#nixos #nix #cargo #build

bin+lib crate2nix

crate2nix generates nix (as in NixOS) build files for rust using cargo

14 releases

0.10.0 Jun 12, 2021
0.10.0-alpha.1 Feb 27, 2021
0.9.0 Feb 27, 2021
0.9.0-alpha.1 Mar 29, 2020
0.7.0-alpha.4 Nov 30, 2019

#76 in Build Utils

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crate2nix generates nix build files for rust crates using cargo.

tests-nix-linux tests-nix-macos Crate

Same dependency tree as cargo: It uses cargo_metadata to obtain the dependency tree from cargo. Therefore, it will use the exact same library versions as cargo and respect any locked down version in Cargo.lock.

Smart caching: It uses smart crate by crate caching so that nix rebuilds exactly the crates that need to be rebuilt. Compare that to docker layers...

Nix ecosystem goodness: You can use all things that make the nix/NixOS ecosystem great, e.g. distributed/remote builds, build minimal docker images, deploy your binary as a service to the cloud with NixOps, ...

Out of the box support for libraries with non-rust dependencies: It builds on top of the buildRustCrate function from NixOS so that native dependencies of many rust libraries are already correctly fetched when needed. If your library with native dependencies is not yet supported, you can customize defaultCrateOverrides / crateOverrides, see below.

Easy to understand nix template: The actual nix code is generated via templates/build.nix.tera so you can fix/improve the nix code without knowing rust if all the data is already there.

Here is a simple example which uses all the defaults and will generate a Cargo.nix file:

# From the project directory.
crate2nix generate

Here is a more elaborate example that uses <nixos-unstable> as the default pkgs path (instead of <nixpkgs>) and specifies both the path to the Cargo.toml file (-f) and the output (-o) file explicitly (usually not needed).

crate2nix generate \
    -n '<nixos-unstable>' \
    -f /some/project/dir/Cargo.toml \
    -o /some/project/dir/Cargo.nix

Use crate2nix help to show all commands and options.


If you are not running, install a recent version of nix by running curl https://nixos.org/nix/install | sh or following the instructions on https://nixos.org/nix/.


# Install the stable version to your user env (with shell completions):
nix-env -i -f https://github.com/kolloch/crate2nix/tarball/0.9.0

NOTE: You can use eigenvalue.cachix.org to get prebuilt binaries for linux.

Development Version (master)

Similarly, you can install crate2nix by

# Install the unstable version to your user env (with shell completions):
nix-env -i -f https://github.com/kolloch/crate2nix/tarball/master

If you want to tweak crate2nix, clone the repository and then

cd crate2nix
# to run crate2nix without installing it
# or to install it in your user environment
nix-env -i -f .

Generating build files

The crate2nix generate command generates a nix file. You can specify the output file with -o. E.g.

crate2nix generate

generates Cargo.nix from the Cargo.lock in the current directory.

Look at the ./crate2nix/Cargo.nix file of this project for a non-trivial example. (How meta!)

Using build files (single binaries)

If your Cargo.nix was generated for a single binary crate (i.e. workspace) then the derivation that builds your binary can be accessed via the rootCrate.build attribute. Use this command to build it and make the result available in the result directory:

nix build -f Cargo.nix rootCrate.build

Within a nix file (e.g. your manually written default.nix), you can access the derivation like this:

let cargo_nix = import ./Cargo.nix { inherit pkgs; };
in cargo_nix.rootCrate.build

Using build files (workspaces)

If your Cargo.nix was generated for a workspace (i.e. not a single binary) then the derivation that builds your binary CANNOT be accessed via the rootCrate attribute. There is no single root crate.

Instead, you can conveniently access the derivations of all your workspace members through the workspaceMembers attribute. Use this command to build one of the workspace members and make the result available in the result directory:

nix build -f Cargo.nix workspaceMembers.${your_crate_name}.build

Within a nix file (e.g. your manually written default.nix), you can access the derivation like this:

let cargo_nix = import ./Cargo.nix { inherit pkgs; };
in cargo_nix.workspaceMembers."${your_crate_name}".build

Choosing a Rust Version

If you want to compile your code with a different rust version than is currently available in your nixpkgs, you can use the awesome overlays of the nixpkgs-mozilla.

