The refactor tractor

This repository contains some scripts for automated refactors of nixpkgs.

It uses devenv.sh. Enter the shell and then:

• Run no-substitute-all <path-to-nixpkgs> --write to get rid of the evil substituteAll. At least the nix-variant of it.
• Run remove-meta-with-lib <path-to-nixpkgs> --write to remove with lib; from meta.
• Run lint-replace-vars <path-to-nixpkgs> to confirm that nobody uses replaceVars in a way they really shouldn't.
• Run build-replace-vars <path-to-nixpkgs> to build all replaceVars derivations in a minimal fashion. See below.

All of those scripts are based on ast-grep.

build-replace-vars

When testing the refactor of 100s of substituteAll to replaceVars we end up with 1000s of rebuilds necessarily. We don't really care about building all those packages, though, because replaceVars has error detection built-in already: We only need to build the replaceVars derivation itself and it will tell us whether any replacements are missing or can be removed.

One approach would be to take the big list of rebuilds and then filter out all those that belong to a replaceVars call. This will still lead to a ton of rebuilds, though, because all the dependencies for each replacement need to be build first - and those are likely part of the rebuilds itself, too.

Here, we take a different approach:

• Extract the call to replaceVars with ast-grep.
• Change all replacements to pass in null.
• Build the resulting code with nix.

If it passes, we have the right replacements. If it doesn't we either miss some replacements, or can remove some.