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Description
Is your feature request related to a problem? Please describe.
lib.filterAttrsallocates a list of attribute names twice.
filterAttrs is called every time attrsOf produces a value, and it needs to, because of mkIf.
Plenty of other usages exist.
Describe the solution you'd like
Add builtins.filterAttrs with the same behavior.
Side benefit: stack traces will be slightly shorter without losing info.
Describe alternatives you've considered
Additional context
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