https://github.com/joshka/brew-cask-zsh-completions

Note: This repo is now deprecated given that these changes were merged into Homebrew at Homebrew/brew#936.

This script is a fork of the Zsh Homebrew Cask completion plugin from Oh My Zsh. It solves the following issues with the original code:

1. Many users use Prezto rather than Oh My Zsh.
2. The plugin worked by intercepting the brew command rather than the brew completion calling this completion script. These changes require Homebrew to merge brew#407 before it will work.
3. It was missing a bunch of completions (mostly options).

Symlink or copy the completion script to a directory in your fpath. Homebrew installs its completions into $(brew --prefix)/share/zsh/site-functions which is in the default fpath for the version of zsh installed by Homebrew.

$ git clone https://github.com/joshka/brew-cask-zsh-completions.git
$ ln -s brew-cask-zsh-completions/_brew_cask $(brew --prefix)/share/zsh/site-functions