- Now `pkg.outputUnspecified = true` but this attribute is missing in
every output, so we can recognize whether the user chose or not.
If (s)he didn't choose, we put `pkg.bin or pkg.out or pkg` into
`systemPackages`.
- `outputsToLink` is replaced by `extraOutputsToLink`.
We add extra outputs *regardless* of whether the user chose anything.
It's mainly meant for outputs with docs and debug symbols.
- Note that as a result, some libraries will disappear from system path.
This is like callPackageWith, except that it expects the supplied
function to return a *set* of packages. It will then make the
individual packages overridable.
It's unused, and also a bad idea: because it recursively recomputes
every function argument and there is no sharing, you can get an
exponential (?) blowup in evaluation time. For example, evaluating
‘linuxPackages.kernel’ takes 0.09s and ~13 MiB, but evaluating
‘linuxPackages.kernel.deepOverride {}’ takes 3.6s and ~305 MiB.
It now strictly evaluates all remaining attributes, preventing
unevaluated thunks that cannot be garbage-collected. It's also applied
to all jobs in Nixpkgs' release.nix.
This reduces hydra-eval-jobs' memory consumption on the 14.12
release-combined jobset from 5.1 GB to 2.0 GB.
