---
title: Rust
author: Matthias Beyer
date: 2017-03-05
---

# User's Guide to the Rust Infrastructure

To install the rust compiler and cargo put

```
rustc
cargo
```

into the `environment.systemPackages` or bring them into
scope with `nix-shell -p rustc cargo`.

> If you are using NixOS and you want to use rust without a nix expression you
> probably want to add the following in your `configuration.nix` to build
> crates with C dependencies.
>
>     environment.systemPackages = [binutils gcc gnumake openssl pkgconfig]

For daily builds (beta and nightly) use either rustup from
nixpkgs or use the [Rust nightlies
overlay](#using-the-rust-nightlies-overlay).

## Compiling Rust applications with Cargo

Rust applications are packaged by using the `buildRustPackage` helper from `rustPlatform`:

```
rustPlatform.buildRustPackage rec {
  name = "ripgrep-${version}";
  version = "0.4.0";

  src = fetchFromGitHub {
    owner = "BurntSushi";
    repo = "ripgrep";
    rev = "${version}";
    sha256 = "0y5d1n6hkw85jb3rblcxqas2fp82h3nghssa4xqrhqnz25l799pj";
  };

  cargoSha256 = "0q68qyl2h6i0qsz82z840myxlnjay8p1w5z7hfyr8fqp7wgwa9cx";

  meta = with stdenv.lib; {
    description = "A fast line-oriented regex search tool, similar to ag and ack";
    homepage = https://github.com/BurntSushi/ripgrep;
    license = licenses.unlicense;
    maintainers = [ maintainers.tailhook ];
    platforms = platforms.all;
  };
}
```

`buildRustPackage` requires a `cargoSha256` attribute which is computed over
all crate sources of this package. Currently it is obtained by inserting a
fake checksum into the expression and building the package once. The correct
checksum can be then take from the failed build.

When the `Cargo.lock`, provided by upstream, is not in sync with the
`Cargo.toml`, it is possible to use `cargoPatches` to update it. All patches
added in `cargoPatches` will also be prepended to the patches in `patches` at
build-time.

## Compiling Rust crates using Nix instead of Cargo

### Simple operation

When run, `cargo build` produces a file called `Cargo.lock`,
containing pinned versions of all dependencies. Nixpkgs contains a
tool called `carnix` (`nix-env -iA nixos.carnix`), which can be used
to turn a `Cargo.lock` into a Nix expression.

That Nix expression calls `rustc` directly (hence bypassing Cargo),
and can be used to compile a crate and all its dependencies. Here is
an example for a minimal `hello` crate:


    $ cargo new hello
    $ cd hello
    $ cargo build
     Compiling hello v0.1.0 (file:///tmp/hello)
      Finished dev [unoptimized + debuginfo] target(s) in 0.20 secs
    $ carnix -o hello.nix --src ./. Cargo.lock --standalone
    $ nix-build hello.nix -A hello_0_1_0

Now, the file produced by the call to `carnix`, called `hello.nix`, looks like:

```
# Generated by carnix 0.6.5: carnix -o hello.nix --src ./. Cargo.lock --standalone
{ lib, stdenv, buildRustCrate, fetchgit }:
let kernel = stdenv.buildPlatform.parsed.kernel.name;
    # ... (content skipped)
in
rec {
  hello = f: hello_0_1_0 { features = hello_0_1_0_features { hello_0_1_0 = f; }; };
  hello_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
    crateName = "hello";
    version = "0.1.0";
    authors = [ "pe@pijul.org <pe@pijul.org>" ];
    src = ./.;
    inherit dependencies buildDependencies features;
  };
  hello_0_1_0 = { features?(hello_0_1_0_features {}) }: hello_0_1_0_ {};
  hello_0_1_0_features = f: updateFeatures f (rec {
        hello_0_1_0.default = (f.hello_0_1_0.default or true);
    }) [ ];
}
```