For managing the git dependencies, I recommend niv. To create an initial "sources.nix" in the nix directory run:

niv init
niv add mozilla/nixpkgs-mozilla

Later on, you can update your dependencies with niv update.

To pin your nixpkgs with the appropriate overlays, place a nixpkgs.nix file into you nix directory that was created by niv (if necessary):

  # Manage this with https://github.com/nmattia/niv
  # or define { nixpkgs = ...; nixpkgs-mozilla = ...; }
  # yourself.
  sources = import ./sources.nix;

  rustChannelsOverlay = import "${sources.nixpkgs-mozilla}/rust-overlay.nix";
  # Useful if you also want to provide that in a nix-shell since some rust tools depend
  # on that.
  rustChannelsSrcOverlay = import "${sources.nixpkgs-mozilla}/rust-src-overlay.nix";

in import sources.nixpkgs {
    overlays = [
      (self: super: {
        # Replace "latest.rustChannels.stable" with the version of the rust tools that
        # you would like. Look at the documentation of nixpkgs-mozilla for examples.
        # NOTE: "rust" instead of "rustc" is not a typo: It will include more than needed
        # but also the much needed "rust-std".
        rustc = super.latest.rustChannels.stable.rust;
        inherit (super.latest.rustChannels.stable) cargo rust rust-fmt rust-std clippy;

In your default.nix or other nix files, you can use the following to refer to that pinned pkgs with the rust version of your choice:

{ pkgs ? import ./nix/nixpkgs.nix }:

let cargoNix = import ./Cargo.nix { inherit pkgs; };
in cargoNix.rootCrate.build

Dynamic feature resolution

The enabled features for a crate now are resolved at build time! That means you can easily override them:

  1. There is a "rootFeatures" argument to the generated build file which you can override when calling it from the command line:

    nix build -f ....nix --arg rootFeatures '["default" "other"]' rootCrate.build
  2. Or when importing the build file:

    let cargo_nix = import ./Cargo.nix { inherit pkgs; rootFeatures = ["default" "other"]; };
        crate2nix = cargo_nix.rootCrate.build;
    in ...
  3. Or by overriding them on the rootCrate or workspaceMembers:

    let cargo_nix = import ./Cargo.nix { inherit pkgs; };
        crate2nix = cargo_nix.rootCrate.build.override { features = ["default" "other"]; };
    in ...

Note that only dependencies for the default features are included in the build. If you want full flexibility, you can use crate2nix generate --all-features to generate the most general build file. If you want to strip down the generated build file, you may want to use crate2nix generate --no-default-features --features "feature1 feature2".

Patching crate derivations with crateOverrides

NixOS comes with defaultCrateOverrides which specifies mostly some additional native buildInputs for various popular crates. If you are using a rust crate with native dependencies which is not yet covered, you can add additional buildInputs with the crateOverride parameter (similar to features):

  customBuildRustCrateForPkgs = pkgs: pkgs.buildRustCrate.override {
    defaultCrateOverrides = pkgs.defaultCrateOverrides // {
      funky-things = attrs: {
        buildInputs = [ pkgs.openssl ];
  generatedBuild = import ./crate2nix/Cargo.nix {
    inherit pkgs;
    buildRustCrateForPkgs = customBuildRustCrateForPkgs;
in generatedBuild.rootCrate.build

Or obviously you can use the power of nix to add a dependency conditionally:

  customBuildRustCrateForPkgs = pkgs: pkgs.buildRustCrate.override {
    defaultCrateOverrides = pkgs.defaultCrateOverrides // {
      cssparser-macros = attrs: {
        buildInputs =
            [ pkgs.darwin.apple_sdk.frameworks.Security ];
  generatedBuild = import ./crate2nix/Cargo.nix {
    inherit pkgs;
    buildRustCrateForPkgs = customBuildRustCrateForPkgs;
in generatedBuild.rootCrate.build

crateOverrides are not restricted to buildInputs however. You should also be able to add patches and the like! (I didn't try that, though.)

crateOverrides are a feature of the underlying buildRustCrate support in NixOS that crate2nix uses.

