concat initial_dir fn
-    else
-      fn
+let absolute fn =
+  if is_local fn then
+    Filename.concat initial_cwd fn
+  else
+    fn
 
 let reach t ~from =
   match is_local t, is_local from with

src/path.mli

@@ -57,6 +57,8 @@ val is_local : t -> bool
 
 val relative : ?error_loc:Loc.t -> t -> string -> t
 
+(** Create an external path. If the argument is relative, assume it is
+    relative to the initial directory jbuilder was launched in. *)
 val absolute : string -> t
 
 val reach : t -> from:t -> string