ocamldep should detect ppx rewriter errors reported via [%%ocaml.error]

[vicuna]

Original bug ID: 6996
Reporter: @db4
Assigned to: @diml
Status: closed (set by @xavierleroy on 2017-02-16T14:18:08Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.1
Fixed in version: 4.03.0+dev / +beta1
Category: tools (ocaml{lex,yacc,dep,debug,...})
Duplicate of: #6795
Monitored by: @gasche @diml

Bug description

Suppose I have a file

[-- test.ml --]
open Printf
type t = int [@@deriving create]
[-- end of test.ml --]

which I feed to ocamldep:

$ DERIVING_DIR=ocamlfind query ppx_deriving
$ ocamlfind ocamldep -modules -ppx "${DERIVING_DIR}/ppx_deriving" test.ml
test.ml:

ocamldep does not report an error, it just does not see any dependencies. Of course, here I forgot to specify ppx_deriving_create plugin and ppx rewriter reports that:

$ ocamlfind ppx_tools/rewriter -ppx "${DERIVING_DIR}/ppx_deriving" -impl test.ml
[%%ocaml.error let _ = "Cannot locate deriver create"
let _ = ""]

but ocamldep just silently ignores this ocaml.error extension. I believe it should be fixed and an error should be reported to user.

I didn't try that with the latest OCaml distribution but at the first glance the problem is still there.