don't evaluate the default lockfile at all in certain cases

README.md

@@ -88,6 +88,33 @@ BUNDLE_LOCKFILE=rails-7.0 bundle exec rspec
 You can also dynamically select it in your Gemfile, and pass `current: true`
 to (exactly one!) `lockfile` method.
 
+In some cases, you may want to essentially disable bundler-multilock's
+syncing behavior, while still allowing the Gemfile to select the active
+lockfile. For example, if you have gems in the default lockfile that cannot
+be installed under a certain Ruby version, but still want to be able to CI
+against that Ruby version, you may set it up like this:
+
+```ruby
+lockfile active: RUBY_VERSION >= "2.7" do
+  gem "debug", "~> 1.9"
+end
+
+lockfile "ruby-2.6", active: RUBY_VERSION < "2.7" do
+  gem "debug", "~> 1.8.0"
+end
+```
+
+However, you cannot run a regular `bundle install` while running Ruby 2.6 in
+this situation, since simply evaluating the default lockfile's block, even
+for checking that the lockfiles are valid, will raise an exception that debug
+1.9.0 require Ruby 2.7. You could set `BUNDLE_LOCKFILE=ruby-2.6` to have
+bundler-multilock only consider the one lockfile, but then your CI may need
+additional logic to _not_ set it for other Ruby versions. Instead, you can
+set `BUNDLE_LOCKFILE=active`, which will not override the Gemfile's selection
+of which lockfile is active, but still behave as if `BUNDLE_LOCKFILE` is set,
+bypassing any other syncing logic and not evaluating the default lockfile in
+our example.
+
 ## Comparison to Appraisal
 
 [Appraisal](https://github.com/thoughtbot/appraisal) is a gem that might serve

lib/bundler/multilock.rb

@@ -60,7 +60,8 @@ def add_lockfile(lockfile = nil,
           raise ArgumentError, "Lockfile #{lockfile} is already defined"
         end
 
-        env_lockfile = ENV["BUNDLE_LOCKFILE"]&.then { |l| expand_lockfile(l) }
+        env_lockfile = lockfile if active && ENV["BUNDLE_LOCKFILE"] == "active"
+        env_lockfile ||= ENV["BUNDLE_LOCKFILE"]&.then { |l| expand_lockfile(l) }
         active = env_lockfile == lockfile if env_lockfile
 
         if active && (old_active = lockfile_definitions.find { |definition| definition[:active] })

spec/bundler/multilock_spec.rb

@@ -736,6 +736,25 @@
     end
   end
 
+  it "does not evaluate the default lockfile at all if an alternate is active, " \
+     "without specifying that lockfile explicitly" do
+    with_gemfile(<<~RUBY) do
+      gem "inst-jobs", "3.1.13"
+
+      lockfile active: ENV["ALTERNATE"] != "1" do
+        raise "evaluated!" if ENV["ALTERNATE"] == "1"
+      end
+
+      lockfile "alt", active: ENV["ALTERNATE"] == "1" do
+        gem "activerecord-pg-extensions"
+      end
+    RUBY
+      invoke_bundler("install")
+
+      invoke_bundler("install", env: { "ALTERNATE" => "1", "BUNDLE_LOCKFILE" => "active" })
+    end
+  end
+
   private
 
   def create_local_gem(name, content = "", subdirectory: true)