add DSL to generate completions

Library/Homebrew/formula.rb

@@ -1625,6 +1625,21 @@ class Formula
   end
   private :extract_macho_slice_from
 
+  def generate_completions(base_name = name, shells = [:bash, :zsh, :fish], binary = bin/base_name, cmd = "completion", shell_as_flag = false)
+    completion_script_path_map = {
+      :bash => bash_completion/base_name,
+      :zsh => zsh_completion/"_#{base_name}",
+      :fish => fish_completion/"#{base_name}.fish",
+    }
+
+    shells.each do |shell|
+      script_path = completion_script_path_map[shell]
+      shell_parameter = shell_as_flag ? "--#{shell.to_s}" : shell.to_s
+      script_path.dirname.mkpath
+      script_path.write Utils.safe_popen_read(binary, cmd, shell_parameter)
+    end
+  end
+
   # an array of all core {Formula} names
   # @private
   def self.core_names