Spent hours debugging why rustic-format-buffer wasn’t working in Emacs. The error? “Program not found: No such file or directory, cargo”

The Problem

rustic-format-buffer
Error: Program not found: No such file or directory, cargo

Cargo was definitely installed and working in my terminal (zsh). But Emacs couldn’t see it.

The Debug Journey

Tried the systematic approach:

1. Checked if rustic-mode was actually running (yes)
2. Searched rustic repo issues (nothing relevant)
3. Used M-x debug-on-error to trace the error
4. Found it failed in rustic-buffer-workspace - literally couldn’t find cargo
5. Checked Emacs PATH: M-x getenv RET PATH

Aha moment: Emacs PATH didn’t include ~/.cargo/bin!

Why This Happens

Emacs GUI on macOS doesn’t inherit shell environment variables. My zsh had cargo in PATH, but Emacs launched from Dock didn’t.

The Fix

Install exec-path-from-shell:

(use-package exec-path-from-shell
  :config
  (when (memq window-system '(mac ns x))
    (exec-path-from-shell-initialize)))

Restart Emacs. Now rustic-format-buffer works perfectly.

Lesson

Emacs GUI environment ≠ Terminal environment. Always check M-x getenv when programs “aren’t found”.

Found the solution in this Reddit thread.