Is it possible to override cargoSha256 in buildRustPackage?

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This re-invokes the original package (whose path you need to know of course) but overrides the call to rustPlatform.buildRustPackage such that it modifies the args before passing them along.

Packages created with callPackage automatically get a .override function (technically speaking, a functor, an attribute set which behaves like a function thanks to its __functor attribute), since lib.callPackageWith uses lib.makeOverridable to make packages overridable.

nix-repl> pkgs.ffsend.override
{ __functionArgs = { ... }; __functor = «lambda @ /nix/store/gzf4zwcakda1nykn6h0avh45xhjhvsz4-source/lib/trivial.nix:440:19»; }

Since lib.callPackageWith takes either a function or a path,

  callPackageWith = autoArgs: fn: args:
    let
      f = if isFunction fn then fn else import fn;

one can use super.callPackage super.ffsend.override instead of super.callPackage <nixpkgs/pkgs/tools/misc/ffsend>. This avoids the lookup path impurity and allows overlaying packages that don’t have their own dedicated file (e.g. pkgs.xorg.xorgserver) or come from a flake.

I (ab)use this trick significantly when patching out /bin/sh. For example, tectonic can be overlayed like

final: prev:

{
  tectonic-unwrapped = final.callPackage prev.tectonic-unwrapped.override {
    rustPlatform = final.rustPlatform // {
      buildRustPackage = args: final.rustPlatform.buildRustPackage (args // {
        src = final.fetchFromGitHub {
          owner = "tectonic-typesetting";
          repo = "tectonic";
          rev = "78fd97716ee111861bd981a45e6816589d16f504";
          sha256 = "sha256-pKx2fzBllkv3fzUfhv9qKlPqD4JKdEN74+gsLbmwZ/o=";
        };
        cargoHash = "sha256-BcVepSoFogh/OU0DoQNnT8X9bNc74rMDT+3zelmfwkY=";
        # https://github.com/NixOS/nixpkgs/pull/291770
        buildFeatures = [ "external-harfbuzz" ];
      });
    };
  };
}