Features fail to contain optional dependencies that got enabled indirectly

[nagisa]

Consider the following Cargo.toml:

[package]
name = "test-num-features"
version = "0.1.0"
edition = "2018"

[dependencies]
num = { version = "=0.3", default-features = false, features = ["std"] }

It currently fails to build despite succeeding with cargo. The reason is because when crate2nix is building num-rational it somehow drops the feature for an optional num-bigint dependency. This optional dependency ends up being enabled indirectly, through num depending on both num-rational and num-bigint.

Both cargo metadata and the generated Cargo.nix contains num-bigint in the resolvedDefaultFeatures:

{
      "num-rational" = rec {
        crateName = "num-rational";
        version = "0.3.0";
        edition = "2018";
        sha256 = "0f41j1l1kn5jj36a8xdy8kv242wlwq0ka578vm8gnb1n1wvdgd55";
        authors = [<snip>];
        dependencies = [<snip>];
        buildDependencies = [<snip>];
        features = {
          "default" = [ "num-bigint-std" "std" ];
          "num-bigint-std" = [ "num-bigint/std" ];
          "std" = [ "num-integer/std" "num-traits/std" ];
        };
        resolvedDefaultFeatures = [ "num-bigint" "num-bigint-std" "std" ];
      };
}

But the final invocation of buildRustCrate ends up not being provided the feature in the end. And so num-rational is compiled without a --cfg feature="num-bigint" flag.

I admit that this is the first time I’m reading templates/nix/crate2nix/default.nix at all seriously, but from what I can tell it just throws away the information cargo already computed and attempts to compute its own feature sets. Is that correct? Why?

[kolloch]

Hmmm, it is a bit weird because there is even a unit test for something very similar:

https://github.com/kolloch/crate2nix/blob/master/crate2nix/templates/nix/crate2nix/tests/packageFeatures.nix#L98-L104

 "pkg_num" = {
      crateName = "num";
      dependencies = [
        {
          name = "num-bigint";
          packageId = "pkg_num_bigint";
          usesDefaultFeatures = false;
          optional = true;
        }
      ];
      features = {
        "default" = [ "std" ];
        "std" = [ "num-bigint/std" ];
      };
    };

[pandaman64]

When Cargo see num-bigint/std, it implicitly adds num-bigint feature to the rustc invocation.
Although the expected feature set in the test matches the explicitly specified ones, it misses implicit features in this case.

For more detail, please see #165.

[Fuuzetsu]

Is there any work-around for this or planned work? I am hitting this exact issue on this exact crate.

[Fuuzetsu]

For us, I did something like this:

  cargoNix = pkgs.callPackage ./nix/Cargo.nix {
    inherit nixpkgs;
    buildRustCrate = null;
    defaultCrateOverrides = pkgs.defaultCrateOverrides // {
      num-rational = attrs:
        attrs // {
          # https://github.com/kolloch/crate2nix/issues/146
          features = attrs.features ++ [ "num-bigint" ];
        };
    };