A Flake URL can not only be Git repositories. They can also refer to local paths. If you have two projects ~/code/foo and ~/code/bar, and bar depends on foo, you can use the following flake.nix in bar to have it refer to the local foo project:
{
inputs = {
foo.url = "path:/Users/me/code/foo";
};
outputs = inputs: { ... };
}flake.lock
Whenever you modify files under ~/code/foo, you must run update the flake.lock hash in ~/code/bar by running:
nix flake update foo
The alternative is to pass --override-input foo ~/code/foo to nix build or nix develop commands; this will override the hash for “foo” in the flake.lock file.