I had some issues getting rustfmt to work in spacemacs on my mac. Here’s roughly how I got it working.

These two github issues helped a lot: 1 , 2.

In particular, these are the most important steps:

1. Confirm you’re using rustfmt-nightly
2. Export LD_LIBRARY_PATH
3. Use exec-path-from-shell in your spacemacs config

Confirm you’re using rustfmt-nightly

You can use rustup show to see whether you’re using the nightly version.

You can install rustfmt-nightly with cargo install rustfmt-nightly --force

Run rustup run nightly rustfmt --help to confirm that your nightly version of rustfmt works.

Export LD_LIBRARY_PATH

For whatever reason, macs don’t like linking the LD_LIBRARY_PATH stuff by default. This causes problems for rustfmt.

Run export LD_LIBRARY_PATH=$(rustc --print sysroot)/lib:$LD_LIBRARY_PATH, and then try running rustfmt --help to see if it worked.

If it doesn’t, then try also using export DYLD_LIBRARY_PATH=$(rustc --print sysroot)/lib and running rustfmt again. If that doesn’t work, then I’m sorry as I didn’t have issues after that.

Assuming it does work, you now want to put those exports into your .bash_profile so they get loaded every time you start a shell.

Source your .bash_profile, and then confirm it works by running rustfmt in a new shell.

Use exec-path-from-shell in your spacemacs

Spacemacs won’t know how to load those exports by default. So we need to tell it to get those paths somehow.

You can use exec-path-from-shell to do just this. Put the following into your .spacemacs in the user-config section:

(require 'exec-path-from-shell)
(exec-path-from-shell-initialize)
;; This is used for rustfmt
(exec-path-from-shell-copy-env "LD_LIBRARY_PATH")

You might also need to add the DYLD_LIBRARY_PATH as well (I didn’t)

Now if you restart spacemacs, you should be able to use rustfmt in it.