I'm maintaining a docker image to cross-compile at https://hub.docker.com/r/fabricedesre/rustpi2/. It's Rasberry Pi2 specific since it relies on a tweaked sysroot to link against OpenSSL and a few other libraries.
The way I'm using it is: docker run --name rustpi2 -v `pwd`:/home/rustpi2/dev/source -v $HOME/.cargo:/home/rustpi2/.cargo fabricedesre/rustpi2 cargopi build (cargopi is just equivalent to cargo --target=armv7-unknown-linux-gnueabihf)
A summary is "every Rust compiler is a cross-compiler, but you need a copy of the standard library for your target platform available." Getting said copy is what takes the work. In the future, we want it to be as easy as a call to the command-line, but for now, https://github.com/japaric/rust-cross is a good resource.
See also https://github.com/japaric/rust-everywhere , which is very interesting.