Would this make changes like #190215 obsolete?

After this PR, you have to write it like:

pkgs.runCommand "validate-vector-conf" { nativeBuildInputs = [ pkgs.vector ]; } ''
  vector validate --no-environment "${file}"
  ln -s "${file}" "$out"
''

You do not need to add buildPackages, but you have to put vector into nativeBuildInputs.

Following derivation fail to cross compile right now:

• https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/web-apps/hedgedoc.nix#L19
• https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/monitoring/thanos.nix#L66
• https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/databases/dgraph.nix#L9
• https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/misc/redmine.nix#L29
• https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/databases/influxdb.nix#L99
• ... and many more.

They get fixed automatically with this PR.

You do not need to add buildPackages, but you have to put vector into nativeBuildInputs.

Would you be so kind to add a fix for this to this PR? Otherwise with my limited knowledge I think this sounds good.

Existing code will not break with this PR. Your PR is compatible with this PR. If you want, you can do a cleanup in one line
from

nativeBuildInputs = [ pkgs.buildPackages.vector ];

to

nativeBuildInputs = [ pkgs.vector ];

This PR allows you to write derivations as you do it for nixpkgs.

I didn't receive an answer from @Ericson2314 so lets try it

Thanks! We will see if we face issues.

I'd like to understand this. @ck3d can you explain why __splicedPackages is needed?

I'd like to understand this. @ck3d can you explain why __splicedPackages is needed?

So that the packages will have a __spliced attrset which contains buildHost hostTarget etc which are then selected by mkDerivation

With callPackage the packages are gotten from the spliced set

Ah I see now, normally packages are obtained via callPackage ({ hello, ... }: ...) which takes care of the splicing.

Is there a reason that import nixpkgs { ... } does not return the spliced packages by default? Does this affect performance?