Zsh completion of brew formulas

Try installing the zsh-completions formula.

$ 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

It also looks like zsh may have some compatibility with bash completions. You could try that in conjunction with the bash-completions formula.

Homebrew provides it's own completion. You just need to link the file $(brew --prefix)/Library/Contributions/brew_zsh_completion.zsh to _brew somewhere in your $fpath.(See comment at the start of https://github.com/Homebrew/homebrew/blob/master/Library/Contributions/brew_zsh_completion.zsh)

zsh-completions recently removed its _brew completion in favour of that file, see https://github.com/zsh-users/zsh-completions/issues/305.

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Update 2016-04-19: At some point(1) Homebrew updated its handling of zsh completion again. Now if you install zsh via Homebrew it should "just work". The file is installed to /usr/local/share/zsh/site-functions/_brew.