# Rust {#rust}

To install the rust compiler and cargo put

```nix
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](#using-the-rust-nightlies-overlay).

## Compiling Rust applications with Cargo

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

```nix
{ 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](https://www.w3.org/TR/SRI/) hashes. For example:

```nix
  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:

```nix
  cargoSha256 = lib.fakeSha256;
```

For `cargoHash` you can use:

```nix
  cargoHash = lib.fakeHash;
```

Per the instructions in the [Cargo Book](https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html)
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`:

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

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

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

  # ...
}
```


### 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:
   ```nix
   import <nixpkgs> {
     crossSystem = (import <nixpkgs/lib>).systems.examples.armhf-embedded // {
       rustc.config = "thumbv7em-none-eabi";
     };
   }
   ```
   will result in:
   ```shell
   --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:
   ```nix
   import <nixpkgs> {
     crossSystem = (import <nixpkgs/lib>).systems.examples.armhf-embedded // {
       rustc.config = "thumb-crazy";
       rustc.platform = { foo = ""; bar = ""; };
     };
   }
   ```
   will result in:
   ```shell
   --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`:

```nix
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:

```nix
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/`](https://doc.rust-lang.org/cargo/guide/build-cache.html),
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.

```nix
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:

```nix
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.

```nix
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:

```nix
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.

### 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](https://github.com/PyO3/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:

```nix
{ 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`.

```nix

{ 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](https://github.com/PyO3/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.

```nix
{ 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:

```nix
# 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:

```nix
# 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](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.

```nix
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:

```nix
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:

```nix
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:

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

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

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

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

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

- Extra arguments to be passed to `rustc`:

  ```nix
  (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:

  ```nix
  (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:

```nix
(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:

```nix
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:
```ShellSesssion
$ 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`

```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:
```ShellSession
$ 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 [#sec-overlays-install](the manual on installing overlays).

#### Imperative rust overlay installation

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

### Declarative rust overlay installation

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

```nix
{ 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](https://www.rustup.rs/).
