#nixos #nix #cargo #build

bin+lib crate2nix

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

9 releases

✓ Uses Rust 2018 edition

new 0.7.1 Feb 15, 2020
0.7.0 Feb 15, 2020
0.7.0-beta.1 Jan 31, 2020
0.7.0-alpha.4 Nov 30, 2019

#61 in Cargo plugins

Download history 18/week @ 2019-11-28 5/week @ 2019-12-05 7/week @ 2019-12-12 1/week @ 2019-12-19 3/week @ 2019-12-26 3/week @ 2020-01-02 44/week @ 2020-01-09 13/week @ 2020-01-16 16/week @ 2020-01-23 16/week @ 2020-01-30 25/week @ 2020-02-06 96/week @ 2020-02-13

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crate2nix generates nix build files for rust crates using cargo.

tests-nix workflow status 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 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.

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.


NOTE: It is only tested on Linux for now!

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/0.7.1

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 .

Nixpkgs Version

This uses a pinned version nixos-unstable because at the time of writing this, it contains a necessary fix.

If that doesn't work for you, you can override the pkgs argument, e.g.:

nix-env --arg pkgs 'import <nixos> {config = {}; }' -i -f https://github.com/kolloch/crate2nix/tarball/master

Generating build files

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

crate2nix generate -o Cargo.nix

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 = callPackage ./Cargo.nix {};
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 = callPackage ./Cargo.nix {};
in cargo_nix.workspaceMembers."${your_crate_name}".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 with "callPackage":

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

    let cargo_nix = callPackage ./Cargo.nix {};
        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):

let generatedBuild = callPackage ./crate2nix/Cargo.nix {
    defaultCrateOverrides = pkgs.defaultCrateOverrides // {
      funky-things = attrs: { 
        buildInputs = [openssl]; \
in generatedBuild.rootCrate.build

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

let generatedBuild = callPackage ./crate2nix/Cargo.nix {
    defaultCrateOverrides = pkgs.defaultCrateOverrides // {
      cssparser-macros = attrs: { 
        buildInputs = 
            [ darwin.apple_sdk.frameworks.Security ]; 
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.

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.

  • 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.
  • There is only experimental support for running tests Before 0.7.x: No support for building and running tests, see nixpkgs, issue 59177.
  • Before 0.6.x: Renamed crates with an explicit package name don't work yet.
  • 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...
  • 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.
  • 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.

I consider this "Works as intended" but don't hesitate to tell me if you run into restrictions in popular crates because of this:

  • 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.

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 does not work in sandboxes in some cases where cargo needs to write to lock files (I should file bugs for this).

regenerate_cargo_nix.sh should do what you want. Additional diffs in packageIds are, unfortunately, expected.

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.


~214K SLoC