libexpr: Add filterAttrs builtin

[not-ronjinger]

Motivation

The implementation in nixpkgs is slightly inefficient:

filterAttrs = pred: set: removeAttrs set (filter (name: !pred name set.${name}) (attrNames set));

Namely, it has to create a temporary list of names attrNames set, then has to scan through the entries with filter, apply the predicate which needs to dereference the value, then pass it to removeAttrs, which then needs to do another scan of names which failed the predicate. It's likely as good as you can get without creating a specific builtin.

With a native builtin, we should be able to eliminate the need for generating the temporary list, the initial scan should be a bit faster as there is less indirection with looking up the value, and the need to do another scan for removeAttrs should be eliminated altogether.

Re-introducing this to nixpkgs/lib should be easy with a builtins ? filterAttrs check.

Context

Gotta go fast 🚀

[wolfgangwalther]

This user was banned.

[Lillecarl]

Is it possible to reopen this after NixOS/org#229 got merged?

[philiptaron]

I do want builtins.filterAttrs to land (the results in the DetSys PR are quite nice) but Jon's ban is not based on which specific person is enforcing it, Carl, as well you likely know.