Crane

A Nix library for building cargo projects.

• Source fetching: automatically done using a Cargo.lock file
• Incremental: build your workspace dependencies just once, then quickly lint, build, and test changes to your project without slowing down
• Composable: split builds and tests into granular steps. Gate CI without burdening downstream consumers building from source.

Features

Examples can be found here. Detailed API docs are available, but at a glance, the following are supported:

• Automatic vendoring of dependencies in a way that works with Nix
  • Alternative cargo registries are supported (with a minor configuration change)
  • Git dependencies are automatically supported without additional configuration.
    • Cargo retains the flexibility to only use these dependencies when they are actually needed, without forcing an override for the entire workspace.
• Reusing dependency artifacts after only building them once
• clippy checks
• rustfmt checks
• cargo-tarpaulin for code coverage

Getting Started

The easiest way to get started is to initialize a flake from a template:

# Start with a comprehensive suite of tests
nix flake init -t github:ipetkov/crane#quick-start

# Or if you want something simpler
nix flake init -t github:ipetkov/crane#quick-start-simple

# If you need a custom rust toolchain (e.g. to build WASM targets):
nix flake init -t github:ipetkov/crane#custom-toolchain

# If you need to use another crate registry besides crates.io
nix flake init -t github:ipetkov/crane#alt-registry

For an even more lean, no frills set up, create a flake.nix file with the following contents at the root of your cargo workspace:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    crane.url = "github:ipetkov/crane";
    crane.inputs.nixpkgs.follows = "nixpkgs";
    flake-utils.url = "github:numtide/flake-utils";
    flake-utils.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = { self, nixpkgs, crane, flake-utils, ... }:
    flake-utils.lib.eachDefaultSystem (system: {
      defaultPackage = crane.lib.${system}.buildPackage {
        src = ./.;

        # Add extra inputs here or any other derivation settings
        # doCheck = true;
        # buildInputs = [];
        # nativeBuildInputs = [];
      };
    });
}

Philosophy

Crane is designed around the idea of composing cargo invocations such that they can take advantage of the artifacts generated in previous invocations. This allows for both flexible configurations and great caching (à la Cachix) in CI and local development builds.

Here's how it works at a high level: when a cargo workspace is built its source is first transformed such that only the dependencies listed by the Cargo.toml and Cargo.lock files are built, and none of the crate's real source is included. This allows cargo to build all dependency crates and prevents Nix from invalidating the derivation whenever the source files are updated. Then, a second derivation is built, this time using the real source files, which also imports the cargo artifacts generated in the first step.

This pattern can be used with any arbitrary sequence of commands, regardless of whether those commands are running additional lints, performing code coverage analysis, or even generating types from a model schema. Let's take a look at two examples at how very similar configurations can give us very different behavior!

Example One

Suppose we are developing a crate and want to run our CI assurance checks via nix flake check. Perhaps we want the CI gate to be very strict and block any changes which raise warnings when run with cargo clippy. Oh, and we want to enforce some code coverage too!

Except we do not want to push our strict guidelines on any downstream consumers who may want to build our crate. Suppose they need to build the crate with a different compiler version (for one reason or another) which comes with a new lint whose warnings we have not yet addressed. We don't want to make their life harder, so we want to make sure we do not run cargo clippy as part of the crate's actual derivation, but at the same time, we don't want to have to rebuild dependencies from scratch.

Here's how we can set up our flake to achieve our goals:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    crane.url = "github:ipetkov/crane";
    crane.inputs.nixpkgs.follows = "nixpkgs";
    flake-utils.url = "github:numtide/flake-utils";
    flake-utils.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = { self, nixpkgs, crane, flake-utils, ... }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs {
          inherit system;
        };

        craneLib = crane.lib.${system};
        src = ./.;

        # Build *just* the cargo dependencies, so we can reuse
        # all of that work (e.g. via cachix) when running in CI
        cargoArtifacts = craneLib.buildDepsOnly {
          inherit src;
        };

        # Run clippy (and deny all warnings) on the crate source,
        # resuing the dependency artifacts (e.g. from build scripts or
        # proc-macros) from above.
        #
        # Note that this is done as a separate derivation so it
        # does not impact building just the crate by itself.
        my-crate-clippy = craneLib.cargoClippy {
          inherit cargoArtifacts src;
          cargoClippyExtraArgs = "-- --deny warnings";
        };

        # Build the actual crate itself, reusing the dependency
        # artifacts from above.
        my-crate = craneLib.buildPackage {
          inherit cargoArtifacts src;
        };

        # Also run the crate tests under cargo-tarpaulin so that we can keep
        # track of code coverage
        my-crate-coverage = craneLib.cargoTarpaulin {
          inherit cargoArtifacts src;
        };
      in
      {
        defaultPackage = my-crate;
        checks = {
         inherit
           # Build the crate as part of `nix flake check` for convenience
           my-crate
           my-crate-clippy
           my-crate-coverage;
        };
      });
}

When we run nix flake check the following will happen:

1. The sources for any dependency crates will be fetched
2. They will be built without our crate's code and the artifacts propagated
3. Our crate, the clippy checks, and code coverage collection will be built, each reusing the same set of artifacts from the initial source-free build. If enough cores are available to Nix it may build all three derivations completely in parallel, or schedule them in some arbitrary order.

