mac/diagnostic: add check_for_prerelease_xcode

Closes #524.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -4,6 +4,7 @@ module Homebrew
       def all_development_tools_checks
         %w[
           check_for_unsupported_osx
+          check_for_prerelease_xcode
           check_for_bad_install_name_tool
           check_for_installed_developer_tools
           check_xcode_license_approved
@@ -32,6 +33,21 @@ module Homebrew
         EOS
       end
 
+      def check_for_prerelease_xcode
+        return if ARGV.homebrew_developer?
+        # Running a pre-release Xcode on a pre-release OS is expected
+        # and likely to cause less problems than a stable Xcode will.
+        return if OS::Mac.prerelease?
+        return unless MacOS::Xcode.installed?
+        return unless MacOS::Xcode.prerelease?
+
+        <<-EOS.undent
+          You are using a pre-release version of Xcode.
+          You may encounter build failures or other breakages.
+          Please create pull-requests instead of filing issues.
+        EOS
+      end
+
       # TODO: distill down into single method definition a la BuildToolsError
       if MacOS.version >= "10.9"
         def check_for_installed_developer_tools