[pshirshov]

> eventually the nixpkgs of the project is garbage collected

You should manually create a garbage collection root (that pesky 'result' symlink, --out-link)

[sshine]

Are you saying that the presence of a result/ symlink means the sources are kept, and if I remove the result/ symlink, it gets gc’ed?

[pshirshov]

Yes, gc root links are accounted at the moment they are written by nix and they retain the graph while they are continue to exist.