Here’s how to setup multiple rust toolchains setup on El Capitan.

First uninstall any old toolchains you might have (I just had a vanilla install of stable 1.8):

$ /usr/local/lib/rustlib/

Then trust brson and grab multirust, this will use the stable toolchain by default:

$ curl -sf | sh

Get GPG so rust binaries can be verified, then add the nightly toolchain:

$ brew install gpg
$ multirust update nightly

Now start a new project (the diesel demo, in my case) and use the nightly-2016-04-09 toolchain:

$ cargo new diesel_demo
$ cd diesel_demo
$ multirust override nightly-2016-04-09

I previously had some path modifications in my ~/.zshrc, I needed to get rid of them and just let multirust manage everything, then I could install the diesel CLI:

$ cargo install diesel_cli
$ export PATH=$PATH:~/.multirust/toolchains/nightly-2016-04-09/cargo/bin
$ diesel --version
diesel 0.6.1

Read more about using alternative toolchains on the multirust project page.