figsoda a3925908e4 doc: rust: improve documentation on cargoLock.lockFile

2021-08-01 17:01:13 -04:00

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Rust

To install the rust compiler and cargo put

environment.systemPackages = [
  rustc
  cargo
];

into your configuration.nix or bring them into scope with nix-shell -p rustc cargo.

For other versions such as daily builds (beta and nightly), use either rustup from nixpkgs (which will manage the rust installation in your home directory), or use Mozilla's Rust nightlies overlay.

Compiling Rust applications with Cargo

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

{ lib, rustPlatform }:

rustPlatform.buildRustPackage rec {
  pname = "ripgrep";
  version = "12.1.1";

  src = fetchFromGitHub {
    owner = "BurntSushi";
    repo = pname;
    rev = version;
    sha256 = "1hqps7l5qrjh9f914r5i6kmcz6f1yb951nv4lby0cjnp5l253kps";
  };

  cargoSha256 = "03wf9r2csi6jpa7v5sw5lpxkrk4wfzwmzx7k3991q3bdjzcwnnwp";

  meta = with 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 ];
  };
}

buildRustPackage requires either the cargoSha256 or the cargoHash attribute which is computed over all crate sources of this package. cargoHash256 is used for traditional Nix SHA-256 hashes, such as the one in the example above. cargoHash should instead be used for SRI hashes. For example:

  cargoHash = "sha256-l1vL2ZdtDRxSGvP0X/l3nMw8+6WF67KPutJEzUROjg8=";

Both types of hashes are permitted when contributing to nixpkgs. The Cargo hash is obtained by inserting a fake checksum into the expression and building the package once. The correct checksum can then be taken from the failed build. A fake hash can be used for cargoSha256 as follows:

  cargoSha256 = lib.fakeSha256;

For cargoHash you can use:

  cargoHash = lib.fakeHash;

Per the instructions in the Cargo Book best practices guide, Rust applications should always commit the Cargo.lock file in git to ensure a reproducible build. However, a few packages do not, and Nix depends on this file, so if it is missing you can use cargoPatches to apply it in the patchPhase. Consider sending a PR upstream with a note to the maintainer describing why it's important to include in the application.

The fetcher will verify that the Cargo.lock file is in sync with the src attribute, and fail the build if not. It will also will compress the vendor directory into a tar.gz archive.

The tarball with vendored dependencies contains a directory with the package's name, which is normally composed of pname and version. This means that the vendored dependencies hash (cargoSha256/cargoHash) is dependent on the package name and version. The cargoDepsName attribute can be used to use another name for the directory of vendored dependencies. For example, the hash can be made invariant to the version by setting cargoDepsName to pname:

rustPlatform.buildRustPackage rec {
  pname = "broot";
  version = "1.2.0";

  src = fetchCrate {
    inherit pname version;
    sha256 = "1mqaynrqaas82f5957lx31x80v74zwmwmjxxlbywajb61vh00d38";
  };

  cargoHash = "sha256-JmBZcDVYJaK1cK05cxx5BrnGWp4t8ca6FLUbvIot67s=";
  cargoDepsName = pname;

  # ...
}

Importing a `Cargo.lock` file

Using cargoSha256 or cargoHash is tedious when using buildRustPackage within a project, since it requires that the hash is updated after every change to Cargo.lock. Therefore, buildRustPackage also supports vendoring dependencies directly from a Cargo.lock file using the cargoLock argument. For example:

rustPlatform.buildRustPackage rec {
  pname = "myproject";
  version = "1.0.0";

  cargoLock = {
    lockFile = ./Cargo.lock;
  }

  # ...
}

This will retrieve the dependencies using fixed-output derivations from the specified lockfile. Note that setting cargoLock.lockFile doesn't add a Cargo.lock to your src, and a Cargo.lock is still required to build a rust package. A simple fix is to use:

postPatch = ''
  cp ${./Cargo.lock} Cargo.lock
'';

The output hash of each dependency that uses a git source must be specified in the outputHashes attribute. For example:

rustPlatform.buildRustPackage rec {
  pname = "myproject";
  version = "1.0.0";

  cargoLock = {
    lockFile = ./Cargo.lock;
    outputHashes = {
      "finalfusion-0.14.0" = "17f4bsdzpcshwh74w5z119xjy2if6l2wgyjy56v621skr2r8y904";
    };
  };

  # ...
}

If you do not specify an output hash for a git dependency, building the package will fail and inform you of which crate needs to be added. To find the correct hash, you can first use lib.fakeSha256 or lib.fakeHash as a stub hash. Building the package (and thus the vendored dependencies) will then inform you of the correct hash.

