cargo2nix

Nixify your Rust projects today with cargo2nix, bringing you reproducible builds and better caching.

This repository hosts two components:

• A Nixpkgs overlay, located at the /overlay directory, providing utilities to build and test your Cargo workspace.

• A utility written in Rust to generate version pins of crate dependencies.

Together, these components will take an existing Cargo.lock and delegate the process of fetching and compiling your dependencies (generated by Cargo) using the deterministic Nix package manager.

Install

This project assumes that the Nix package manager is already installed on your machine. Run the command below to install cargo2nix:

nix-env -iA package -f https://github.com/cargo2nix/cargo2nix/tarball/master

How to use this for your Rust projects

As a build system

The basic process of converting an existing Cargo project to cargo2nix boils down to the following steps:

1. Generate a Cargo.nix file by running cargo2nix -f at the root of your Cargo workspace.
2. Create a default.nix file which imports Nixpkgs with the cargo2nix and nixpkgs-mozilla overlays and builds your project using the Cargo.nix file from earlier.
3. Run nix-build to compile and/or test your project.

Check out our series of example projects which showcase how to use cargo2nix in detail.

Declarative debug & development shell

You can load a nix-shell for any crate derivation in the dependency tree. The advantage of this shell is that in this environment users can develop their crates and be sure that their crates builds in the same way that cargo2nix overlay will build them.

To do this, first run nix-shell -A 'rustPkgs.<registry>.<crate>."x.y.z"' default.nix. For instance, the following command being invoked in this repository root drops you into such a development shell.

# When a crate is not associated with any registry, such as when building locally,
# the registry is "unknown" as shown below:
nix-shell -A 'rustPkgs.unknown.cargo2nix."0.9.0"' default.nix

# This crate is a dependency that we may be debugging. Use the --pure switch if
# impurities from your current environment may be polluting the nix build:
nix-shell --pure -A 'rustPkgs."registry+https://github.com/rust-lang/crates.io-index".openssl."0.10.30"' default.nix

# If you are working on a dependency and need the source (or a fresh copy) you
# can unpack the $src variable. Through nix stdenv, tar is available in pure 
# shells
mkdir debug
cp $src debug
cd debug
tar -xzfv $(basename $src)
cd <unpacked source>

You will need to override your Cargo.toml and Cargo.lock in this shell, so make sure that you have them backed up if your are directly using your clone of your project instead of unpacking fresh sources like above.

Now you just need to run the $configurePhase and $buildPhase steps in order. You can find additional phases that may exist in overrides by running env | grep Phase

echo $configurePhase 
# runHook preConfigure runHook configureCargo runHook postConfigure

runHook preConfigure
runHook configureCargo
runHook postConfigure

echo $buildPhase
# runHook overrideCargoManifest runHook setBuildEnv runHook runCargo

runHook overrideCargoManifest  # This overrides your .cargo folder, e.g. for setting cross-compilers
runHook setBuildEnv  # This sets up linker flags for the `rustc` invocations
runHook runCargo

If runCargo succeeds, you will have a completed output ready for the (usually) less interesting $installPhase. If there's a problem, inspecting the env or reading the generated Cargo.lock etc should yield clues. If you've unpacked a fresh source and are using the --pure switch, everything is identical to how the overlay builds the crate, cutting out guess work.

Common issues

1. When building sys crates, native dependencies that build.rs scripts may themselves attempt to provide could be missing. See the overlay/overrides.nix for patterns of common solutions for fixing up specific deps.

   To provide your own override, pass a modified packageOverrides to pkgs.rustBuilder.makePackageSet':

     rustPkgs = pkgs.rustBuilder.makePackageSet' {
       # ... required arguments not shown
     
       # Use the existing all list of overrides and append your override
       packageOverrides = pkgs: pkgs.rustBuilder.overrides.all ++ [
       
         # parentheses disambiguate each makeOverride call as a single list element
         (pkgs.rustBuilder.rustLib.makeOverride {
             name = "fantasy-zlib-sys";
             overrideAttrs = drv: {
               propagatedNativeBuildInputs = drv.propagatedNativeBuildInputs or [ ] ++ [
                 pkgs.zlib.dev
               ];
             };
         })
         
       ];
     };

2. When re-vendoring nixpkgs-mozilla or cargo2nix, pay attention to the revs of nixpkgs, the nixpkgs-mozilla overlay, and the cargo2nix overlay. Certain non-release versions of nixpkgs-mozilla have shipped with a rustc that doesn't include zlib in its runtime dependencies.

3. Many crates.io public crates may not build using the current Rust compiler, unless a lint cap is put on these crates. For instance, cargo2nix caps all lints to warn by default.

4. Toml parsing / conversion issues Error: Cannot convert data to TOML (Invalid type <class 'NoneType'>)

   jq and remarshal are used to read & modify toml files in some cases. Lines of the form: [key."cfg(foo = \"a\", bar = \"b\"))".path] could produce breakage when jq output was fed back to remarshal. There are workarounds in place to catch many cases. See #149 for more information and report any newly found breakage until a total solution is in place.

5. Git dependencies and crates from alternative Cargo registries rely on builtins.fetchGit to support fetching from private Git repositories. This means that such dependencies cannot be evaluated with restrict-eval applied.

   Also, if your Git dependency is tied to a Git branch, e.g. master, and you would like to force it to update on upstream changes, you should append --option tarball-ttl 0 to your nix-build command.

Design

This Nixpkgs overlay builds your Rust crates and binaries by first pulling the dependencies apart, building them individually as separate Nix derivations and linking them together. This is achieved by passing custom linker flags to the cargo invocations and the underlying rustc and rustdoc invocations.

In addition, this overlay takes cross-compilation into account and build the crates onto the correct host platform configurations with the correct platform-dependent feature flags specified in the Cargo manifests and build-time dependencies.

Credits

The design for the Nix overlay is inspired by the excellent work done by James Kay, which is described here and here. His source is available here. This work would have been impossible without these fantastic write-ups. Special thanks to James Kay!

License

cargo2nix is free and open source software distributed under the terms of the MIT License.