Add --mask-directive option

lib/yard/cli/yardoc.rb

@@ -785,6 +785,12 @@ def tag_options(opts)
         opts.on('--non-transitive-tag TAG', 'Marks a tag as not transitive') do |tag|
           Tags::Library.transitive_tags -= [tag.to_sym]
         end
+
+        opts.on('--mask-directive DIRECTIVE',
+                'Masks a directive to suppress any callback ' \
+                ' and the "unknown directive" warning') do |tag|
+          Tags::Library.define_directive(tag, Tags::NilDirective)
+        end
       end
     end
   end

lib/yard/tags/directives.rb

@@ -621,5 +621,11 @@ def call
         end
       end
     end
+
+    # Does nothing but suppresses "unknown directive" warnings, if
+    # a missing directive definition was masked with this class.
+    class NilDirective < Directive
+      def call; end
+    end
   end
 end

spec/cli/yardoc_spec.rb

@@ -806,6 +806,11 @@ def tag_hidden(tag)
       @yardoc.parse_arguments('--non-transitive-tag', 'foo')
       expect(Tags::Library.transitive_tags).not_to include(:foo)
     end
+
+    it "accepts --mask-directive" do
+      @yardoc.parse_arguments('--mask-directive', 'foo')
+      expect(Tags::Library.factory_method_for_directive(:foo)).to eq Tags::NilDirective
+    end
   end
 
   describe "Safe mode" do

spec/tags/directives_spec.rb

@@ -461,3 +461,15 @@ def baz; end
     end if YARD::Parser::SourceParser.parser_type == :ruby
   end
 end
+
+RSpec.describe YARD::Tags::NilDirective do
+  describe '#call' do
+    before { Tags::Library.define_directive(:unknown, YARD::Tags::NilDirective) }
+
+    after { Registry.clear }
+
+    it "does nothing if directive is unknown" do
+      tag_parse("@!unknown")
+    end
+  end
+end