Cross compilation

By default, Rust packages are compiled for the host platform, just like any other package is. The --target passed to rust tools is computed from this. By default, it takes the stdenv.hostPlatform.config and replaces components where they are known to differ. But there are ways to customize the argument:

To choose a different target by name, define stdenv.hostPlatform.rustc.config as that name (a string), and that name will be used instead.

For example:

import <nixpkgs> {
  crossSystem = (import <nixpkgs/lib>).systems.examples.armhf-embedded // {
    rustc.config = "thumbv7em-none-eabi";
  };
}

will result in:

--target thumbv7em-none-eabi

To pass a completely custom target, define stdenv.hostPlatform.rustc.config with its name, and stdenv.hostPlatform.rustc.platform with the value. The value will be serialized to JSON in a file called ${stdenv.hostPlatform.rustc.config}.json, and the path of that file will be used instead.

For example:
```
import <nixpkgs> {
  crossSystem = (import <nixpkgs/lib>).systems.examples.armhf-embedded // {
    rustc.config = "thumb-crazy";
    rustc.platform = { foo = ""; bar = ""; };
  };
}
```
will result in:
```
--target /nix/store/asdfasdfsadf-thumb-crazy.json # contains {"foo":"","bar":""}
```

Finally, as an ad-hoc escape hatch, a computed target (string or JSON file path) can be passed directly to buildRustPackage:

pkgs.rustPlatform.buildRustPackage {
  /* ... */
  target = "x86_64-fortanix-unknown-sgx";
}

This is useful to avoid rebuilding Rust tools, since they are actually target agnostic and don't need to be rebuilt. But in the future, we should always build the Rust tools and standard library crates separately so there is no reason not to take the stdenv.hostPlatform.rustc-modifying approach, and the ad-hoc escape hatch to buildRustPackage can be removed.

Note that currently custom targets aren't compiled with std, so cargo test will fail. This can be ignored by adding doCheck = false; to your derivation.

Running package tests

When using buildRustPackage, the checkPhase is enabled by default and runs cargo test on the package to build. To make sure that we don't compile the sources twice and to actually test the artifacts that will be used at runtime, the tests will be ran in the release mode by default.

However, in some cases the test-suite of a package doesn't work properly in the release mode. For these situations, the mode for checkPhase can be changed like so:

rustPlatform.buildRustPackage {
  /* ... */
  checkType = "debug";
}

Please note that the code will be compiled twice here: once in release mode for the buildPhase, and again in debug mode for the checkPhase.

Test flags, e.g., --features xxx/yyy, can be passed to cargo test via the cargoTestFlags attribute.

Another attribute, called checkFlags, is used to pass arguments to the test binary itself, as stated (here)[https://doc.rust-lang.org/cargo/commands/cargo-test.html].

Tests relying on the structure of the `target/` directory

Some tests may rely on the structure of the target/ directory. Those tests are likely to fail because we use cargo --target during the build. This means that the artifacts are stored in target/<architecture>/release/, rather than in target/release/.

This can only be worked around by patching the affected tests accordingly.

Disabling package-tests

In some instances, it may be necessary to disable testing altogether (with doCheck = false;):

If no tests exist -- the checkPhase should be explicitly disabled to skip unnecessary build steps to speed up the build.
If tests are highly impure (e.g. due to network usage).

There will obviously be some corner-cases not listed above where it's sensible to disable tests. The above are just guidelines, and exceptions may be granted on a case-by-case basis.

However, please check if it's possible to disable a problematic subset of the test suite and leave a comment explaining your reasoning.

Setting `test-threads`

buildRustPackage will use parallel test threads by default, sometimes it may be necessary to disable this so the tests run consecutively.

rustPlatform.buildRustPackage {
  /* ... */
  dontUseCargoParallelTests = true;
}

Building a package in `debug` mode

By default, buildRustPackage will use release mode for builds. If a package should be built in debug mode, it can be configured like so:

rustPlatform.buildRustPackage {
  /* ... */
  buildType = "debug";
}

In this scenario, the checkPhase will be ran in debug mode as well.

Custom `build`/`install`-procedures

Some packages may use custom scripts for building/installing, e.g. with a Makefile. In these cases, it's recommended to override the buildPhase/installPhase/checkPhase.

Otherwise, some steps may fail because of the modified directory structure of target/.

Building a crate with an absent or out-of-date Cargo.lock file

buildRustPackage needs a Cargo.lock file to get all dependencies in the source code in a reproducible way. If it is missing or out-of-date one can use the cargoPatches attribute to update or add it.

rustPlatform.buildRustPackage rec {
  (...)
  cargoPatches = [
    # a patch file to add/update Cargo.lock in the source code
    ./add-Cargo.lock.patch
  ];
}

Compiling non-Rust packages that include Rust code

Several non-Rust packages incorporate Rust code for performance- or security-sensitive parts. rustPlatform exposes several functions and hooks that can be used to integrate Cargo in non-Rust packages.