Splitting up our builds like this also gives us the benefit of granular control over what is rebuilt. Suppose we change our mind and decide to adjust the clippy flags (e.g. to allow certain lints or forbid others). Doing so will only rebuild the clippy derivation, without having to rebuild and rerun any of our other tests!

Example Two

Let's take an alternative approach to the example above. Suppose instead that we care more about not wasting any resources building certain tests (even if they would succeed!) if another particular test fails. Perhaps binary substitutes are readily available so that we do not mind if anyone building from source is bound by our rules, and we can be sure that all tests have passed as part of the build.

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    crane.url = "github:ipetkov/crane";
    crane.inputs.nixpkgs.follows = "nixpkgs";
    flake-utils.url = "github:numtide/flake-utils";
    flake-utils.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = { self, nixpkgs, crane, flake-utils, ... }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs {
          inherit system;
        };

        craneLib = crane.lib.${system};
        src = ./.;

        # Build *just* the cargo dependencies, so we can reuse
        # all of that work (e.g. via cachix) when running in CI
        cargoArtifacts = craneLib.buildDepsOnly {
          inherit src;
        };

        # First, run clippy (and deny all warnings) on the crate source.
        my-crate-clippy = craneLib.cargoClippy {
          inherit cargoArtifacts src;
          cargoClippyExtraArgs = "-- --deny warnings";
        };

        # Next, we want to run the tests and collect code-coverage, _but only if
        # the clippy checks pass_ so we do not waste any extra cycles.
        my-crate-coverage = craneLib.cargoTarpaulin {
          inherit src;
          cargoArtifacts = my-crate-clippy;
        };

        # Build the actual crate itself, _but only if the previous tests pass_.
        my-crate = craneLib.buildPackage {
          cargoArtifacts = my-crate-coverage;
          inherit src;
        };
      in
      {
        defaultPackage = my-crate;
        checks = {
         inherit
           # Build the crate as part of `nix flake check` for convenience
           my-crate
           my-crate-coverage;
        };
      });
}

When we run nix flake check the following will happen:

1. The sources for any dependency crates will be fetched
2. They will be built without our crate's code and the artifacts propagated
3. Next the clippy checks will run, reusing the dependency artifacts above.
4. Next the code coverage tests will run, reusing the artifacts from the clippy run
5. Finally the actual crate itself is built

In this case we lose the ability to build derivations independently, but we gain the ability to enforce a strict build order. However, we can easily change our mind, which would be much more difficult if we had written everything as one giant derivation.

Compatibility Policy

Breaking changes can land on the master branch at any time, so it is recommended you use a versioning strategy when consuming this library (for example, using something like flakes or niv).

Tagged releases will be cut periodically and changes will be documented in the CHANGELOG. Release versions will follow Semantic Versioning.

The test suite is run against the latest stable nixpkgs release, as well as nixpkgs-unstable. Any breakage on those channels is considered a bug and should be reported as such.

FAQs

I want to use a custom version of nixpkgs or another specific system

The crane library can be instantiated with a specific version of nixpkgs as follows. For more information, see the API docs for mkLib.

crane.mkLib (import nixpkgs { system = "armv7l-linux"; })

I want to override a particular package used by the crane library

Specific inputs can be overridden for the entire library via the overrideScope' API as follows. For more information, see the API docs for mkLib or checkout the custom-toolchain example.

crane.lib.${system}.overrideScope' (final: prev: {
  cargo = myCustomCargoVersion;
})

My custom rust flags are getting ignored

If you are using a build.rustflags definition in .cargo/config.toml, consider turning off source prefix remapping by adding doRemapSourcePathPrefix = false; in your derivation.

See the API docs for remapSourcePathPrefixHook for more information.

Nix is complaining about IFD (import from derivation)

If a derivation's pname and version attributes are not explicitly set, crane will inspect the project's Cargo.toml file to set them as a convenience to avoid duplicating that information by hand. This works well when the source is a local path, but can cause issues if the source is being fetched remotely, or flakes are not being used (since flakes have IFD enabled on by default).

One easy workaround for this issue (besides enabling the allow-import-from-derivation option in Nix) is to explicitly set { pname = "..."; version = "..."; } in the derivation.

You'll know you've run into this issue if you see error messages along the lines of:

• cannot build '/nix/store/...-source.drv' during evaluation because the option 'allow-import-from-derivation' is disabled
• a 'aarch64-darwin' with features {} is required to build '/nix/store/...', but I am a 'x86_64-linux' with features {}

License

This project is licensed under the MIT license.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion by you, shall be licensed as MIT, without any additional terms or conditions.