Running rust tests

There is some experimental support for running tests of your rust crates. All of the crates in the workspace will have their tests executed. When enabling test execution (runTests = true;), failing tests will make the whole build fail unless you explicitly disable this via test hooks: see the section below.

      let cargo_nix = import ./Cargo.nix { inherit pkgs; };
          crate2nix = cargo_nix.rootCrate.build.override {
	    runTests = true;
	    testInputs = [ pkgs.cowsay ];
      in ...

testInputs is optional and allows passing inputs to the test execution that should be in scope. Defaults to an empty list and is ignored when runTests equals false.

Custom pre/post test hooks

Want to customize your test execution? Use the testPreRun and testPostRun crate attributes(next to runTests in the example above). crate2nix executes the bash snippets in testPreRun and testPostRun directly before and after the actual test command, and in the same shell. Some example use-cases include:

  • Setting some environment variable that is needed for the test.

  • Setting (and then unsetting) the bash set +e option to not fail the derivation build even if a test fails. This is quite useful if your tests are not flaky and you want to cache failures.


I get a warning about buildRustCrate being deprecated in favor of buildRustCrateForPkgs

First, this warning only matters if you care about cross-compilation.

You are calling ./Cargo.nix with the option buildRustCrate:

  • either implicitely with callPackage (because nixpkgs contains an attribute called buildRustCrate) callPackage ./Cargo.nix {}: use import instead and pass the pkgs argument explicitely: import ./Cargo.nix { inherit pkgs; }
  • or explicitly (import ./Cargo.nix { inherit pkgs; buildRustCrate = ...; }). In this case you should migrate to buildRustCrateForPkgs: import ./Cargo.nix { inherit pkgs; buildRustCrateForPkgs = package_set: ...; }. buildRustCrateForPkgs should be a function taking a package set package_set and returning the same value as buildRustCrate but taking its constituent derivations in package_set. The default value is only package_set: package_set.buildRustCrate and the value you used to pass as buildRustCrate should be equal to buildRustCrateForPkgs pkgs.

Known Restrictions

crate2nix makes heavy use of buildRustCrate in nixpkgs. So we potentially depend on features in a recent version of nixpkgs. Check nix/sources.json for the version of nixpkgs that crate2nix is tested against.

If you feel limited by these restrictions, please do not hesitate to file an issue! That gives me a feeling of what is worth working on.

  • There is only experimental support for running tests Before 0.7.x: No support for building and running tests, see nixpkgs, issue 59177.
  • Target-specific features do not work automatically, see #129. You should be able to enable the required features manually, however.
  • A crate will only have access to its own source directory during build time and not e.g. to other directories in the same workspace. See crate2nix, issue 17. I used to consider this "works as intended" but now I think that we should use the "workspaceMember" argument of buildRustCrate for this.
  • It does translates target strings to nix expressions. The support should be reasonable but probably not complete - please let me know if you hit problems. Before 0.2.x: Filters all dependencies for the hard-coded "Linux x86_64" target platform. Again, it should be quite easy to support more platforms. To do so completely and at build time (vs build generation time) might be more involved.

Former restrictions, now supported:

  • Before 0.8.x: Since cargo exposes local paths in package IDs, the generated build file also contain them as part of an "opaque" ID. They are not interpreted as paths but maybe you do not want to expose local paths in there... The full opaque package ID will only be used if you have the same package with the same version multiple times. That should be very rare.
  • Before 0.6.x: Renamed crates with an explicit package name don't work yet.
  • Git sources are now also supported. Starting with 0.7 sub modules also work. Finding crates in arbitrary sub directories of git sources (which cargo supports!)is not supported, see #53.
  • Before 0.4.x: Only default crate features are supported. It should be easy to support a different feature set at build generation time since we can simply pass this set to cargo metadata. Feature selection during build time is out of scope for now.

Feedback: What is needed for a 1.0 release?

I would really appreciate your thoughts. Please add comments to issue #8.

