overrideAttrs and buildRustPackage

I’m trying to create an overlay inside a flake.nix for a package like so:

  description = "Polkadot overlay";

  inputs = {
    nixpkgs.url = github:NixOS/nixpkgs/nixpkgs-unstable;
    polkadot = {
      url = github:paritytech/polkadot/release;
      flake = false;

  outputs = { self, nixpkgs, polkadot }: {

    overlay = final: prev:
        pkgs = nixpkgs.legacyPackages.${final.system};
        # this returns v.0.8.29
        vers = pkgs.lib.strings.fileContents ./version;
          polkadot-unstable = pkgs.polkadot.overrideAttrs (_: {
            pname = "polkadot-unstable";
            # remove v
            version = with builtins; substring 1 (stringLength vers) vers;
            src = polkadot;
            cargoSha256 = pkgs.lib.fakeSha256;

    defaultPackage.x86_64-linux =
        pkgs = import nixpkgs { system = "x86_64-linux"; overlays = [ self.overlay ]; };


The build fails with:

error: --- Error ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- nix
builder for '/nix/store/74p8x7h9dah5bld1w2fymdjb300khb4x-polkadot-0.8.29.drv' failed with exit code 1; last 10 log lines:

  ERROR: cargoSha256 is out of date

  Cargo.lock is not the same in polkadot-0.8.28-1-vendor.tar.gz

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

I don’t understand why the vendor tarball derivation is for the wrong package version (0.8.28-1 instead of 0.8.29).

There’s also the issue of getting the right sha256 hash, but I can deal with that once this first issue is fixed.

There seems to be something I don’t understand about how overrideAttrs works, (at least) in conjunction with buildRustPackage's machinery.

Any ideas?

Buildrustpackage is a bit complicated in that regard…

The override of the vendorsha isn’t passed to the vendor derivation.

You need to override the vendored dependency to make it work.

I don’t have any public sources available though as far as I remember, the post linked below helped me solving the issue:

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