[ParetoOptimal]

> It copies every time you build something the whole darn project into the Nix store. Fine when you have a small project, but with large projects this becomes a significant overhead.

Isn't copying the repo to a read only location just inherent to the problem of reproducibility?

[grumbel]

Not necessarily, most build systems allow you to separate source input and build output. So grabbing the source from the current directly is a possibility, if the build script is clean and doesn't do anything weird in the source directory.

In Nix you can get that via 'nix develop', which gives you a shell with access to the individual build phases and with a bit of tweaking, allows you to run them from your source directory without copying the source around first (see 'declare -f genericBuild').