This pushes check-meta evaluation to derivation evaluation step, leaving all other
attributes accessible.
Before this commit:
> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen --argstr system aarch64-linux
> Package ‘xen-4.5.5’ in pkgs/applications/virtualization/xen/generic.nix:226 is not supported on ‘aarch64-linux’, refusing to evaluate.
as expected
> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen.name --argstr system aarch64-linux
> Package ‘xen-4.5.5’ in pkgs/applications/virtualization/xen/generic.nix:226 is not supported on ‘aarch64-linux’, refusing to evaluate.
> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen.meta.description --argstr system aarch64-linux
> Package ‘xen-4.5.5’ in pkgs/applications/virtualization/xen/generic.nix:226 is not supported on ‘aarch64-linux’, refusing to evaluate.
which is unfortunate since its impossible to use packages in autogenerated
documentation on all platforms.
After this commit:
> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen --argstr system aarch64-linux
still fails
> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen.name --argstr system aarch64-linux
> "xen-4.5.5"
> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen.meta.description --argstr system aarch64-linux
> "Xen hypervisor and related components (vanilla)"
The reason is that if cross compiling (or for other reasons) python
bindings as a whole are turned off. Those two lines then trigger
assertion errors unless manually overridden for cross compilation.
This way:
1. The `enableNumpy` default respects the `enablePython deafult.
2. Cross works by default
3. Absurd manual overrides still break as they should
4. The `>= 1.65` logic is direct and not a maintaince gotcha.
