This is a work in progress! See the open issues on this repo for what we're working on. Feel free to get involved, or join the #rust-lang channel
on our Tessel Slack community 
You can read documentation for Tessel crates published on docs.rs:
The tessel crate is all you need to start talking to low-level hardware APIs.
Rust can be compiled on a remote cross-compilation server or locally.
Local compilation with Tessel requires the Rust, Cargo, and the Tessel 2 CLI >= v0.1.0 (installed via npm):
npm install -g t2-cli
cargo tessel sdk install
You can then create an deploy a blinking lights example using the CLI:
mkdir quickstart
cd quickstart
t2 init --lang=rust
t2 run blinky
You can use the remote compilation server to compile your code quickly and run
it on Tessel. If you don't have Rust and Cargo installed on your machine, or if
cargo tessel sdk install says you are using an unsupported version of Rust or
an unsupported platform, this is the quickest way to get started:
mkdir quickstart
cd quickstart
t2 init --lang=rust
t2 run blinky --rustcc
The --rustcc argument invokes the remote cross-compilation server, which bundles
and then locally deploys code to your Tessel.
The Tessel Standard Library is the library that gets 'loaded' into a user program (runs on T2) and presents an API for configuring the hardware (LEDs, module ports, network interfaces, etc.). You can see the JavaScript version of the Tessel Standard Library here. The most important function is communication with module ports which takes places by writing to a Unix Domain Socket always running on OpenWRT. See the technical overview or previously linked JS Standard Library for more detailed information on how that works. Everything sent to the domain socket gets sent to the microcontroller. There is a simple protocol between the MediaTek (running OpenWRT) and the coprocessor to coordinate hardware operations.
See the rust-compilation-server repo for how to develop for the remote compilation server.
MIT or Apache-2.0, at your option.