Runtime Dependencies

crate2nix use cargo metadata / nix-prefetch-url at runtime so they need to be in the PATH. The default.nix adds the built-time nix/cargo binaries as fallback to the path.

Currently, crate2nix is only tested with nixpkgs-unstable since it depends on some new features and bug fixes.

Project Overview / Terminology

If you want to hack on this, it is useful to know that build file generation is broken up into multiple phases:

  1. cargo metadata: Calling cargo metadata via the cargo_metadata crate.
  2. indexing metadata: Indexing the metadata by package ID to enable easy joining of "Node" and "Package" information, resulting in metadata::IndexedMetadata.
  3. resolving: Using the indexed metadata to actually resolve the dependencies and join all needed build information into resolve::CrateDerivation.
  4. pre-fetching: Pre-fetching crates.io packages to determine their sha256, see prefetch module.
  5. rendering: Rendering the data via the build.nix.tera template, see render module.

Related Projects

  • carnix is already widely used in NixOS itself, yet it failed to generate correct builds for my rust projects. After some attempts to fix that, I gave up. That said, big kudos for all the work on buildRustCrate and showing the way!
  • naersk uses cargo to drive the entire build. It builds all dependencies in one derivation and the crate itself in another. Since it relies on hashes from the Cargo.lock file, I don't know how it handles git dependencies with sub modules.
  • tenx-tech/cargo2nix: I haven't used it so take it with a grain of salt but I think
    • it uses its own build logic instead of buildRustCrate but still builds each crate in its own derivation.
    • it has some support for cross building (which is quite weak in crate2nix).
  • cargo-raze generates BUILD files for bazel.


Contributions in the form of documentation and bug fixes are highly welcome. Please start a discussion with me before working on larger features.

I'd really appreciate tests for all new features. Please run ./run_tests.sh before submitting a pull request.

Feature ideas are also welcome -- just know that this is a pure hobby side project and I will not allocate a lot of bandwidth to this. Therefore, important bug fixes are always prioritised.

By submitting a pull request, you agree to license your changes via all the current licenses of the project.

Regenerating Cargo.nix files for tests

If you change crate2nix such that it will produce a different output, you may need to regenerate some of the Cargo.toml files. Not all Cargo.toml files can be generated during test time because crate2nix needs to vendor the dependencies for this to work and support for git submodules is not working yet, see #101.

regenerate_cargo_nix.sh should do what you want and is run as part of run_tests.sh (see below).

Running tests

run_tests.sh will regenerate build files AND run cargo test for you. It will call out to nix to build the sample projects -- so a considerable number of dependencies will be fetched and built. Consecutive runs are much faster.

Hacking on buildRustCrate in nixpkgs

Since crate2nix heavily depends on buildRustCrate in nixpkgs, it makes sense to hack on them together.

To be able to provide pull requests, you probably want to fork nixpkgs first. Once you have done that, clone that fork into some local directory (separate from crate2nix).

Overriding nixpkgs for everything

Now you can run the integration tests of crate2nix against that version of nixpkgs. Let's assume you are in the crate2nix project directory and you cloned nixpkgs to a sibling directory:

nix-build --arg nixpkgs ../nixpkgs -o ./crate2nix/target/nix-results ./tests.nix

Or run just an individual test (in this example "bin_with_lib_dep"):

nix-build --arg nixpkgs ../nixpkgs \
  -o ./crate2nix/target/nix-results ./tests.nix -A bin_with_lib_dep

(The "-o" argument is just to avoid a lot of top-level result directories.)

Overriding nixpkgs for buildTests

The problems is that this method will rebuild everything with your nixpkgs, including crate2nix itself. That can become severely annoying if you want to iterate on one special test case.

If you are fixing an issue in buildRustCrate that you can reproduce with a buildTest in tests.nix, then there is a much better way.

nix-build --arg buildTestNixpkgs ../nixpkgs \
  -o ./crate2nix/target/nix-results ./tests.nix -A bin_with_lib_dep

Notice --arg buildTestNixpkgs instead of --arg nixpkgs. That will not rebuild crate2nix itself with your nixpkgs but it will use buildRustCrate from your nixpkgs for all buildTests.


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