There is a usability problem/bug with the stack-to-nix cache file.

Firstly, users don't understand what it is, and assume that this file is the real product of stack-to-nix (e.g. cardano-foundation/cardano-wallet@17485e1).

Secondly, after running stack-to-nix and then git commit -a -m "regenerate stuff", there may have been new .stack.nix/*.nix files that are not checked in to git and then lost.

Thirdly, after the .nix file is lost, but .stack-to-nix.cache is updated with an entry, the necessary file will never again be generated, and stack-to-nix will not work properly until the cache file is removed.

There need to be additional checks that cache entries are valid before using them.