Vendoring of dependencies

Since network access is not allowed in sandboxed builds, Rust crate dependencies need to be retrieved using a fetcher. rustPlatform provides the fetchCargoTarball fetcher, which vendors all dependencies of a crate. For example, given a source path src containing Cargo.toml and Cargo.lock, fetchCargoTarball can be used as follows:

cargoDeps = rustPlatform.fetchCargoTarball {
  inherit src;
  hash = "sha256-BoHIN/519Top1NUBjpB/oEMqi86Omt3zTQcXFWqrek0=";
};

The src attribute is required, as well as a hash specified through one of the sha256 or hash attributes. The following optional attributes can also be used:

name: the name that is used for the dependencies tarball. If name is not specified, then the name cargo-deps will be used.
sourceRoot: when the Cargo.lock/Cargo.toml are in a subdirectory, sourceRoot specifies the relative path to these files.
patches: patches to apply before vendoring. This is useful when the Cargo.lock/Cargo.toml files need to be patched before vendoring.

If a Cargo.lock file is available, you can alternatively use the importCargoLock function. In contrast to fetchCargoTarball, this function does not require a hash (unless git dependencies are used) and fetches every dependency as a separate fixed-output derivation. importCargoLock can be used as follows:

cargoDeps = rustPlatform.importCargoLock {
  lockFile = ./Cargo.lock;
};

If the Cargo.lock file includes git dependencies, then their output hashes need to be specified since they are not available through the lock file. For example:

cargoDeps = rustPlatform.importCargoLock {
  lockFile = ./Cargo.lock;
  outputHashes = {
    "rand-0.8.3" = "0ya2hia3cn31qa8894s3av2s8j5bjwb6yq92k0jsnlx7jid0jwqa";
  };
};

If you do not specify an output hash for a git dependency, building cargoDeps will fail and inform you of which crate needs to be added. To find the correct hash, you can first use lib.fakeSha256 or lib.fakeHash as a stub hash. Building cargoDeps will then inform you of the correct hash.

Hooks

rustPlatform provides the following hooks to automate Cargo builds:

cargoSetupHook: configure Cargo to use depenencies vendored through fetchCargoTarball. This hook uses the cargoDeps environment variable to find the vendored dependencies. If a project already vendors its dependencies, the variable cargoVendorDir can be used instead. When the Cargo.toml/Cargo.lock files are not in sourceRoot, then the optional cargoRoot is used to specify the Cargo root directory relative to sourceRoot.
cargoBuildHook: use Cargo to build a crate. If the crate to be built is a crate in e.g. a Cargo workspace, the relative path to the crate to build can be set through the optional buildAndTestSubdir environment variable. Additional Cargo build flags can be passed through cargoBuildFlags.
maturinBuildHook: use Maturin to build a Python wheel. Similar to cargoBuildHook, the optional variable buildAndTestSubdir can be used to build a crate in a Cargo workspace. Additional maturin flags can be passed through maturinBuildFlags.
cargoCheckHook: run tests using Cargo. The build type for checks can be set using cargoCheckType. Additional flags can be passed to the tests using checkFlags and checkFlagsArray. By default, tests are run in parallel. This can be disabled by setting dontUseCargoParallelTests.
cargoInstallHook: install binaries and static/shared libraries that were built using cargoBuildHook.

Examples

Python package using `setuptools-rust`

For Python packages using setuptools-rust, you can use fetchCargoTarball and cargoSetupHook to retrieve and set up Cargo dependencies. The build itself is then performed by buildPythonPackage.

The following example outlines how the tokenizers Python package is built. Since the Python package is in the source/bindings/python directory of the tokenizers project's source archive, we use sourceRoot to point the tooling to this directory:

{ fetchFromGitHub
, buildPythonPackage
, rustPlatform
, setuptools-rust
}:

buildPythonPackage rec {
  pname = "tokenizers";
  version = "0.10.0";

  src = fetchFromGitHub {
    owner = "huggingface";
    repo = pname;
    rev = "python-v${version}";
    hash = "sha256-rQ2hRV52naEf6PvRsWVCTN7B1oXAQGmnpJw4iIdhamw=";
  };

  cargoDeps = rustPlatform.fetchCargoTarball {
    inherit src sourceRoot;
    name = "${pname}-${version}";
    hash = "sha256-BoHIN/519Top1NUBjpB/oEMqi86Omt3zTQcXFWqrek0=";
  };

  sourceRoot = "source/bindings/python";

  nativeBuildInputs = [ setuptools-rust ] ++ (with rustPlatform; [
    cargoSetupHook
    rust.cargo
    rust.rustc
  ]);

  # ...
}

In some projects, the Rust crate is not in the main Python source directory. In such cases, the cargoRoot attribute can be used to specify the crate's directory relative to sourceRoot. In the following example, the crate is in src/rust, as specified in the cargoRoot attribute. Note that we also need to specify the correct path for fetchCargoTarball.


