Implement RuboCop for cask `desc` stanza.

Library/.rubocop_cask.yml

 inherit_from: ./Homebrew/.rubocop.yml
 
+Cask/Desc:
+  Description: 'Ensure that the desc stanza conforms to various content and style checks.'
+  Enabled: true
+
 Cask/HomepageMatchesUrl:
   Description: 'Ensure that the homepage and url match, otherwise add a comment. More info at https://github.com/Homebrew/homebrew-cask/blob/HEAD/doc/cask_language_reference/stanzas/url.md#when-url-and-homepage-hostnames-differ-add-a-comment'
   Enabled: true

Library/Homebrew/rubocops/cask/ast/cask_header.rb

@@ -29,7 +29,13 @@ module RuboCop
         end
 
         def cask_token
-          @cask_token ||= pair_node.val_node.children.first
+          @cask_token ||= begin
+            if dsl_version?
+              pair_node.val_node.children.first
+            else
+              method_node.first_argument.str_content
+            end
+          end
         end
 
         def hash_node

Library/Homebrew/rubocops/cask/desc.rb

+# frozen_string_literal: true
+
+require "forwardable"
+require "uri"
+require "rubocops/cask/mixin/on_desc_stanza"
+require "rubocops/shared/desc_helper"
+
+module RuboCop
+  module Cop
+    module Cask
+      # This cop audits `desc` in Casks.
+      # See the `DescHelper` module for details of the checks.
+      class Desc < Cop
+        include OnDescStanza
+        include DescHelper
+
+        def on_desc_stanza(stanza)
+          name = cask_block.header.cask_token
+          desc_call = stanza.stanza_node
+          audit_desc(:cask, name, desc_call)
+        end
+
+        def autocorrect(node)
+          name = cask_block.header.cask_token
+          autocorrect_desc(node, name)
+        end
+      end
+    end
+  end
+end

Library/Homebrew/rubocops/cask/mixin/on_desc_stanza.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Cask
+      # Common functionality for checking desc stanzas.
+      module OnDescStanza
+        extend Forwardable
+        include CaskHelp
+
+        def on_cask(cask_block)
+          @cask_block = cask_block
+
+          toplevel_stanzas.select(&:desc?).each do |stanza|
+            on_desc_stanza(stanza)
+          end
+        end
+
+        private
+
+        attr_reader :cask_block
+
+        def_delegators :cask_block,
+                       :toplevel_stanzas
+      end
+    end
+  end
+end

Library/Homebrew/rubocops/rubocop-cask.rb

@@ -11,6 +11,7 @@ require "rubocops/cask/extend/string"
 require "rubocops/cask/extend/node"
 require "rubocops/cask/mixin/cask_help"
 require "rubocops/cask/mixin/on_homepage_stanza"
+require "rubocops/cask/desc"
 require "rubocops/cask/homepage_matches_url"
 require "rubocops/cask/homepage_url_trailing_slash"
 require "rubocops/cask/no_dsl_version"

Library/Homebrew/test/rubocops/cask/desc_spec.rb

+# frozen_string_literal: true
+
+require "rubocops/rubocop-cask"
+require "test/rubocops/cask/shared_examples/cask_cop"
+
+describe RuboCop::Cop::Cask::Desc do
+  include CaskCop
+
+  subject(:cop) { described_class.new }
+
+  context "with incorrect `desc` stanza" do
+    let(:source) {
+      <<~RUBY
+        cask "foo" do
+          desc "A bar program"
+        end
+      RUBY
+    }
+    let(:correct_source) {
+      <<~RUBY
+        cask "foo" do
+          desc "Bar program"
+        end
+      RUBY
+    }
+    let(:expected_offenses) do
+      [{
+        message:  "Description shouldn't start with an indefinite article, i.e. \"A\".",
+        severity: :convention,
+        line:     2,
+        column:   8,
+        source:   "A",
+      }]
+    end
+
+    include_examples "reports offenses"
+
+    include_examples "autocorrects source"
+  end
+
+  context "with correct `desc` stanza" do
+    let(:source) {
+      <<~RUBY
+        cask "foo" do
+          desc "Bar program"
+        end
+      RUBY
+    }
+
+    include_examples "does not report any offenses"
+  end
+end

Library/Homebrew/test/rubocops/cask/no_dsl_version_spec.rb

@@ -8,31 +8,27 @@ describe RuboCop::Cop::Cask::NoDslVersion do
 
   subject(:cop) { described_class.new }
 
-  context "with header method `cask`" do
-    let(:header_method) { "cask" }
+  context "with no dsl version" do
+    let(:source) { "cask 'foo' do; end" }
 
-    context "with no dsl version" do
-      let(:source) { "cask 'foo' do; end" }
+    include_examples "does not report any offenses"
+  end
 
-      include_examples "does not report any offenses"
+  context "with dsl version" do
+    let(:source) { "cask :v1 => 'foo' do; end" }
+    let(:correct_source) { "cask 'foo' do; end" }
+    let(:expected_offenses) do
+      [{
+        message:  "Use `cask 'foo'` instead of `cask :v1 => 'foo'`",
+        severity: :convention,
+        line:     1,
+        column:   0,
+        source:   "cask :v1 => 'foo'",
+      }]
     end
 
-    context "with dsl version" do
-      let(:source) { "cask :v1 => 'foo' do; end" }
-      let(:correct_source) { "cask 'foo' do; end" }
-      let(:expected_offenses) do
-        [{
-          message:  "Use `cask 'foo'` instead of `cask :v1 => 'foo'`",
-          severity: :convention,
-          line:     1,
-          column:   0,
-          source:   "cask :v1 => 'foo'",
-        }]
-      end
-
-      include_examples "reports offenses"
-
-      include_examples "autocorrects source"
-    end
+    include_examples "reports offenses"
+
+    include_examples "autocorrects source"
   end
 end