It's practically impossible to override the src and hash arguments of rustPlatform.fetchCargoVendor. You can copy code from Nixpkgs and try to reuse vendorStaging, but there's a clear issue here.

The switch from fetchCargoTarball to fetchCargoVendor broke the previous way of using overrideAttrsand setting src and outputHash.

I'm unsure how to amend this myself as the FOD in fetchCargoVendor is wrapped. Maybe something like lib.extendMkDerivation could be used here?

The issue with buildRustPackage not yet fully utilizing finalAttrs is adjacent but not directly in the scope of this issue, but I'll document it here regardless:

let
  pkgs = import <nixpkgs> {};
  myPkg = pkgs.rustPlatform.buildRustPackage {
    name = "example";
    src = pkgs.hello;
    cargoHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
  };
in
(myPkg.overrideAttrs { cargoHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABB="; }).cargoDeps
== myPkg.cargoDeps

# These should be inequal but they aren't

Related:

@9999years @amesgen @ShamrockLee

#194475 (comment) #107070 #382550