Hi Guix,

I have been writing a lot of Rust recently. Racer provides
autocompletion. It uses the source of rust standard libraries among
other tools.

There are two items I paid close attention to:

1. Add a "source" output for rust.  This roughly corresponds to the
rustup component.  Using the rustup toolchain to manage rust tooling the
command would be: rustup component add rust-src. However, using guix, we
already have the source.

2. Racer requires features that are not in the stable branch of rust.
Unfortunately this is not any fault of racer. Racer uses crates from
rustc which uses unstable features.  To get around this, I built
racer using the undocumented and unsupported RUSTC_BOOTSTRAP environment
variable. Perhaps the better future solution would be to provide a
nightly rustc and use that to compile racer.

I hope someone gets some use out of racer!

- John