This is a Heroku buildpack for Rust with support for cargo and rustup, which seems to be the future of Rust deployment as of the fall of 2016. Features include:

• Caching of builds between deployments.
• Automatic updates to the latest stable Rust by default.
• Optional pinning of Rust to a specific version.
• Support for export so that other buildpacks can access the Rust toolchain.
• Support for compiling Rust-based extensions for projects written in other languages.

For an example project, see heroku-rust-cargo-hello.

Using this buildpack

To deploy an application to Heroku, we recommend installing the Heroku CLI.

If you're creating a new Heroku application, cd to the directory containing your code, and run:

heroku create --buildpack https://github.com/emk/heroku-buildpack-rust.git

This will only work if your application has a Cargo.toml and uses git. If you want to set a particular name for application, see heroku create --help first.

To use this as the buildpack for an existing application, run:

heroku buildpacks:set https://github.com/emk/heroku-buildpack-rust.git

You will also need to create a Procfile pointing to the release version of your application, and commit it to git:

web: ./target/release/hello

To deploy your application, run:

git push heroku master

Specifying which version of Rust to use

By default, your application will be built using the latest stable Rust. Normally, this is pretty safe: New stable Rust releases have excellent backwards compatibility.

But you may wish to use nightly Rust or to lock your Rust version to a known-good configuration for more reproducible builds. To do this, create a top-level file named RustConfig in your project specifying either a channel name:

VERSION=nightly

...or a specific version:

VERSION=1.20.0

Combining with other buildpacks

If you have a project which combines both Rust and another programming language, you can insert this buildpack before your existing one as follows:

heroku buildpacks:add --index 1 https://github.com/emk/heroku-buildpack-rust.git

If you have a valid Cargo.toml in your project, this is all you need to do. The Rust buildpack will run first, and your existing buildpack will run second.

But if you only need Rust to build a particular Ruby gem, and you have no top-level Cargo.toml file, you'll need to let the buildpack know to skip the build stage. You can do this by adding the following line to RustConfig:

RUST_SKIP_BUILD=1

Development notes

If you need to tweak this buildpack, the following information may help.

Testing with Docker

To test changes to the buildpack using the included docker-compose-test.yml, run:

./test_buildpack

Then make sure there are no Rust-related *.so files getting linked:

ldd heroku-rust-cargo-hello/target/release/hello

This uses the Docker image heroku/cedar, which allows us to test in an official Cedar-like environment.

We also run this test automatically on Travis CI.