add arch_only to caveats mini-DSL

Relevant to points raised by @goxberry in #2581

CONTRIBUTING.md

@@ -264,6 +264,7 @@ The following methods may be called to generate standard warning messages:
 | `logout`                          | The user should log out and log back in to complete installation
 | `reboot`                          | The user should reboot to complete installation
 | `files_in_usr_local`              | The Cask installs files to `/usr/local`, which may confuse Homebrew
+| `arch_only(list)`                 | The Cask only supports certain architectures.  Currently valid elements of `list` are `intel-32` and `intel-64`
 
 Example:
 

lib/cask/caveats.rb

@@ -77,6 +77,33 @@ class Cask::CaveatsDSL
     EOS
   end
 
+  # minor bug: because output from arch_only is conditional, the
+  # existence of this directive causes "===> Caveats" header to
+  # appear even if no warning is output.  One workaround would
+  # be to spin out arch-detection from caveats into a separate
+  # Cask stanza, and that is probably a sensible design.
+  def arch_only(*supported_arches)
+    known_arches = %w{intel-64 intel-32}
+    supported_arches.each do |arch|
+      unless known_arches.include?(arch)
+        # There ought to be some standard exceptions for Cask validation errors
+        raise "The only valid arguments to caveats arch_only in #{@cask} are: #{known_arches.join(', ')}"
+      end
+    end
+    this_arch = "#{Hardware::CPU.type}-#{Hardware::CPU.bits}"
+    unless supported_arches.include?(this_arch)
+      puts <<-EOS.undent
+      Cask #{@cask} provides binaries for these architectures: #{supported_arches.join(', ')}.
+      But you appear to be running on an unsupported architecture:
+
+        #{this_arch}
+
+      Therefore #{@cask} is not expected to work on your system.
+
+      EOS
+    end
+  end
+
   def method_missing(method, *args)
     poo = <<-EOPOO.undent
       Unexpected method #{method} called on caveats in Cask #{@cask}.

test/support/Casks/with-caveats.rb

@@ -17,4 +17,9 @@ class WithCaveats < TestCask
     puts 'Custom text via puts followed by DSL-generated text:'
     manual_installer('Installer.app')
   end
+  caveats do
+    # since both valid arches are specified, no output should be
+    # generated here during the test
+    arch_only('intel-32', 'intel-64')
+  end
 end