How to `nix-build` again a built store path?

Question

I create my own repository to fetch some git source.

# packages.nix
with (import <nixpkgs> {});

rec {
  rustcSource = fetchgit {
    url = https://github.com/rust-lang/rust;
    rev = "3191fbae9da539442351f883bdabcad0d72efcb6";
    sha256 = "0w1l14kz9kxyj5dw3w9xxk1fzww5xqs3sf8saay0mh7nkmvrdb59";
  };
}

Then I build rustcSource,

sudo nix-env -f package.nix -A rustcSource

It reveals a store path of /nix/store/096fpy9qjbz5r14aadjnq9d2md9ql9cg-rust-3191fba. The problem is, I forgot to download it's submodules, so I change my expression to include submodules,

with (import <nixpkgs> {});

rec {
  rustcSource = fetchgit {
    url = https://github.com/rust-lang/rust;
    rev = "3191fbae9da539442351f883bdabcad0d72efcb6";
    sha256 = "0w1l14kz9kxyj5dw3w9xxk1fzww5xqs3sf8saay0mh7nkmvrdb59";
    leaveDotGit = true;
    fetchSubmodules = true;
  };
}

But, I discovered that nix-build doesn't recalculate the hash and see that the path has been built. So, it ends up not downloading the submodules.

Q: Is it nix bug? How can I "rebuild" or redownloading the repository?

P.S. Actually I create a derivation with fetchgit in it, but it fails because the submodules doesn't being downloaded. So, the above case simplify my problem.

Answer 1

Not a bug, this is by design.

Derivations that specify a hash are called fixed-output derivations and they only check if hash matches any existing derivations in store and use that. So your fetchSubmodules change is ignored.

For more discussion see https://github.com/NixOS/nix/issues/969