bug#43890: ‘package-input-rewriting/spec’ can introduce unnecessary variants

[Ludovic Courtès]

Consider this example:

--8<---------------cut here---------------start------------->8---
$ guix describe
Generacio 162   Oct 01 2020 00:23:38    (nuna)
  guix 7607ace
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 7607ace5091aea0157ba5c8a508129cc5fc4f931
$ guix build inkscape --no-grafts -d
/gnu/store/arjs5hb4wmy6dh5d3y8bbs808ki9abf8-inkscape-1.0.1.drv
$ guix build inkscape --no-grafts -d --with-graft=glib=glib-networking
/gnu/store/zd8mm3w6x9c97anfaly77fz28s5y3i5h-inkscape-1.0.1.drv
$ guix build inkscape --no-grafts -d --with-graft=libreoffice=abiword
/gnu/store/arjs5hb4wmy6dh5d3y8bbs808ki9abf8-inkscape-1.0.1.drv
--8<---------------cut here---------------end--------------->8---

The last one is fine: it has no effect.

The second one is problematic: since we’re using ‘--no-grafts’, the
‘--with-graft’ option should have absolutely no effect; yet, it yields a
different derivation.

On closer inspection, we see that the core issue is that
‘gobject-introspection’ in the second case ends up with ‘libffi’ twice
in its ‘*-guile-builder’ script, a problem similar to
<https://issues.guix.gnu.org/38100>.  (‘libffi’ is propagated by both
‘glib’ and ‘gobject-introspection’.)

Ludo’.