In particular, note that the argument given as `--src` is copied
verbatim to the source. If we look at a more complicated
dependencies, for instance by adding a single line `libc="*"` to our
`Cargo.toml`, we first need to run `cargo build` to update the
`Cargo.lock`. Then, `carnix` needs to be run again, and produces the
following nix file:

```
# Generated by carnix 0.6.5: carnix -o hello.nix --src ./. Cargo.lock --standalone
{ lib, stdenv, buildRustCrate, fetchgit }:
let kernel = stdenv.buildPlatform.parsed.kernel.name;
    # ... (content skipped)
in
rec {
  hello = f: hello_0_1_0 { features = hello_0_1_0_features { hello_0_1_0 = f; }; };
  hello_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
    crateName = "hello";
    version = "0.1.0";
    authors = [ "pe@pijul.org <pe@pijul.org>" ];
    src = ./.;
    inherit dependencies buildDependencies features;
  };
  libc_0_2_36_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
    crateName = "libc";
    version = "0.2.36";
    authors = [ "The Rust Project Developers" ];
    sha256 = "01633h4yfqm0s302fm0dlba469bx8y6cs4nqc8bqrmjqxfxn515l";
    inherit dependencies buildDependencies features;
  };
  hello_0_1_0 = { features?(hello_0_1_0_features {}) }: hello_0_1_0_ {
    dependencies = mapFeatures features ([ libc_0_2_36 ]);
  };
  hello_0_1_0_features = f: updateFeatures f (rec {
    hello_0_1_0.default = (f.hello_0_1_0.default or true);
    libc_0_2_36.default = true;
  }) [ libc_0_2_36_features ];
  libc_0_2_36 = { features?(libc_0_2_36_features {}) }: libc_0_2_36_ {
    features = mkFeatures (features.libc_0_2_36 or {});
  };
  libc_0_2_36_features = f: updateFeatures f (rec {
    libc_0_2_36.default = (f.libc_0_2_36.default or true);
    libc_0_2_36.use_std =
      (f.libc_0_2_36.use_std or false) ||
      (f.libc_0_2_36.default or false) ||
      (libc_0_2_36.default or false);
  }) [];
}
```

Here, the `libc` crate has no `src` attribute, so `buildRustCrate`
will fetch it from [crates.io](https://crates.io). A `sha256`
attribute is still needed for Nix purity.

### Handling external dependencies

Some crates require external libraries. For crates from
[crates.io](https://crates.io), such libraries can be specified in
`defaultCrateOverrides` package in nixpkgs itself.

Starting from that file, one can add more overrides, to add features
or build inputs by overriding the hello crate in a seperate file.

```
with import <nixpkgs> {};
((import ./hello.nix).hello {}).override {
  crateOverrides = defaultCrateOverrides // {
    hello = attrs: { buildInputs = [ openssl ]; };
  };
}
```

Here, `crateOverrides` is expected to be a attribute set, where the
key is the crate name without version number and the value a function.
The function gets all attributes passed to `buildRustCrate` as first
argument and returns a set that contains all attribute that should be
overwritten.

For more complicated cases, such as when parts of the crate's
derivation depend on the the crate's version, the `attrs` argument of
the override above can be read, as in the following example, which
patches the derivation:

```
with import <nixpkgs> {};
((import ./hello.nix).hello {}).override {
  crateOverrides = defaultCrateOverrides // {
    hello = attrs: lib.optionalAttrs (lib.versionAtLeast attrs.version "1.0")  {
      postPatch = ''
        substituteInPlace lib/zoneinfo.rs \
          --replace "/usr/share/zoneinfo" "${tzdata}/share/zoneinfo"
      '';
    };
  };
}
```

Another situation is when we want to override a nested
dependency. This actually works in the exact same way, since the
`crateOverrides` parameter is forwarded to the crate's
dependencies. For instance, to override the build inputs for crate
`libc` in the example above, where `libc` is a dependency of the main
crate, we could do:

```
with import <nixpkgs> {};
((import hello.nix).hello {}).override {
  crateOverrides = defaultCrateOverrides // {
    libc = attrs: { buildInputs = []; };
  };
}
```

### Options and phases configuration

Actually, the overrides introduced in the previous section are more
general. A number of other parameters can be overridden:

- The version of rustc used to compile the crate:

  ```
  (hello {}).override { rust = pkgs.rust; };
  ```

- Whether to build in release mode or debug mode (release mode by
  default):

  ```
  (hello {}).override { release = false; };
  ```

- Whether to print the commands sent to rustc when building
  (equivalent to `--verbose` in cargo:

  ```
  (hello {}).override { verbose = false; };
  ```

- Extra arguments to be passed to `rustc`:

  ```
  (hello {}).override { extraRustcOpts = "-Z debuginfo=2"; };
  ```

