As a result of Cardoes earlier mail we talked a bit about possible
solutions in #gento-portage, and I suggested to let portage
automatically inject the deps based on SRC_URI pattern matching.
A mapping of extensions and their unpack deps would be kept in the tree
(e.g. mapping '.tar.bz2' to '( app-arch/tar app-arch/bzip2 )'

Benefits:
- simplified depstrings for most packages (I assume >90% of the tree)
- easier maintenance if dependencies are changing, or additional
implementations become supported (this could also be achieved with
virtuals though)
- potentially more accurate dependencies, as some common errors would
be eliminated (e.g. proper treatment of use-conditionals, not adding
unpack deps to RDEPEND)
- long-term adds the possibility to remove bzip2, gzip and tar from
@system

Potential problems:
- might cause trouble for some packages that use custom code for
unpacking, or due to circular deps, this could simply be solved with a
new RESTRICT value though.
- automagic deps could be confusing to devs/users
- not available for existing EAPIs (due to the mentioned problems)
- slightly slower dep calculations due to the extra processing
- possible match errors

So, is this something ebuild maintainers would like in general, or does
such a feature cause you nightmares?

Marius
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