Describe the bug
I am currently on the latest stable relese 20.09. Used to be able to pull the correct cargoSha256 from a failed build. After recently trying to override rnix-lsp to the latest git commit, when the build fails, there is no correct cargoSha256 value to paste into the overlay. All it says is:

ERROR: cargoSha256 is out of date

Cargo.lock is not the same in rnix-lsp-0.1.0-vendor.tar.gz

To fix the issue:
1. Use "0000000000000000000000000000000000000000000000000000" as the cargoSha256 value
2. Build the derivation and wait it to fail with a hash mismatch
3. Copy the 'got: sha256:' value back into the cargoSha256 field

However, after reviewing the entire failed build log, there is no longer a line containing the proper hash as their used to be. Even if I follow the instructions above and replace the full hash with 0's, still there is no proper hash in the build log.

To Reproduce
Steps to reproduce the behavior:
Use this overlay in a NixOS 20.09 configuration:

final: prev: {
  rnix-lsp = prev.rnix-lsp.overrideAttrs (o: {

    src = prev.fetchFromGitHub {
      owner = "nix-community";
      repo = "rnix-lsp";
      rev = "23a07432e98a9a34812c4e6bde5ff40a73f290c1";
      hash = "sha256-G8Tn703lTxUzgmQVNm/v1e40QQrGhymBkuzydM/xrho=";
    };

    cargoSha256 = "0000000000000000000000000000000000000000000000000000";

  });
}

Attempt to build rnix-lsp, and review the build log.
Notice how there is no cargoSha256 to be found anywhere in the log.

I have also attached the build log so you can skip the above steps and see for yourself that the hash is nowhere in the log.
log.gz

Expected behavior
On 20.03, it was quite simple to simply pull the proper hash from the failed build log and copy and paste it into the overlay for rust packages.

Notify maintainers
@andir