- Phases, just like in any other derivation, can be specified using
  the following attributes: `preUnpack`, `postUnpack`, `prePatch`,
  `patches`, `postPatch`, `preConfigure` (in the case of a Rust crate,
  this is run before calling the "build" script), `postConfigure`
  (after the "build" script),`preBuild`, `postBuild`, `preInstall` and
  `postInstall`. As an example, here is how to create a new module
  before running the build script:

  ```
  (hello {}).override {
    preConfigure = ''
       echo "pub const PATH=\"${hi.out}\";" >> src/path.rs"
    '';
  };
  ```

### Features

One can also supply features switches. For example, if we want to
compile `diesel_cli` only with the `postgres` feature, and no default
features, we would write:

```
(callPackage ./diesel.nix {}).diesel {
  default = false;
  postgres = true;
}
```

Where `diesel.nix` is the file generated by Carnix, as explained above.


## Setting Up `nix-shell`
Oftentimes you want to develop code from within `nix-shell`. Unfortunately
`buildRustCrate` does not support common `nix-shell` operations directly
(see [this issue](https://github.com/NixOS/nixpkgs/issues/37945))
so we will use `stdenv.mkDerivation` instead.

Using the example `hello` project above, we want to do the following:
- Have access to `cargo` and `rustc`
- Have the `openssl` library available to a crate through it's _normal_
  compilation mechanism (`pkg-config`).

A typical `shell.nix` might look like:

```
with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "rust-env";
  buildInputs = [
    rustc cargo

    # Example Additional Dependencies
    pkgconfig openssl
  ];

  # Set Environment Variables
  RUST_BACKTRACE = 1;
}
```

You should now be able to run the following:
```
$ nix-shell --pure
$ cargo build
$ cargo test
```

### Controlling Rust Version Inside `nix-shell`
To control your rust version (i.e. use nightly) from within `shell.nix` (or
other nix expressions) you can use the following `shell.nix`

```
# Latest Nightly
with import <nixpkgs> {};
let src = fetchFromGitHub {
      owner = "mozilla";
      repo = "nixpkgs-mozilla";
      # commit from: 2018-03-27
      rev = "2945b0b6b2fd19e7d23bac695afd65e320efcebe";
      sha256 = "034m1dryrzh2lmjvk3c0krgip652dql46w5yfwpvh7gavd3iypyw";
   };
in
with import "${src.out}/rust-overlay.nix" pkgs pkgs;
stdenv.mkDerivation {
  name = "rust-env";
  buildInputs = [
    # Note: to use use stable, just replace `nightly` with `stable`
    latest.rustChannels.nightly.rust

    # Add some extra dependencies from `pkgs`
    pkgconfig openssl
  ];

  # Set Environment Variables
  RUST_BACKTRACE = 1;
}
```

Now run:
```
$ rustc --version
rustc 1.26.0-nightly (188e693b3 2018-03-26)
```

To see that you are using nightly.


## Using the Rust nightlies overlay

Mozilla provides an overlay for nixpkgs to bring a nightly version of Rust into scope.
This overlay can _also_ be used to install recent unstable or stable versions
of Rust, if desired.

To use this overlay, clone
[nixpkgs-mozilla](https://github.com/mozilla/nixpkgs-mozilla),
and create a symbolic link to the file
[rust-overlay.nix](https://github.com/mozilla/nixpkgs-mozilla/blob/master/rust-overlay.nix)
in the `~/.config/nixpkgs/overlays` directory.

    $ git clone https://github.com/mozilla/nixpkgs-mozilla.git
    $ mkdir -p ~/.config/nixpkgs/overlays
    $ ln -s $(pwd)/nixpkgs-mozilla/rust-overlay.nix ~/.config/nixpkgs/overlays/rust-overlay.nix

The latest version can be installed with the following command:

    $ nix-env -Ai nixos.latest.rustChannels.stable.rust

Or using the attribute with nix-shell:

    $ nix-shell -p nixos.latest.rustChannels.stable.rust

To install the beta or nightly channel, "stable" should be substituted by
"nightly" or "beta", or
use the function provided by this overlay to pull a version based on a
build date.

The overlay automatically updates itself as it uses the same source as
[rustup](https://www.rustup.rs/).
