[CLI] Warn when running an outdated bundler version

lib/bundler/cli.rb

@@ -13,6 +13,7 @@ def self.start(*)
       Bundler.ui = UI::Shell.new
       raise e
     ensure
+      warn_on_outdated_bundler
       Bundler::SharedHelpers.print_major_deprecations!
     end
 
@@ -579,5 +580,26 @@ def print_command
       command.reject!(&:empty?)
       Bundler.ui.info "Running `#{command * " "}` with bundler #{Bundler::VERSION}"
     end
+
+    def self.warn_on_outdated_bundler
+      return if Bundler.settings[:disable_version_check]
+
+      latest = Fetcher::CompactIndex.
+               new(nil, Source::Rubygems::Remote.new(URI("https://rubygems.org")), nil).
+               send(:compact_index_client).
+               instance_variable_get(:@cache).
+               dependencies("bundler").
+               map {|d| Gem::Version.new(d.first) }.
+               max
+      return unless latest
+
+      current = Gem::Version.new(VERSION)
+      return if current >= latest
+
+      Bundler.ui.warn "The latest bundler is #{latest}, but you are currently running #{current}.\nTo update, run `gem install bundler#{" --pre" if latest.prerelease?}`"
+    rescue
+      nil
+    end
+    private_class_method :warn_on_outdated_bundler
   end
 end

lib/bundler/settings.rb

@@ -11,14 +11,15 @@ class Settings
       disable_exec_load
       disable_local_branch_check
       disable_shared_gems
+      disable_version_check
+      force_ruby_platform
       frozen
       gem.coc
       gem.mit
       ignore_messages
       major_deprecations
       no_install
       no_prune
-      force_ruby_platform
       only_update_to_newer_versions
       plugins
       silence_root_warning

man/bundle-config.ronn

@@ -169,6 +169,9 @@ learn more about their operation in [bundle install(1)][bundle-install].
 * `disable_checksum_validation` (`BUNDLE_DISABLE_CHECKSUM_VALIDATION`):
    Allow installing gems even if they do not match the checksum provided by
    RubyGems.
+* `disable_version_check` (`BUNDLE_DISABLE_VERSION_CHECK`):
+   Stop Bundler from checking if a newer Bundler version is available on
+   rubygems.org.
 
 In general, you should set these settings per-application by using the applicable
 flag to the [bundle install(1)][bundle-install] or [bundle package(1)][bundle-package] command.

spec/bundler/cli_spec.rb

@@ -61,6 +61,69 @@
       expect(out).to start_with("Running `bundle config --verbose` with bundler #{Bundler::VERSION}")
     end
   end
+
+  describe "printing the outdated warning" do
+    shared_examples_for "no warning" do
+      it "prints no warning" do
+        bundle "fail"
+        expect(err + out).to eq("Could not find command \"fail\".")
+      end
+    end
+
+    let(:bundler_version) { "1.1" }
+    let(:latest_version) { nil }
+    before do
+      simulate_bundler_version(bundler_version)
+      if latest_version
+        info_path = home(".bundle/cache/compact_index/rubygems.org.443.29b0360b937aa4d161703e6160654e47/info/bundler")
+        info_path.parent.mkpath
+        info_path.open("w") {|f| f.write "#{latest_version}\n" }
+      end
+    end
+
+    context "when there is no latest version" do
+      include_examples "no warning"
+    end
+
+    context "when the latest version is equal to the current version" do
+      let(:latest_version) { bundler_version }
+      include_examples "no warning"
+    end
+
+    context "when the latest version is less than the current version" do
+      let(:latest_version) { "0.9" }
+      include_examples "no warning"
+    end
+
+    context "when the latest version is greater than the current version" do
+      let(:latest_version) { "2.0" }
+      it "prints the version warning" do
+        bundle "fail"
+        expect(err + out).to eq(<<-EOS.strip)
+The latest bundler is #{latest_version}, but you are currently running #{bundler_version}.
+To update, run `gem install bundler`
+Could not find command "fail".
+        EOS
+      end
+
+      context "and disable_version_check is set" do
+        before { bundle! "config disable_version_check true" }
+        include_examples "no warning"
+      end
+
+      context "and is a pre-release" do
+        let(:latest_version) { "2.0.0.pre.4" }
+        it "prints the version warning" do
+          bundle "fail"
+          expect(err + out).to eq(<<-EOS.strip)
+The latest bundler is #{latest_version}, but you are currently running #{bundler_version}.
+To update, run `gem install bundler --pre`
+Could not find command "fail".
+          EOS
+        end
+      end
+    end
+  end
 end
 
 describe "bundler executable" do