{ buildPythonPackage
, fetchPypi
, rustPlatform
, setuptools-rust
, openssl
}:

buildPythonPackage rec {
  pname = "cryptography";
  version = "3.4.2"; # Also update the hash in vectors.nix

  src = fetchPypi {
    inherit pname version;
    sha256 = "1i1mx5y9hkyfi9jrrkcw804hmkcglxi6rmf7vin7jfnbr2bf4q64";
  };

  cargoDeps = rustPlatform.fetchCargoTarball {
    inherit src;
    sourceRoot = "${pname}-${version}/${cargoRoot}";
    name = "${pname}-${version}";
    hash = "sha256-PS562W4L1NimqDV2H0jl5vYhL08H9est/pbIxSdYVfo=";
  };

  cargoRoot = "src/rust";

  # ...
}

Python package using `maturin`

Python packages that use Maturin can be built with fetchCargoTarball, cargoSetupHook, and maturinBuildHook. For example, the following (partial) derivation builds the retworkx Python package. fetchCargoTarball and cargoSetupHook are used to fetch and set up the crate dependencies. maturinBuildHook is used to perform the build.

{ lib
, buildPythonPackage
, rustPlatform
, fetchFromGitHub
}:

buildPythonPackage rec {
  pname = "retworkx";
  version = "0.6.0";

  src = fetchFromGitHub {
    owner = "Qiskit";
    repo = "retworkx";
    rev = version;
    sha256 = "11n30ldg3y3y6qxg3hbj837pnbwjkqw3nxq6frds647mmmprrd20";
  };

  cargoDeps = rustPlatform.fetchCargoTarball {
    inherit src;
    name = "${pname}-${version}";
    hash = "sha256-heOBK8qi2nuc/Ib+I/vLzZ1fUUD/G/KTw9d7M4Hz5O0=";
  };

  format = "pyproject";

  nativeBuildInputs = with rustPlatform; [ cargoSetupHook maturinBuildHook ];

  # ...
}

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
{ 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
{ 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. A sha256 attribute is still needed for Nix purity.

Handling external dependencies

Some crates require external libraries. For crates from 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 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) 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";
  nativeBuildInputs = [
    rustc cargo

    # Example Build-time Additional Dependencies
    pkg-config
  ];
  buildInputs = [
    # Example Run-time Additional Dependencies
    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: 2019-05-15
      rev = "9f35c4b09fd44a77227e79ff0c1b4b6a69dff533";
      sha256 = "18h0nvh55b5an4gmlgfbvwbyqj91bklf1zymis6lbdh75571qaz0";
   };
in
with import "${src.out}/rust-overlay.nix" pkgs pkgs;
stdenv.mkDerivation {
  name = "rust-env";
  buildInputs = [
    # Note: to use stable, just replace `nightly` with `stable`
    latest.rustChannels.nightly.rust

    # Add some extra dependencies from `pkgs`
    pkg-config 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.

Rust overlay installation

You can use this overlay by either changing your local nixpkgs configuration, or by adding the overlay declaratively in a nix expression, e.g. in configuration.nix. For more information see the manual on installing overlays.

Imperative rust overlay installation

Clone nixpkgs-mozilla, and create a symbolic link to the file 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

Declarative rust overlay installation

Add the following to your configuration.nix, home-configuration.nix, shell.nix, or similar:

{ pkgs ? import <nixpkgs> {
    overlays = [
      (import (builtins.fetchTarball https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz))
      # Further overlays go here
    ];
  };
};

Note that this will fetch the latest overlay version when rebuilding your system.

Rust overlay usage

The overlay contains attribute sets corresponding to different versions of the rust toolchain, such as:

latest.rustChannels.stable
latest.rustChannels.nightly
a function rustChannelOf, called as (rustChannelOf { date = "2018-04-11"; channel = "nightly"; }), or...
(nixpkgs.rustChannelOf { rustToolchain = ./rust-toolchain; }) if you have a local rust-toolchain file (see https://github.com/mozilla/nixpkgs-mozilla#using-in-nix-expressions for an example)

Each of these contain packages such as rust, which contains your usual rust development tools with the respective toolchain chosen. For example, you might want to add latest.rustChannels.stable.rust to the list of packages in your configuration.

Imperatively, the latest stable version can be installed with the following command:

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

Or using the attribute with nix-shell:

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

Substitute the nixpkgs prefix with nixos on NixOS. 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.

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