This guide will show how Rust programs can be cross compiled for the Raspberry Pi using Cargo. These instructions may or may not work for your particular system, so you may have to adjust the procedure to fit your configuration.

There is also a docker container version of the Rust cross-compiler for the Raspberry: https://github.com/Ragnaroek/rust-on-raspberry-docker. If you are using docker this may be a easier solution for cross-compiling your project.

These instructions are based on the rusty-pi guide, but with some additions and adaptations for the current build systems.

The first step is to download the Raspberry Pi toolchain. It's a collection of binaries and libraries for cross compiling. Begin by installing git if it's not already installed:

sudo apt-get install git

(replace apt-get install with the install command for your favorite package manager)

and procede by cloning the toolchain repository:

git clone https://github.com/raspberrypi/tools.git ~/pi-tools

The next step is to compile the Rust compiler and standard libraries. The standard libraries are particularly important, since they have to be compiled for the ARM platform to make them usable in your program. See Appendix A for information on how to add more libraries.

start by cloning the Rust repository and cd into it:

git clone http://github.com/rust-lang/rust.git
cd rust

You may want to check out the same revision as your current copy of rustc to keep everything in sync. You can find the revision hash by running rustc with the -V flag:

$ rustc -V
rustc 1.0.0-nightly (30e1f9a1c 2015-03-14) (built 2015-03-15)
                     ^-------^ This is what you are looking for

Copy the hash and use it to reset the repository to the same revision:

git reset --hard 30e1f9a1c

Alright, finally time to build it. Begin by adding the binary directory from the Raspberry Pi toolchain to your path. Use the following command to use the 64 bit tools:

export PATH=~/pi-tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin:$PATH

or use this command for the 32 bit tools:

export PATH=~/pi-tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin:$PATH

Configure the compiler to build everything for ARM Linux and set the install destination to $HOME/pi-rust:

./configure --target=arm-unknown-linux-gnueabihf --prefix=$HOME/pi-rust

And build+install!

make -j4 && make install

(Change the 4 to your preferred number of parallel build processes)

This is will take a while, so go and grab some coffee or take a walk while waiting.

Is it done? Great! Now, move on to the next part.

We are almost ready to actually build things. You may actually be able to use rustc directly, but we want more, right? We want the convenience of Cargo! But Cargo doesn't come without demands. It has to know what we are using to link our program, so let's tell it. Cargo will be looking for configuration files where we can specify what to use when building ARM programs.

You may already have a directory in your $HOME called .cargo. Create one, if you don't have it. Now, create a file called config, or edit an existing one, and add the following lines to associate the target triple arm-unknown-linux-gnueabihf with our cross compilers:

[target.arm-unknown-linux-gnueabihf]
ar = "arm-linux-gnueabihf-gcc-ar"
linker = "gcc-sysroot"

Wait a minute! What is gcc-sysroot? Well, that is a semi ugly hack and we are going to use it.

The thing is that Cargo has some problems when it comes to cross compiling while depending on share libraries. These libraries are placed somewhere in the sysroot and gcc is using ld to link them. The problem is that the default sysroot is where the host libraries are located and those are not built for ARM. There is currently no good way to tell Cargo to tell rustc to tell gcc to tell ld to look somewhere else, so we have to do it for them.

The Raspberry Pi toolchain contains a directory with various system directories filled with common libraries (if you want to add more libraries, see Appendix A). This is where we want ld to look for things, so we are going to use a simple script to tell gcc to tell it where this directory can be found. Create a file in the binary directory you added to your $PATH before (~/pi-tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin if you used the 64 bit tools or ~/pi-tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin if you prefer the 32 bit tools) and call it gcc-sysroot.

Add the following lines in gcc-sysroot:

#!/bin/bash
arm-linux-gnueabihf-gcc --sysroot=$HOME/pi-tools/arm-bcm2708/arm-bcm2708hardfp-linux-gnueabi/arm-bcm2708hardfp-linux-gnueabi/sysroot "$@"

This is basically an alias for arm-linux-gnueabihf-gcc, but with the --sysroot set to where the libraries are kept. The "$@" part is there to pass all the incoming argument forwards to arm-linux-gnueabihf-gcc. Alright, make the file executable:

chmod +x ~/pi-tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin/gcc-sysroot

or

chmod +x ~/pi-tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/gcc-sysroot

If your crate requires building C++ code, then you'll need to create a g++-sysroot just like gcc-sysroot, substituting arm-linux-gnueabihf-g++ for arm-linux-gnueabihf-gcc inside.

And you should be done! Or, kind of done. There is one thing left.

It would be nice to just be able to run cargo build and be done with it, but the reality is not exactly that nice. Almost, but not exactly. You will have to add the binaries to your $PATH every time you open a new terminal and get ready for compiling. You may also have to define some other variables to satisfy some other systems. Sound tedious and boring, right? Well, that's what we have scripts for.

This repository contains two versions of the same script (cross32 for 32 bit and cross64 for 64 bit). They can be used as a substitute for Cargo, like this:

./cross64 cargo-command path/to/rust path/to/pi/toolchain

Examples:
./cross64 build ~/some-other-rust ~/some-other-pi-tools
./cross64 doc
./cross64 "build --release"

The first argument is what you would normally pass to cargo. It can be single commands, like build or doc, or multiple arguments like "build --release". Note that the combined commands have to be passed as a single string. path/to/rust and path/to/pi/toolchain are optional and should only be used if you want to use tools from somewhere else than what this guide recommends.

These scripts will set up the required environment variables and run cargo for you. All without polluting the external environment. You can include them in your projects and modify them however you want.

That's it! You should now be ready to cross compile your Raspberry Pi projects.

Have fun!

Let's say your project uses some crate that depends on having openssl installed on the system. In this case you have to install the package manually into the ARM toolset.

Get these packages either from the raspberry, or download them online.

We'll assume that you're running Raspbian (i.e. deb files), but it should be straight forward to adapt the steps to your dist/packages.

If you do apt-cache show libssl1.0.0 on the raspberry, you'll see this in the output:

Filename:    pool/main/o/openssl/libssl1.0.0_1.0.1e-2+rvt+deb7u17_armhf.deb

You should be able to find a match for that under ftp.debian.org/debian/pool, so the resulting URL in this case is

http://ftp.debian.org/debian/pool/main/o/openssl/libssl-dev_1.0.1e-2+deb7u17_armhf.deb

If it's not there, see if it is still on the raspberry under /var/cache/apt/archive.

If you still can't find it, try searching for the filename online.

When you have the dependencies downloaded, extract them into the ARM toolchain:

# cd into the sysroot of the arm toolchain
cd ~/pi-tools/arm-bcm2708/arm-bcm2708hardfp-linux-gnueabi/arm-bcm2708hardfp-linux-gnueabi/sysroot/

# move the deb files here and use `ar` to extract the contents:
ar p libssl1.0.0_1.0.1e-2+rvt+deb7u17_armhf.deb data.tar.gz | tar zx

# Repeat for any other dependencies you may have..
ar p libssl-dev_1.0.1e-2+rvt+deb7u17_armhf.deb data.tar.gz | tar zx
ar p zlib1g_1.2.7.dfsg-13_armhf.deb data.tar.gz | tar zx
ar p zlib1g-dev_1.2.7.dfsg-13_armhf.deb data.tar.gz | tar zx

Now you're ready to build the project.

Pull requests with enhancements, corrections or additional instructions are very much appreciated. Good luck!