This fixes a bug whereby "guix build texlive -n" would report:
0.0 MB would be downloaded:
/gnu/store/…-texlive-20180414
instead of:
The following derivation would be built:
/gnu/store/…-texlive-texmf-20180414.drv
2,595.2 MB would be downloaded:
/gnu/store/…-texlive-20180414-texmf.tar.xz
/gnu/store/…-texlive-20180414
where 'texlive-texmf' is a non-substitutable dependency of 'texlive'.
* guix/derivations.scm (dependencies-of-substitutables): New procedure.
(derivation-build-plan): When 'input-substitutable-info' returns true,
append the subset of DEPS that corresponds to SUBSTITUABLES to the first
argument of 'loop'.
* guix/ui.scm (show-what-to-build): Remove half-baked traversal of
DOWNLOAD.
* tests/derivations.scm ("derivation-build-plan and substitutes,
non-substitutable dep"): New test.
