$ brew install em<TAB>

$ brew install emacs

$ brew info zsh-completions
zsh-completions: stable 0.10.0, HEAD
https://github.com/zsh-users/zsh-completions
Not installed
From: https://github.com/Homebrew/homebrew/commits/master/Library/Formula/zsh-completions.rb
==> Caveats
To activate these completions, add the following to your .zshrc:

  fpath=(/usr/local/share/zsh-completions $fpath)

You may also need to force rebuild `zcompdump`:

  rm -f ~/.zcompdump; compinit

Additionally, if you receive "zsh compinit: insecure directories" warnings when attempting
to load these completions, you may need to run this:

  chmod go-w /usr/local/share