[rubygems/rubygems] Add bundle lock --update --bundler

bundle lock --update can do everything that bundle update can do, but
it doesn't actually install gems. This is especially useful for
generating a lockfile on a machine that doesn't have the libraries
available to be able to build native extensions.

But, there was no parallel for bundle update --bundler. So let's add
one.

https://github.com/rubygems/rubygems/commit/7fc00bd2a5

lib/bundler/cli.rb

@@ -672,6 +672,8 @@ module Bundler
       "If updating, do not allow any gem to be updated past latest --patch | --minor | --major"
     method_option "conservative", :type => :boolean, :banner =>
       "If updating, use bundle install conservative update behavior and do not allow shared dependencies to be updated"
+    method_option "bundler", :type => :string, :lazy_default => "> 0.a", :banner =>
+      "Update the locked version of bundler"
     def lock
       require_relative "cli/lock"
       Lock.new(options).run

lib/bundler/cli/lock.rb

@@ -22,12 +22,15 @@ module Bundler
 
       update = options[:update]
       conservative = options[:conservative]
+      bundler = options[:bundler]
 
       if update.is_a?(Array) # unlocking specific gems
         Bundler::CLI::Common.ensure_all_gems_in_lockfile!(update)
         update = { :gems => update, :conservative => conservative }
-      elsif update
-        update = { :conservative => conservative } if conservative
+      elsif update && conservative
+        update = { :conservative => conservative }
+      elsif update && bundler
+        update = { :bundler => bundler }
       end
       definition = Bundler.definition(update)
 

spec/bundler/commands/lock_spec.rb

@@ -212,6 +212,26 @@ RSpec.describe "bundle lock" do
     end
   end
 
+  it "updates the bundler version in the lockfile without re-resolving", :rubygems => ">= 3.3.0.dev" do
+    build_repo4 do
+      build_gem "rack", "1.0"
+    end
+
+    install_gemfile <<-G
+      source "#{file_uri_for(gem_repo4)}"
+      gem "rack"
+    G
+    lockfile lockfile.sub(/(^\s*)#{Bundler::VERSION}($)/, '\11.0.0\2')
+
+    FileUtils.rm_r gem_repo4
+
+    bundle "lock --update --bundler"
+    expect(the_bundle).to include_gem "rack 1.0"
+
+    allow(Bundler::SharedHelpers).to receive(:find_gemfile).and_return(bundled_app_gemfile)
+    expect(the_bundle.locked_gems.bundler_version).to eq v(Bundler::VERSION)
+  end
+
   it "supports adding new platforms" do
     bundle "lock --add-platform java x86-mingw32"